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The development of a practical heat stress assessment methodology for use in UK industry
Prepared by Loughborough University for the Health and Safety Executive 2002
RESEARCH REPORT 008
Damian Bethea & Ken Parsons Department of Human Sciences Loughborough University Leicestershire EH8 9SU United Kingdom
A series of ten experiments were conducted at the Human Thermal Environments Laboratory at
Loughborough University to compare the validity of ISO 7933 Required Sweat Rate
Predicted Heat Strain (PHS) index predictions with observed physiological data. Comparisons were
made between the predicted and observed sweat rates and the time it took core temperature to reach
38°C (Duration Limit Exposures). The results showed that neither the
model were valid predictors of Duration Limit Exposures (DLE) and predicted sweat rate for people wearing protective clothing in warm humid environments. The PHS DLE predictions were more representative of the ISO predictions that the observed DLEs in all but one experiment. Although the PHS model predictions of sweat rate were an improvement on the ISO predictions. The model also significantly underestimated the observed sweat rates. Following the validity study, a heuristic evaluation of the usability of ISO 7933 was also performed. This found that the standard is unnecessarily scientific and may not encourage users to use it. The format and the information provided do not satisfy ergonomic guidelines of usability. The need for a more practical heat stress assessment methodology was identified. A framework was formulated for the design, development and evaluation of such a methodology. This framework consisted of three stages; an Exploratory Stage, a Design and Development Stage and an Evaluation Stage. This resulted in the development of a practical heat stress assessment methodology.
SW req nor the modified SW req
) and
(SW req
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.
HSE BOOKS
© Crown copyright 2002 Applications for reproduction should be made in writing to:
Copyright Unit, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ
ISBN 0 7176 2533 8
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording or otherwise) without the prior written permission of the copyright owner.
The authors would like to thank and acknowledge all those who contributed and took part in this project, including:
· Members of the British, European and International Standards Committees;
· The researchers involved in the BIOMED II heat stress project;
· The advisory committee for the project (Professor Rainer Goldsmith, Robin Howie, Geoff Crockford, Dr W R Withey);
· The subjects who took part in the laboratory experiments,
· The workers who took part in the field experiments,
· The managers and safety officers who contributed to discussion groups and made workplaces available for assessment (particularly those in the Sittingbourne paper mills and the steel mills in Teesside as well as firemen, foresters and others across the country);
· The members of the British Occupational Hygiene Society *BOHS) who participated in discussion groups, interviews and questionnaires;
· Those members of industry who gave practical advice and completed the postal questionnaires from which the final version of the practical heat stress assessment method was completed.
Finally, we would like to thank Len Morris Chris Quarrie of the Health and Safety Executive, Bootle, for providing invaluable support and advice throughout the project.
It has long been known that work in the heat can compromise health and safety and lead directly or indirectly to worker injury and death. This project, sponsored by the Health and Safety Executive in the United Kingdom, was conducted to ensure that developments in heat stress assessment were evaluated and where to advantage, included in assessment methods appropriate for use in British industry. The project was conducted by the Human Thermal Environments Laboratory at Loughborough University who are nationally and internationally active in the field and have been involved in heat stress research for over 20 years.
The project presented in this report is unique in that it addresses the issues of both validity and usability. It was conducted in parallel with a number of national and international initiatives into heat stress assessment. Professor Ken Parsons was particularly well placed to ensure that the project could act on up-to-date developments. As Chairman of ISO TC 159 SC5 'Ergonomics of the Physical Environment', Convenor of CEN TC 122 WG11 'Ergonomics of the thermal environment' and Chairman of BSI PH9/1 'Ergonomics of the thermal environment', he was involved in the revision of relevant standards. Professor Parsons and Damian Bethea were also involved in the production (through the Department of Trade and Industry and the Institute of Occupational Medicine, Edinburgh) of the standard BS 7963 (2000) Ergonomics of the thermal
environment – Guide to the assessment of heat strain in workers wearing personal
protective equipment'. BS 7963 improved the application of the method used in ISO 7933 (1989), based upon the calculation of required sweat rate, to take account of protective clothing and equipment. The revision of ISO 7933 is underway and the proposal is to replace the required sweat rate index (SWreq) with the Predicted Heat Strain index (PHS). We are fortunate to have had access to full documentation of this work as well as to the related heat stress assessment strategy proposed by Malchaie et al. (2000). This is because Loughborough were also participants in the European Union BIOMED II project which reviewed and extensively revised the method presented in ISO 7933 (1989).
The BIOMED II research team was co-ordinated by Professor Jacques Malchaire (Universitée Catholique de Louvain, Belgium) and involved the following researchers and laboratories: Alain Pettite (Université Catholique de Louvain), Professor Barbara Griefahn, Peter Menhert (IfaDo, Dortmund, Germany), Professor Ingvar Holmér, Hakan Nielsson (National Institute for Working Life, Stockholm, Sweden), Dr George Havenith (now at Loughborough), Emile Den Haag (TNO, Sousterburg, Netherlands), Professor Ken Parsons, Damian Bethea (HTEL, Loughborough, UK), Dr Bernhard Kampmann (German coal mines, Dortmund, Germany), Dr Hans Jurgen Gehardt (Wuppertaal, Germany) and Professor Gaetano Alfano (University of Naples, Italy).
The present project achieved its aim of investigating the validity and usability of existing (ISO 7933; 1989) and proposed (PHS and risk assessment strategy) methods. It also provided a proposal for a heat stress assessment strategy that could be effectively used by British Industry.
Ken Parsons and Damian Bethea Loughborough, January, 2001.
The usability of ISO 7933 The Design, Development and Evaluation of a Practical Heat Stress Assessment Methodology
Chapter 1 Introduction & Literature Survey
1.2.1 Heat Stress Indices
1.2.2 Research Objectives
1.3 Literature Survey
Human Thermoregulation
Heat Stress in Industry
Risk Assessment & Management Considerations
Health Surveillance in the UK – Specific to the Thermal Stress
Heat Stress Indices
British, European and International Standards: Ergonomics of the thermal environment
Supporting standards
ISO 7243. Hot Environments – Estimation of heat stress on a working man, based on
the WBGT –Index (wet bulb globe temperature).
(BS EN 27243)
I.3.10.1
ISO 7933, Hot Environments – Analytical determination and interpretation of thermal
stress using calculation of Required Sweat Rate (SW req ) (BS EN 12515)
Literature review of the Validity of the ISO 7933 SW req index
BIOMED HEAT and the Predicted Heat Strain (PHS) Model
Chapter 2 Laboratory evaluations of the validity of the ISO 7933 and PHS models
2.2 Introduction to the Validity Study
2.3 Aims of studies
2.4 ISO SW req and PHS indices used in validation
2.4.1 ISO 7933 SW req index
2.4.2 ISO 7933 SW req (modified) index
2.4.3 PHS index
2.5 Data considerations
2.5.1 ISO and ISO(mod)
2.5.2 PHS model
2.6 Analysis of Results
Statistical Analysis to be used
2.7 Study 1 (Experiments 1 to 6): Validity of ISO and PHS models for people wearing boiler suits in a
range of hot conditions
2.7.1 Method – Study 1
2.7.2 Procedure
2.8 Study 2 (Experiments 7 and 8): Validity of ISO and PHS models for people wearing clothing with
different vapour permeability
2.9 Study 3 (Experiments 9 and 10): Validity of ISO and PHS models for tea leaf pickers in a simulated hot environment
2.10 Results of the 3 Studies - Comparison of ISO 7933, ISO(mod), PHS and Observed Physiological
Comparison of Estimated and Measured Metabolic Rates
Comparison of within model predictions for estimated and measured metabolic rate
Comparison between ISO and PHS model sweat rate predictions and observed data64
Comparison between ISO and PHS model DLE predictions and observed data
Comparison between models
2.11 Discussion
2.12 Conclusions
Chapter 3 Usability of ISO 7933
3.2 Heuristic evaluation of ISO 7933
3.2.2 Heuristic Analysis
3.2.3 ISO Usability -Paper
3.2.4 ISO Usability -Program
3.2.5 ISO Usability – Practical Example
3.2.6 Conclusions of Heuristic Evaluation of ISO 7933
Chapter 4 Design, development and evaluation of a practical heat stress assessment methodology
4.2 Stage 1 – Exploratory Stage
4.2.1 Aims
4.2.2 Methods
4.2.3 Ergonomics techniques for data collection
4.2.4 Informal exploratory
4.2.5 Hierarchical Task Analysis (HTA) of Heat Stress
4.2.6 Field Trials – Heat Stress Assessment in a Paper Mill
4.2.7 CONCLUSIONS OF STAGE 1 – Exploratory Stage
4.3 Stage 2: Design and Development Stage
4.3.2 Rating and Ranking Methods for Risk Assessment
4.3.3 Structured Group Discussions
4.3.4 Postal Questionnaire Survey
4.3.5 CONCLUSIONS OF STAGE 2 – DESIGN Stage
4.3.6 The proposed Observation Checklist for the Assessment of Heat Stress
4.4 Stage 3 – Evaluation Stage
4.4.1 User Trials
4.4.4 Method
4.4.6 Results
Chapter 5 A Practical Heat Stress Assessment Methodology
5.1 Part 1: Checklist to identify possible personal risk
5.2 Observation Checklist for Heat Stress Risk Assessment
5.3 Thermal Environment Considerations
5.3.1 Air temperature
5.3.2 Thermal Radiation
5.3.3 Air velocity
5.3.4 Humidity
5.3.5 Work Rate
5.3.6 Clothing
5.4 Results of Observation Checklist for Heat Stress Risk Assessment
Chapter 6 Appendices
6.1 Tables for estimating metabolic rate from its components.(Taken from ISO 8996)
6.2 Malchaire's Observation methodology
Chapter 8 Glossary of Terms
Printed and published by the Health and Safety Executive
Methods for the assessment of hot environments have been developed at national, European and international levels. It is generally recognised however, that they are limited in their application. BS EN ISO 27243 has been the accepted as a simple assessment method since 1984. It is based upon the wet-bulb globe temperature index (WBGT). ISO 7933 (BS EN 12515) presents an analytical approach to heat stress assessment called the Required Sweat Rate (SW req ) index. The SW req index evolved from the Heat Stress Index (HSI – Belding and Hatch, 1955). A revision of the SW req index has lead to the development of the Predicted Heat Strain (PHS) model.
Criticisms of existing methods for the assessment of heat stress are mainly in three areas:
· The SCOPE of the methods is often not sufficient to meet the varied realities of industrial heat stress. For example, methods do not account for the effects of protective clothing and equipment, yet workers in hot environments often wear this sort of clothing;
· The VALIDITY of methods has been questioned. Does the method really relate to the heat strain on the workers?
· In recent years the USABILITY of methods has been recognised as important. Methods may be valid and sufficient in scope but if they are too complex or not presented in a way that is complementary to the context, people and organisations who use them then they may not be used correctly, if at all.
A series of ten experiments were conducted at the Human Thermal Environments Laboratory at Loughborough University to compare the ISO 7933 Required Sweat Rate SW req and PHS predictions with observed physiological data. The SW req model was also modified to provide a modified skin temperature for conditions where subjects wore clothing with clothing insulation values of greater that 0.6 clo (as required in the standard). This was called the ISO mod model. Comparisons were made between the predicted and observed sweat rates and the time it took core temperature to reach 38°C (Duration Limit Exposures). The results showed that neither the SW req nor the modified SW req model were valid predictors of Duration Limit Exposures (DLE) and predicted sweat rate for people wearing protective clothing in warm humid environments. The PHS DLE predictions were more representative of the ISO predictions that the observed DLEs in all but one experiment. Although the PHS model predictions of sweat rate were an improvement on the ISO predictions. The model also significantly underestimated the observed sweat rates.
One area of concern was that in industry users of the indices would probably predict the metabolic rate. As such, comparisons were made between the results when estimated metabolic rate and measured metabolic rate were inputted into the models. Insignificant differences in metabolic rate inputs (p<0.42) had a greater impact on the predicted DLEs in the ISO models, with little effect evident in the PHS predictions. These differences between observed and predicted DLE within each model were insignificant. However, the differences in metabolic rate inputs had a significant effect on the SW p in the PHS model (p<0.00) and not on the ISO (p<0.25) and ISO mod (p<0.24) SW p predictions.
THE USABILITY OF ISO 7933
A heuristic evaluation of the usability of ISO 7933 was performed. Three main areas of the
standard were investigated; the paper based standard, the computer program, and the practical use
of the model to determine safe work times. The paper based standard had poor or inadequate
usability in a number of areas; simplicity, structure, consistency, speaking the user's language, providing adequate information and minimising user memory load. The standard appears unnecessarily scientific and would not encourage users to use it. The format and the information provided do not satisfy ergonomic guidelines of usability. Users should not be expected to use the equations presented in the standard and to work through them manually. This standard can only be used as a computer program and, as such, much of the information provided in the body of the standard is superfluous to their requirements.
THE DESIGN, DEVELOPMENT AND EVALUATION OF A PRACTICAL HEAT STRESS ASSESSMENT METHODOLOGY
The following framework for the design, development and evaluation of a practical heat stress
assessment methodology was formulated. The overall strategy of the system life cycle was adopted
to establish and define the processes that would be employed in the design, development and
evaluation of the methodology to meet the specific requirements of this research. The life cycle consisted of the three stages:
Exploratory Stage - Involved the background research into the subject area with a literature
review of those topics that would be investigated during the research. Knowledge of ISO 7933 SW req was gained through a series of validity studies and a heuristic analysis of the usability of the standard. Design and Development Stage - During this stage a number of methods were used to establish a formal definition of who the users would be so that their requirements of a practical heat stress assessment methodology could be identified. A further requirement was to define what the functional specification of the methodology should be. This information, along with the findings of detailed literature survey would provide the basis for the development of the methodology. Evaluation Stage - Production of Final Prototype will be followed by a formal comparative usability evaluation of the prototype methodology with the three stage model developed by the BIOMED project.
The structure of the system's lifecycle was dynamic and underwent a number of alterations due to the difficulties encountered as the project progressed. A number of ergonomic methodologies were used to elicit data to enable the development of user requirements and a functional specification of a practical methodology as well as its evaluation. The methods used included; hierarchical task analysis, informal interviews, questionnaires, structured discussion groups and user trials. The result was that a practical heat stress assessment methodology has been developed and is proposed for further detailed evaluation in UK industry.
The aims of this chapter are to introduce the topic of heat stress risk assessment, and to identify the context and issues within which the development of a valid and usable heat stress assessment method can be carried out.
Chapter 1 includes Sections 1.2 (Introduction) and 1.3 (Literature Review) of the Report. In Section 1.3, the concept of a heat stress index and current British, European and International standards are introduced. Research objectives were identified in relation to the evaluation of heat stress based on the validity and usability of the existing SW req model and the proposed predicted heat strain PHS model. The outcome of the project was to be the user-oriented design, development and evaluation of a practical heat stress assessment methodology for UK industry.
The aim of the literature review was to use past research and current knowledge and activity to provide direction for the research. It was concluded that there is growing evidence that the current international standard ISO 7933 (1989) has limited validity and is not usable. A BIOMED II European Union project has produced a Predicted Heat Strain (PHS) index. This index has only been validated by its developers and should be investigated further through independent evaluation.
A heat stress index integrates the effects of relevant environmental and personal factors into a single number that varies in direct relation to the degree of heat strain on people.
The validity of an index is an indication of how well the index value relates to heat strain and the usability of the index relates to how easy it is to apply by users; for example, in an industrial context. There are a number of possible benefits to be obtained from a valid and usable index, for example;
· to indicate the degree of heat strain expected in workers,
· to monitor work to ensure that there is acceptable risk,
· to determine acceptable exposure times to work in the heat.
· to provide limits beyond which unacceptable heat strain would occur.
"An optimal heat stress index should provide an accurate prediction of the worker's physiological state at any time of exposure, thus allowing the occupational hygienist to assess the permissible duration of exposure and the duration of rest breaks. This objective implies that the index value at a given time takes proper account of the characteristics of past exposure and the response-time constant of the physiological variable considered. This feature of the index variation can only be studied in well controlled conditions where both the input parameters (metabolic rate, climatic
parameters) and the output variables (sweat rate, body temperature, heart rate) are measured with accuracy," Mariaux and Malchaire, 1995.
Since the industrial revolution and the resultant proliferation and advancement of industrial technology, people have been at risk from exposure to occupational hazards that would have adverse effects on their health and well being. One such occupational hazard is heat. The thermal environment in industry is a direct result of the advances in the mechanisation of the production process and differs greatly from the ambient weather-dependent climatic conditions (Eissing, 1995). Since all humans are susceptible to the stresses of exposure to heat, knowledge of the magnitude of the environmental heat load would enable the consequences of the exposure to be predicted (Belding and Hatch, 1955). To estimate the magnitude of the heat load, the six human thermal environmental variables (parameters) - air temperature (t a ), radiant temperature (t r ), air velocity (Va), humidity (h), clothing properties and metabolic rate are critical.
A great deal of research has resulted in the development of practical methodologies for the design
and evaluation of hot working environments. The integration of this knowledge has resulted in the concept of the heat stress index to specify safe working conditions for hot environments. Belding
and Hatch expanded upon the six basic parameters and described 13 parameters which they divided into 2 subgroups:
Factors determining imposed heat: Temperature of the air, of walls etc, of the skin, water vapour pressure, air velocity, metabolic heat production, body surface area exposed and postural attitude, and finally clothing.
Factors determining resulting strains: Heat tolerance and the consequences of increased deep body temperature, exposure duration, skin wettedness and other consequences of sweating, vasodilatation of blood vessels at the skin and the consequences of this increased blood flow to the skin.
is from these expanded parameters that the principles of rational heat stress indices have been
based, as these parameters cannot be treated separately. The development of heat stress indices has
also been aided by an improved understanding of the interaction of these parameters, along with technical advances and an increase in the use of computers. Computer models of thermoregulation and the development of international and national standards has created the mechanisms for thermal audits and education and the methodologies for their execution (Parsons, 1995). Standardisation of these methods has provided exposure limits for the worker who may be at risk from heat stress.
number of influential bodies throughout the world that have been responsible for the development
these standardised methods. These bodies include the International Standards Organisation
(ISO), British Standards Institute (BSI), American Conference of Governmental Industrial Hygienists (ACGIH), American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) and more. The most significant recent development has been the international co operation that led to the development of ISO standards for the ergonomics assessment of human thermal environments. Many of these standards have also been adopted as European Standards by CEN (European Standards body) and nationally by the relevant bodies, such as the British Standards Institution (BSI). The European Coal and Steel Community (ECSC) has developed and used ergonomics methodologies and principles extensively, to the point where it now plays a major
role in the occupational health and safety of both industries (Parsons, 1995). An important consideration in understanding the development of ergonomics within the ECSC framework is that much of the impetus of the programmes' development was 'action oriented' and industry based. The creation of wealth is not threatened by the improvements in the quality of working life and health and safety. The prevailing philosophy behind this development has been that action and research activities must be differentiated. Thus, the research aim should be to sustain the action by providing relevant data.
Many of the research activities of the ECSC led to the development of ISO 7933. Hot Environments, Analytical determination and interpretation of the thermal stresses using calculation
of required sweat rate (SWreq). This standard is also a British standard (BS EN 12515), however as this is almost identical to ISO 7933, for the purpose of this report it will be referred to as ISO
7933. The SW req index is a rational heat stress index, although it's validity has been questioned and
consequently a European project called BIOMED HEAT was conducted. The objective of the BIOMED project was to co-ordinate the work of eight European laboratories specialising in human
thermal environments to improve the assessment of hot working environments. Malchaire (1999) provides the following list of the specific objectives of the BIOMED project:
1. "To design and validate a strategy for the assessment of the strain related to hot working conditions, strategy that can be used by practitioners in the field to determine maximum allowable exposure durations and to optimise the improvement of the working environment;
2. To extend the validity of the present modelling of the role of clothing;
3. To improve the validity of the present indices in cases of high radiation, high humidity or high air velocity;
4. To better define the criteria for the determination of the maximum allowable exposure duration and in particular the inter-individual differences in sweating rate, evaporation efficiency, water loss and increase in core temperature".
This resulted in a new rational model of heat stress called the Predicted Heat Strain (PHS) index and a three-stage approach to the assessment of heat stress involving Observation, Analysis and Expert Stages.
According to Parsons, these heat stress standards can potentially contribute to a reduction in heat stress casualties, although their implementation has not had as large an impact as may have been hoped. To this end, this project is concerned with the development of practical guidelines for the use of the appropriate standards paying particular attention to the requirements of UK industry.
There were four main objectives:
1.) To evaluate the Validity of the ISO 7933 Required Sweat rate (SW req ) and Predicted Heat Strain (PHS) models (see Chapter 2) 2.) To evaluate the Usability of ISO 7933 (see Chapter 3) 3.) To design, develop and evaluate a practical heat stress assessment methodology (see Chapter 4) 4.) To produce a practical heat stress assessment methodology (see Chapter 5)
LITERATURE SURVEY
The aim of the Literature Review is to provide an overview of past research and current knowledge of heat stress relevant to ISO 7933 and those other Standards and methods concerned with the assessment of hot working environments. The survey is extensive and provides a rationale for the approach taken to achieve the project's aim to develop a valid, usable and practical method for the assessment of heat stress in British industry.
1.3.2 Human Thermoregulation
The temperature of the human body is an important indicator of the state or condition that it is in. Leitheid and Lind (1964) recalled work by Claude Bernard in 1878, who proposed the concept of the "milieu interior" where an effective thermoregulation mechanism helps maintain the mechanisms of the internal body organs. This thermoregulation occurs when receptors, sensitive to change, send messages via the Central Nervous System to the hypothalamus, from where it is believed the regulation of body temperature is controlled. The ultimate control and co-ordination of thermoregulation in humans is controlled by the autonomic system which is that neural system within the human body that controls those bodily functions which are not usually under voluntary control. When set point is exceeded, blood vessels widen (vasodilatation) and blood is pumped to the skin, thereby loosing heat from the blood to the skin. This in turn reduces the gradient between skin temperature and the air temperature, reducing the gain of heat through convection and conduction. Through this process of thermoregulation, humans maintain their deep body temperature at or about 37°C and any deviation of a degree or more (plus or minus) from 37°C can have serious consequences.
The concept of heat stress can be confusing with Leithead and Lind (1964) pointing out that "the term heat stress expresses an easily recognised concept which has a practical value but which is difficult to define". Belding (1955) also identified the possibility for confusion, but in terms of heat exhaustion and heat stroke. What is appreciated is that discomfort and fatigue are caused by low levels of heat stress, performance impairment by higher levels, while a heat that exceeds tolerance levels may be a health hazard (Rodahl and Guthe, 1988).
The Heat Balance Equation
For the internal body temperature to be maintained at around 37°C, there must be an equilibrium between the amount of heat generated within the body and the heat transferred to or from it. This equilibrium, or balance, is by no means constant, but is as dynamic as the conditions within which the body is working. The concept of the heat balance equation for the human body explains and provides an understanding of how 37°C internal body temperature is maintained. Parsons (1993) points out that all heat balance equations have the same underlying concept: heat generation within the body, heat transfer, heat storage. Equations 1 and 2 show the conceptual heat balance equation
where metabolic rate (M) provides energy enabling the body to perform mechanical work (W). The remainder of the energy is given off as heat (M-W). There are a number of ways that heat transfer can be achieved: evaporation (E), radiation (R), convection (C) and conduction (K). The resultant heat production and loss provide the storage (S), where in heat balance, S = zero.
M - W = E + R + C + K + S
when S=0
M - W - E - R - C - K = 0
Core Temperature Limits
Deep body temperature (core temperature) can rise to 39°C in controlled conditions (NIOSH 1986). It is not therefore a forgone conclusion that a worker will become a heat stress casualty when their core temperature reaches 38°C or even 39°C. According to NIOSH, 38°C provides a "modest safety margin" because as core temperature exceeds 38°C, so the risk of heat stress occurring increases. This is further complicated by the degree of accuracy with which the actual environmental and work rate loads are assessed may be suspect, with this safety margin allowing some degree of error in the assessment. If insufficient heat is lost, deep body temperature will continue to rise to between 38°C and 39°C (where collapse may occur) and 41°C (where heat stroke may occur).
The Importance of Metabolic Rate
Since heat is produced in proportion to the work rate, deep body temperature has been found to be more closely related to metabolic rate than to the rate at which body heat had to be eliminated (Nielsen, 1967). Therefore, metabolic rate is a major contributor to heat stress even when environmental conditions would suggest that worker is not at risk. Core temperature in a steady state is dependent upon work rate, while under severe environmental conditions this thermoregulation fails. Graveling and Morris (1994) stated that the criteria of many heat stress documents, e.g., ISO 7243 1989, ISO 7933 1989 and NIOSH 1986, operate on the principle that an increase in work rate should be compensated for by a reduction in the environmental heat load. It is here that the criteria for the setting of limits in an industrial setting are set. These "limit" conditions must be cooler than those that cause heat stress.
Effects on Cardiac Output
Work under heat stress conditions results in a competition for cardiac output because less blood is returned to the deep body due to vasodilatation of the blood vessels in the skin (NIOSH). The blood therefore is not only carrying oxygen to the muscles but it is also acting as a cooling fluid. As a result, heart rate increases to maintain the same cardiac output and at a sub-maximum work rate, thermoregulatory requirements override the working muscle's requirements for oxygen. Consequently, heart rate increases during heat stress compared to the same work rate in neutral conditions. This difference decreases as VO 2 nears maximum (Shepherd and Webb-Peploe (1970) cite work by Rowell et al (1966). The phenomenon of this heat-induced increase in heart rate is known as cardiac beats or thermal drift. This, when coupled with the demands placed upon the
heart due to work rate, amplifies the problems even in healthy workers and can manifests itself in heat exhaustion or collapse, due to the inadequate cardiac output providing an insufficient blood supply to the brain (Rodahl and Guthe, 1988).
Due to its ease of use and its apparent wide use in industry, Vogt et al (1983) investigated whether the use of heart rate in the setting of industrial work-rest schedules in the heat was a possible solution. However, they found that even when rest schedules were observed, the heart rate at the end of the experiment was the same as that found when continuous work was observed. This suggested that the thermal component of the subject's heart rate did not decrease sufficiently during the rest schedules. They also found that the use of heart rate alone was not a valid determinant for the establishment of self regulated work-rest schedules even though heart rate may be closely linked to the reason the worker stopped working.
Therefore, although heart rate is an excellent indicator of the general stress imposed on the body, it is not an accurate measure of heat stress because the resultant heart rate will be affected by the thermal strain under which the person is working. Heart rate is only really useful in heat stress assessment if other influencing factors are controlled. Heart rate closely follows core temperature in a general way, but may parallel skin temperature, especially under high radiant temperatures (Rodahl and Guthe, 1988).
The action of sweating itself does not contribute to the loss of heat. Rather, it is the evaporation of the sweat from the skin that drives cooling and in hot environments the evaporation of sweat is the dominant mechanism for maintaining a steady core temperature for a given metabolic rate. In humid environments the driving force encouraging sweat evaporation is diminished due to the resultant decrease in the difference between the partial vapour pressure at the skin and in the environment. As such, in humid environments the effectiveness of heat loss through sweating is reduced as the capacity for evaporation is reduced. This can lead to profuse sweating where dripping of sweat from the skin occurs resulting in insignificant loss of heat with a further reduction in body fluid. A decrease in sweating may occur as the deep body temperature continues to rise and the skin is completely wet. The situation is further complicated when a worker is wearing protective clothing. An example of the power of sweating is provided by NIOSH (1985) which states that each litre of evaporated sweat will provide a loss of 675W of energy. During sweating, salt is lost at about 4g per litre for unacclimatised workers and 1 g per litre in acclimatised workers. Rodahl and Guthe (1988) state that "prolonged exposure to heat and/or prolonged exercise almost always causes hypohydration". Additionally, rectal temperature which is used as an indication of core temperature is always significantly higher in dehydrated subjects. The increase in heart rate due to thermal stress is further exacerbated by the loss of water through sweating because large sweat losses reduce the body's water content (hypohydration) and therefore reduces the blood volume.
Sweating efficiency increases with acclimation because maximum sweat production is greatly increased and a sub-maximum sweat production level is achieved at a much lower t sk and t c . Another consideration is that the distribution of sweat is improved with an overall improvement with respect to the total surface area where sweat is being produced and the amount of sweat
produced. Acclimation is achieved with repeatedly exposing an individual to a hot environment over a number of days.
1.3.3 Heat Stress in Industry
Leithead and Lind (1964) wrote that "for the most part", industrial work presents only low levels of heat stress although in many cases those environmental conditions found naturally are exceeded greatly. Meyer and Rapp (1995) stated that despite technological advances "heat stress remains one of the most frequent complaints of French workers". This contrasts with work they cite by Millican et al (1981) who stated that heat stress "comes low on the list of hazards" although a survey by the French Ministry of Social Affairs reported a daily or frequent occurrence of heat stress in 16.6% of workers. These results may show an overall decrease in complaints since a similar survey conducted in 1979 suggested 19.4% of workers suffered heat stress. Meyer and Rapp reported that these results would seem to be applicable to other western countries where one of the most frequently reported stress factors is working in the heat. (For a detailed review of heat stress in industry see the literature review by Rodahl and Guthe, 1988.)
One of the features associated with the assessment of heat stress in industry appears to be that the exposures are often infrequent and may be for short periods. This is important, because the assessors of the risks may be conducting heat stress assessments so infrequently that they find it difficult to transfer their knowledge from one assessment to the other. Meyer and Rapp (1995) provide three examples of what they call "unusual field situations".
1. Bin incinerator workers cleaning a furnace, where protective clothing is worn (including fresh air supply), high metabolic rates are observed, with exposures lasting between 4 and
2. Maintenance of food sterilising conveyors where the work has to be carried out during normal operation. Workers may work in a confined area for time periods of between 10 to
3. Workers checking the operation and maintenance in paper drying rooms with exposures
lasting for between one and two minutes.
Here the definition of risk provided by Covello and Merkhofer (see Section 0) suggests the need to consider "uncertainty" in the heat stress risk assessment procedure.
The following list of industries where workers may be at risk from heat stress has been compiled from literature reviews by Meyer and Rapp (1995), NIOSH (1986) and Rodhal and Guthe (1988) who also included a review of their own work:
· Glass products manufacturing plants;
· Drying operation in glass-wool manufacture;
· Potash Mines, Coal mines, gold mines, etc;
· Steam and compressed air tunnels;
· Conventional and Nuclear power plants – Sweeper tasks and maintenance tasks;
· Iron, steel, aluminium and other non-ferrous foundries and smelting operations;
· Brick-firing and ceramics operations;
· Plants producing rubber and rubber products;
· Electrical utilities (particularly boiler rooms);
Bakeries, confectioneries and catering kitchens;
Laundries and Ironing in dry cleaning shops;
Food canneries.
Personal Protective Equipment is considered to be a "last resort" to protect workers from the hazards in the workplace (HSE, 1992). Wherever possible, engineering controls and safe systems of work should be provided. In practice though, Crockford (personal correspondence) found that PPE is often used as a first resort as it is cheaper than introducing engineering controls. This is mainly because the effects the environment has on worker performance are clearly less severe than they are on safety.
PPE and Heat Transfer
The very nature of PPE means that it interferes with the body's ability to lose heat from the skin to the environment because of the insulation provided by the clothing and the micro-climate within the garment (Kerslake, 1972; Goldman, 1988; Kenney et al, 1988; Parsons, 1993; Bethea and Parsons, 1998; Bernard and Metheen, 1999). The thermal insulation required to maintain a thermal balance for a human seated in a room at 21°C (air and radiant temperature), humidity less than 50% and air velocity equal to 0.1 m.s -1 was defined as 1 clo by Gagge et al in 1941. One clo is equal to a clothing insulation of 0.155°C.m 2 .W -1 (the boundary layer of air around the human body is equal to 0.12 °C.m 2 .W -1 ).
Evaporation of heat for humans under warm or hot working environments provides a powerful cooling mechanism. "Clothing both inhibits evaporation by producing a humid microclimate and diminishes the cooling effect of the evaporation that does take place" (Nunnely, 1989). This means that heat stress in wearers of protective clothing occurs at lower environmental temperature and humidity values than for those of nude subjects.
The ability to transfer heat from the deep body tissue to the skin is achieved through the transfer of heat by blood to the skin and a key factor in heat stress amongst wearers of PPE is the convergence of skin temperature and core temperature. A skin temperature of 33°C and core temperature at about 37°C is usually observed when seated at rest. It is this 4°C difference between core temperature and skin temperature that enables heat to be transported to the skin. Therefore, as skin temperature increases so the amount of heat that can be transferred per unit of blood will reduce linearly (Goldman, 1988). As a result there is an insulation induced rise in skin temperature and the resultant limited ability to dissipate heat will causes a rise in deep body temperature.
According to NIOSH (1986), the skin temperature must be maintained at least 1°C below the core temperature if the blood is to be cooled before returning to the body core. Due to this convergence of t sk and t c , heart rate increases in an attempt to maintain cardiac output while the volume of blood pumped during each beat decreases. According to Goldman, "if it does not occur beforehand, heat exhaustion collapse is almost certain to occur at or before the point where skin temperature reaches deep body temperature". Here Goldman cites work by himself and Pandolf (1978) which showed that collapse may occur with core temperatures "as low as 38.2°C with skin temperatures in the 37°C range". The skin temperature though is affected by the insulative properties of clothing.
This inability to lose heat from the body when PPE is worn may even result in many workers either wearing the PPE incorrectly or not at all because it makes them feel hot (McCullough, 1983). Meyer and Rapp (1995) cite Millican et al (1981) and Badet et al (1983) who stated that protective clothing was seen more as a handicap than as protection. This is illustrated by Dukes-Dobos et al (1986) who reported that bin incinerator workers in the Aluminium plants that they studied seldom wore their protective clothing. Thus, these workers are exposing themselves to the hazards from which the PPE was supposed to be protecting them, thereby rendering their PPE ensembles redundant.
PPE and Metabolic Rate increase
Duggan (1988) suggested that when estimating metabolic rate for wearers of PPE, the extra metabolic heat production caused by the PPE needs to be considered and the practical implications of this concerns job design. Here, if a worker is to perform a particular task while wearing PPE, they will have a higher metabolic rate than when doing the same task with no PPE. Therefore, it is necessary (where possible) to redesign jobs so that the worker does not have to work at the same intensity as they would if they were not wearing the clothing. This is an important point because this resultant increase in metabolic heat production has two possible consequences:
The onset of fatigue will occur sooner than it would without PPE
Increased heat production increases the need for heat dissipation from the worker and the clothing ensemble.
Teitlebaum and Goldman (1972) reported that in a series of experiments, the PPE worn increased the work rate more than when the equivalent weight of the PPE was worn on a belt. They reported an increase of 18%, while they cite other authors as reporting varying increases in metabolic rate from 5 to 10% (During et al, 1966) and 11 to 13% (Consolazio et al, 1963).
More detailed estimates of the increases in metabolic rate associated with the wearing off PPE are provided in BS 7963 (2000). The table below is reproduced from Table 2 of BS 7963 (2000).
Table 1: Estimated Increases in Metabolic rate due to wearing PPE
INCREASE IN METABOLIC RATE DUE TO WEARING RPE
(W.M-2)
Resting Low metabolic rate
Safety shoes/short boots 0
Safety boots (long) Respirator (low/moderate performance e.g. P1, P2) Respirator (high performance e.g. P3) 5 Self contained breathing apparatus Light, water vapour
permeable chemical coverall 5 (e.g. Disposable) Chemical protective water vapour impermeable ensemble [e.g. polyvinyl 10 chloride (PVC)] with hood, gloves and boots Highly insulating, water vapour semi-permeable ensemble (e.g. firefighter's
gear consisting of helmet, tunic, over trousers, gloves and boots)
According to BS 7963: 2000, the following needs to be considered:
· Metabolic rate values have been rounded off to the nearest 5 W.m -2 .
· Respirator classification defined in BS EN 143 (1991).
· As can be seen from the estimated increases in metabolic rate, very high metabolic rate cannot be maintained when wearing some forms of PPE. Here job redesign would probably be necessary if engineering controls were not possible.
· Do not add footwear induced increased in metabolic rate for sedentary tasks.
· The table presents empirical data. If more accurate methods of obtaining metabolic rate values are required, refer to BS EN 28996.
· If metabolic rate is measured instead of estimated, corrections for the increased metabolic rate are not necessary.
1.3.5 Risk Assessment & Management Considerations
The development of a practical heat stress assessment methodology requires that not only are the heat stress issues considered, but that the role such a methodology may play in the overall risk management and assessment process is understood. To this end, a detailed review was conducted on risk assessment and risk management literature and appropriate principles have been adapted to meet the very specific requirements of not only this project but heat stress assessment as a whole. Primarily though, this has been done from a user-oriented approach and not from an industry or risk specific perspective in order to better understand how health and safety professionals, occupational hygienists etc. may tackle a general risk assessment, what their expectations may be, and what their training tends to cover. This will provide a benchmark by which the current processes of heat stress risk assessment are compared to identify if there are any areas where current heat stress risk assessment strategies do not meet those of generic risk assessment procedures.
The Process of Risk Assessment
"In many countries the requirement to provide a safe environment is a requirement by law" (Osborne and Gruneberg, 1983). The UK is no different, where there is legal requirement for companies to identify hazards and to evaluate the risks that these hazards impose. The regulations for risk assessment are provided in the Management of Health and Safety at Work Regulations, 1992 consisting of the following documents:
· Manual Handling Operations Regulations 1992 (Manual Handling Regulations);
· Personal Protective Equipment at Work Regulations 1992 (PPE);
· Health and Safety (Display Screen Equipment) Regulations 1992 (Display Screen Regulations);
· Noise at Work Regulations 1989 (Noise Regulations);
· Control of Substances Hazardous to Health Regulations 1994 (COSHH);
· Control of Asbestos at Work Regulations 1987 (Asbestos Regulations); and
· Control of Lead at Work Regulations 1980 (Lead Regulations).
To comply with current health and safety legislation, employers and the self-employed are required by Regulation 3 of the Management of Health and Safety at Work Regulations 1992 to carry out a risk assessment to identify the measures they need to take.
Hazards and Risks
The most common definition for a hazard describes something that has "the potential to cause harm" while a risk is "the likelihood of that the harm will be realised". HSE (1991) provides the well-published equation for estimating risk where hazards are scored based on severity and probability,
Risk = hazard severity x likelihood of occurrence
Everley (1994) added to this equation by multiplying risk by a number that represents the number of people who may be exposed to the risk. Recently though this definition has been considered to be somewhat restrictive and an alternative has been provided by Covello and Merkhofer (nd) where; "Risk. A characteristic of a situation or action wherein two or more outcomes are possible, the particular outcome that will occur is unknown, and at least one of the possibilities is undesired."
Although this appears to be a somewhat cumbersome definition, it highlights an important
principle. Risk is a two dimensional concept where one of two possible options may be found:
1. "The possibility of an adverse outcome
2. Uncertainty over the occurrence, timing or magnitude of that adverse outcome."
A risk exists therefore, if there is uncertainty about the risk. Risk therefore is something that is both
uncertain and undesired. The traditional definition of risk as the product of occurrence probability and consequence magnitude is relatively simple to define when one considers the cause-and-effect
of single parameter hazards such as chemicals, noise etc. The risk of heat stress is extremely
difficult to define due to the interaction of the six basic parameters. Added to this, the interpersonal
differences in physiological responses further complicate the risk estimation.
ISO definition of heat stress
A formal definition of heat stress is defined by the standards as "heat stress to which a person
exposed to a hot environment is subjected, in particular, is dependent upon the production of heat
inside the body as a result of physical activity and the characteristics of the environment governing heat transfer between the atmosphere and the body." Although this report refers to International Standards, where a standard has been ratified as a British Standard, the standard's reference number
is provided in brackets.
Definition of Heat Stress Risk Assessment Methodology
From this definition the following definition for a Heat Stress Risk Assessment Methodology was
developed:
Heat Stress Risk Assessment Methodology: "A self-contained, systematic procedure which in part, or full, achieves the goals of the specified heat stress risk assessment."
HSE Five Steps to Risk Assessment
The Health and Safety Executive (HSE) has produced a "Five Steps to Risk Assessment" procedure that provides an easy to use, practical approach to risk assessment. The five steps are:
STEP 1: Look for the hazards; STEP 2: Decide who might be harmed and how; STEP 3: Evaluate the risks and decide whether the existing precautions are adequate or whether more should be done; STEP 4: Record your findings; STEP 5: Review your assessment and revise it if necessary.
Therefore the methodology developed for this project must meet the overall strategy of the "five steps" approach.
Decision Making Sequence
According to Glendon (1987) there are three main areas in human error that may lead to a system failure (when health and safety is viewed as a system):
· Cognition: This includes areas such as the perception of hazard and of risk, the ability to label hazards and risks, the development and learning processes involved and ascribing responsibility, cause and blame;
· Behaviour: How people respond to hazards, cause accidents, make errors and correct their behaviour as a result of those errors. This author however would add "the macho factor" where the worker is not willing to admit that they may be experiencing discomfort or stress;
· Environment: Valid indicators of the risk associated to hazard exposure, the culture of safety, training and education and communication.
The training objectives for both the individual and the organisation should follow a specific decision making sequence according to Glendon (1995). This is an important consideration for this project as it may dictate how any practical methodology may be implemented. His approach has been adapted specifically for application in environments where workers may be at risk to heat stress, and is discussed in further detail in Section 4.3.5 on Page 163
1.3.6 Health Surveillance in the UK – Specific to the Thermal Stress
Honey et al (1996) conducted an HSE sponsored research project to investigate health surveillance by employers in the United Kingdom. Their study consisted of a postal questionnaire of 5000 employers (35% response rate) and over 30 interviews. One of the areas that they considered was hot and cold working environments. Although there was no separation of this category into hot environments and cold environments, their results are discussed in the section as they provide some insight into the risk assessment and control of health surveillance for employees working in extreme thermal environments. It is recognised that many of the respondents may have been from companies with cold working environments, however since the philosophies for the assessment of both types of environment are similar, their findings may be relevant to both. All data reported were weighted according to the base percentages of responses that could have been given. If the responses were below the 50% base they were presented in brackets to indicate that the data should be treated with caution, and below 25% base response "(no data)" will be shown. Some findings of the survey by Honey et al are presented below:
· 13.7% of respondents identified hot and cold environments as a hazard from which their employees were at risk. The following are industry sectors that identified thermal hazards (the proportion of each is also presented): Agriculture (no data); Energy/Water 30.3%; Metals/Minerals 16.3%; Engineering 11.3%; Other manufacturing 8.7%; Construction 30.6%; Distribution/Hotels 1.5%; Transport/Comms 23.6%; Business Services (3.4%); Other Services 23.4%; All Establishments 13.7%. The sector with the highest percentages was construction, followed by Energy/Water;
· Of these, 5.9% of respondents classified thermal stress as being the most serious hazard in the workplace of which only (34.1%) conducted risk assessments specific to the thermal environment. (Note: the bracket shows that this was for less than 50% base of respondents who
reported thermal issues.) (65.4%) did not conduct thermal risk assessments and (0.6%) did not know if thermal assessments were conducted. The (34.1%) was the lowest percentage of risk assessment for any category, with all the other categories having risk assessments by at least two thirds of respondents;
· Another finding of concern was that only (13.7%) of respondents reported pre-employment assessments (such as medical examinations and self completion questionnaires) for workers working in environment where hot/cold conditions were found. The rest, (86.3%), conducted no pre-employment assessments. This is a concern because the health status of the worker is critical in hot working environments. Unfortunately, Honey et al also found that (81.1%) reported that health surveillance was not undertaken for hot and cold conditions. From the (18.9%) where it was conducted, insufficient data (i.e. less than 25% base of respondents) were available for different health surveillance strategies. The strategies they categorised were; "inspection of readily detectable conditions, enquiries about symptoms, medical surveillance by a qualified doctor, biological effects monitoring, biological monitoring." One high-risk population is pregnant women. The study reported that (52.3%) of respondents did not make provisions for pregnant women, while (47.2%) stated that they were not at risk. Only (0.3%) did make provisions;
· Considering the importance of health status and other factors such as experience, skill etc. when working in thermal extremes, it was concerning to find that the lowest percentages for any of the hazards were reported in the areas of employee (37.9%) and management (47.2%) training in hot/cold environments. Record keeping was another area where hot and cold hazards were not being adequately addressed, with (93.1%) of respondents stating that no records were maintained.
Although no direct conclusions with regard to hot/cold environments were drawn from the postal survey, a number of thermal environment conclusions (for hot/cold) can be drawn:
1.) Of the respondents that listed hot/cold environments as a hazard, less than 50% answered many of the questions, as was evident by the percentages given in brackets. This may be for two reasons:
i.)
Thermal stress is not high on their list of high-risk hazards and as such is considered
secondary to the other risks to which their workers may be exposed; ii.) Respondents may not have had sufficient knowledge of thermal stress risk assessment and/or the consequences of exposure to thermal hazards; 2.) Health surveillance, risk assessment and pre-employment screening of thermal stress were inadequately performed in those industries where heat stress was considered a risk; 3.) Record keeping needs to be improved so that the effectiveness of controls, health status of workers and the accuracy of thermal risk assessments can be monitored.
Many attempts have been made over the years to develop an index which, through a single figure, is able to provide an indication of the risk of heat stress (Kerslake, 1972). However, a thermal index has yet to be developed which can accurately predict a person's physiological strain to all environments (NIOSH, 1986). Goldman (1988) and Morris (1994) say that there is a growing body of opinion that computer modelling provides the best solution to the prevention of heat stress.
Belding provides the following recommendations to anyone who is "so bold as to improve on existing standards":
· One of the major problems seems to be the establishment of criteria which are representative of physiological stress and strain;
· To accurately predict the level of strain in such a way as to provide a practical application;
· The "ultimate index should provide a means for rating time-limited exposures";
· The use of sweat rate is only really justified in terms of dehydration levels and salt depletion;
· Mean skin temperature is the result of the effects of environment and metabolic heat loads on the circulatory efficiency of the blood and the evaporative cooling effectiveness.
A heat stress index is a model of human thermoregulation. Models (and indices) are, by their very
nature, limited in their functionality and as such the model (whether it be graphical or numeric representation, mathematical equations) will never be a perfect representation. Therefore, when investigating models, errors or deviations from the observed are expected but the performance
criteria is not so much how accurate it is, but rather whether or not these inaccuracies are significant
in terms of the application or situation to which it is intended to be applied. This is very much the
case in Human Modelling because there is such a wide variation between individuals in their physiological responses. This is still further complicated by the fact that much of our knowledge of human responses to thermal environments is incomplete. This however, by no means negates the potential that human modelling has in the development of research methods and practical applications to address human responses to thermal environments.
Types of heat stress indices
There are generally three types of methods used for the assessment of hot environments:
· Data from laboratory studies provided data that makes it possible to predict the likely effects an environment will have on a human, (i.e. Physiological responses);
· Standardised measuring instruments are used to measure environmental parameters such as globe temperature;
· Calculations of the heat exchanges between the human and the environment provide a method to predict the human responses.
Empirical:
Rational:
These methods all have the same criteria in common, in that their purpose is to define or establish the physiological responses of humans to their environment. Figure 1 shows a simplification of the method of calculating climatic indices, which results in a simplified value or combined measure which can represent the large possible combinations of the parameters that make up the human thermal environment. According to Eissing (1995), this simple index-value allows for a simple comparison between environments, different working situations and different clothing ensembles to be made.
ENVIRONMENTAL PARAMETERS:
Air Temperature Radiant Temperature Humidity Air Velocity
PERSONAL PARAMTERS:
INDEX VALUE
Figure 1: A diagrammatic representation of calculating climatic indices as described by Eissing (1995)
Effective temperature (ET) and corrected effective temperature (CET)
The Effective Temperature (ET) scales were originally devised by Hougton and Yaglogou in 1923 as comfort indices and, in 1927, Yaglou realised that it would be a good physiological index of stress (Leithead and Lind, 1964). ET takes wet bulb temperature, dry bulb temperature and air velocity into account but does not take into account radiant heat. Bedford in 1946 proposed the replacement of an air temperature measure with the use of globe temperature to provide a measure of radiant heat. This produced the Corrected Effective Temperature (CET). The CET has been used extensively within the coal mines in the UK.
The Heat Stress Index (HSI)
The Heat Stress Index (HSI), was developed by Belding and Hatch (1955) as an analytical index that provides an expression on a scale of 0 to 100 that represents heat stress and hence heat strain and thereby the amount of time a worker can be exposed to a hot environment. Table 2 provides the equations used in the calculation of HSI and the resultant Allowable Exposure Times (AET), while Table 3 provides an interpretation of the HSI values.
Table 2. Equations used in the calculation of the (HSI) and Allowable Exposure Times (AET), (from Parsons, 1993)
UNCLOTHED
Radiation Loss (W.m -2 ) Convection Loss (W.m -2 )
R = k 1 (35 - t r ) C = k 2 v 0.6 (35 - t a ) E max = k 3 v 0.6 (56 - P )
for k 1 = 4.4 for k 2 = 4.6
E max (W.m -2 )
for k 3 = 7.0
(upper limit of 390
W.m -2 )
E req (W.m -2 ) Heat Stress Index Allowable Exposure Time
E req = M-R-C HSI = (E req / E max ) X 100 AET = 2440/(E req – E max ) mins
Table 2 shows the equations for the Heat Stress Index, which is based on a comparison of E req and E max . Although it is derived independently, the HSI value is related to the required skin wettedness (w req ) value (which is derived from E req /E max ). As such, it describes strain in terms of sweating, and because it multiplies the w req value by a 100, it describes a prescriptive zone between 0 and 100 (see Table 3). When a value is obtained that is greater than 100, an AET is produced because it effectively means that a skin wettedness value greater than that capable of being produced (remember it is related to w req ) is required.
Table 3: Effects of environments at different HIS values to an eight hour exposure
HSI VALUE
EFFECT OF EIGHT HOUR EXPOSURE
Mild cold strain (e.g. recovery from heat exposure)
No thermal strain Mild to moderate heat strain – Little effect on physical work but possible effect on skill. Severe heat strain, involving threat to health unless physically fit – Acclimation required Very severe heat strain – Personnel should be selected by medical examination, adequate water and salt intake must be ensured. Maximum strain tolerated daily by fit acclimatised young men. Exposure time limited by rise in deep body temperature.
Table 3 describes the prescriptive zone as calculated by HSI. Note how as the HSI value increases so the strain imposed on the worker increases.
Due to it's reliance on E max , HSI cannot be applied if temperature and humidity are high (BOHS,
The American Conference of Governmental Industrial Hygienists (ACGIH)
The ACGIH provide an annual book for practical use by trained industrial hygienists called "Threshold Limit Values and Biological Exposure Limits." These include Threshold Limits Values (TLVs) for heat stress. NIOSH state that "these Threshold Limit Values (TLVs) refer to the levels of physical agents and represent conditions under which it is believed that nearly all workers may be repeatedly exposed day after day without adverse effect." Further practical information and advice regarding working practices in hot environments supplement the TLVs. The TLVs however, provide a method of determining heat stress in terms of WBGT values that are based on physically fit, acclimatised workers wearing summer clothing. Recently the ACGIH TLVs have included corrective values based on different types of PPE which are subtracted from the reference value so as to take into account the effects the PPE may have on the wearer.
Recommended Exposure Limits (REL) and Recommended Alert Limits (RAL)
These are two methods developed by NIOSH (1986) to provide Recommended Exposure Limits (REL) and Recommended Alert Limits (RAL) for workers exposed to a combination of environmental heat (WBGT) and metabolic heat. They are represented by limiting curves of one hour, time weighted averages for both environment and metabolic heat. Both are based on a standard worker who is healthy, acclimatised, normally clothed and limited to a deep body temperature of 38°C. The REL is based on an acclimatised worker while the RAL is based on an
unacclimatised worker. It is again evident that these methods are also based on normally clothed workers, and no account has been made for the "non-normal" nature of PPE.
1.3.8 British, European and International Standards: Ergonomics of the thermal environment
As part of the standardisation of information produced by the International Standards Organisation (ISO), Ergonomics standards have been developed to provide information to people concerned with the design, development and assessment of products, systems, equipment and workplaces. According to Nachreiner (1991) the purpose of these Ergonomics standards is to promote health and safety, wellbeing and worker efficiency both in terms of the performance of the worker as well as the performance of the system as a whole. As part of the Ergonomics standards, a series of documents intended for the assessment, control and management of the risk in of hot working environments. These heat stress documents are part of a larger scope of documents developed for the assessment of human thermal environments as a whole.
Aims of the ISO Standards - Ergonomics of the thermal environment
Parsons (1995) stated that the combined experiences and knowledge of those people who are experts/experienced in human thermal environments (e.g. environmental ergonomists and engineers, occupational hygienists and medics, thermal physiologists etc.) enhance and enable the evolution and development of these standards. ISO Standards for Ergonomics of the Thermal Environment list a number of aims, which for this series of Standards in particular are (see Figure 2, for a diagrammatic representation of standards for thermal environments):
· To provide a definition of terms that are to be used in the measurement, testing and interpretation methodologies, taking into account existing standards and those which are being drafted;
· The "drafting of specifications relating to the method of the measurement for physical parameters characterising thermal environments";
· To provide the reader with a selection of methods of interpretation of the parameters;
· In instances where exposure to thermal environments in areas such as comfort and extreme environments, the standards establish recommended or maximum levels of exposure;
· The "drafting specifications relating to the method of the measurement of the efficiency of devices or procedures for individual or collective protection against heat and cold".
The purpose and methodologies of the Standards concerned with heat stress are briefly explained below.
PMV/PPD Subjective
Ü Introduction (ISO 11399)
Ü Clothing (BS EN 29920)
Ü Metabolic Rate (BS EN 28996)
WBGT
Ü Physiological Methods (ISO 9886)
Ü Subjective Methods (ISO 10551)
Physio og ca
Ü Instruments (BS EN 27726)
Ü Subject Screening (ISO 12894)
Ü Units (ISO 7726)
Ü Medical supervision (ISO 12894)
Ü PPE
Ü Surface temperature
Surface contact
Hot/moderate/cold
Figure 2: A diagrammatic representation of ISO standards concerned with human thermal environments (updated from Parsons 1993, p. 253)
1.3.9 Supporting standards
ISO 11399, Ergonomics of the thermal environment - Principles and application of relevant International Standards
This standard provides the user with information for the "correct, effective and practical use of International Standards concerned with the ergonomics of the thermal environment". This is achieved by describing the relevant standards and how they complement each other when assessing the thermal environment. Brief descriptions of the underlying principles of each standard and also the underlying principles of ergonomics of the thermal environment are provided. This, therefore, should allow the users to decide upon, and develop an appropriate strategy that would be suitable for the environment, the clothing the worker is wearing and nature of the work that they are to assess. This standard covers those standards that should be used when assessing hot, moderate and cold environments, human contact with solid surfaces, and those complimentary standards that include the measurement of physiological responses, the use of subjective responses, the estimation of the clothing insulation etc. Only those standards that are applicable to the assessment of heat stress will be discussed further in this document.
At the time of writing (December 2000), this standard has been recommended for revision to bring it up-to-date, as a number of the standards are currently under revision.
ISO 7726, Thermal environments - Instruments and methods for measuring physical quantities (BS EN 27726)
ISO 7726 specifies the minimum characteristics of instruments for the measurement of the physical quantities that characterise the human thermal environment. Its aim is to standardise the recording of information in order to ensure that these measurements follow recognised procedures, thereby enabling the user to use the other standards in the series to obtain an overall index of thermal comfort or strain. It is also meant as a source of reference for both the manufacturers and users of measuring equipment and instruments used in the measurement of the physical parameters of the environment. It applies to all human thermal environments, whether hot, comfortable or cold.
ISO 8996, Ergonomics of the thermal environment – Estimation of metabolic heat production (BS EN 28996)
This standard provides fundamental support to the other standards in this series, by providing information and data for estimating and calculating metabolic heat production. The standard describes three types of methods for obtaining values for metabolic heat production:
1. Tables for estimating metabolic rate – Different Tables are presented:
· General description (Low, Medium, High);
· Specific descriptions of occupations (bricklayer etc);
· Summing components of tasks (basal metabolic rate + posture component + movement component).
2. Heart rate –Total heart rate is considered as the sum of several components. Heart rate (at greater than 120 beats per minute) is linearly related to metabolic rate;
3. Calculation of metabolic rate from oxygen consumption and carbon dioxide production during both activity and rest.
The estimation of metabolic heat production from activity is an influential part of heat stress assessment. It is difficult to provide an accurate estimate; however, a revision of the standard is underway.
ISO 9920, Estimation of the thermal characteristics of a clothing ensemble (BS EN 9920)
This standard provides a comprehensive database of clothing items and ensembles, materials, insulation values etc. It enables the user to choose a garment or clothing item, then to obtain a measure of the insulation based on the material of the clothing. The purpose of this is to provide the user with an estimate of Intrinsic Clothing Insulation (I cl ). One problem with the database however, is that it is actually quite difficult to use.
Recent studies have provided a more detailed understanding of the thermal properties of clothing. The use of thermal manikins, vapour resistance properties of clothing and the pumping or bellows effects of clothing are all topics to be included in a revision of the standard.
ISO 9886, Evaluation of thermal strain by physiological measurements
This standard describes methods for measuring and interpreting different physiological measures such as heart rate, core temperature, skin temperature and body mass loss. It covers a number of aspects to their application, such as technical complexity, accuracy and risks, applicability as heat stress measures, etc. Its main purpose is to provide a reference for the monitoring of physiological responses to extreme environments. This enables the user to make an informed decision on what measurements may be required, how to measure them and then to interpret them.
ISO 9886 has been revised to consider diary methods of assessments. It is at present under international voting and will eventually become a revised British, European and International Standard. The British Standards Institution committee PH9/1 'Ergonomics of the Thermal Environment' has sponsored the development of a draft standard through the Department of Trade and Industry (DTI) that is concerned with the specification of physiological measuring instruments. The work is being contracted by the Defence Evaluation Research Association (DERA) and a draft standard and report will be produced by Summer 2001.
1.3.10 ISO 7243. Hot Environments – Estimation of heat stress on a working man, based on the WBGT –Index (wet bulb globe temperature). (BS EN 27243)
The WBGT index is an empirical index which represents the heat stress to which an individual is exposed. It was developed during the 1950s by US military as part of a applied programme to reduce heat stress casualties in the US Marine Corps and was evaluated by Yaglou and Minard (1957) as a climatic index to replace the Corrective Effective Temperature (CET). The purpose of the WBGT was to provide a method that could be easily used in an industrial setting allowing a fast diagnosis. It is widely recognised that this has been done as a compromise between the need for an precise index and the need to be able to easily control measurements in an industrial setting (Parsons, 1994). This need for an easy to use method meant that the adoption of the WBGT as an International Standard was heavily influenced by the Threshold Limit Values (TLVs) set out by the ACGIH. A consequence of this compromise between ease of use and accuracy is that it applies to the evaluation of the mean effect of the heat during the period of the worker's activity. It does not however apply to those occasions when the worker may be exposed only for short periods, nor where the heat stress limits are close to the zone of comfort. It also makes no provisions for estimating the effect of PPE. Therefore, the WBGT index is to be used to estimate whether or not a problem exists, by identifying whether the reference values are exceeded. If this occurs, the more advanced Standard (ISO 7933) is to be used to provide a more accurate estimation of stress.
The WBGT-index combines the measurement of two derived parameters; natural wet-bulb temperature (t nw ) and globe temperature (t g ), and a direct parameter air temperature (t a ). These measures are applied using the Equations 4 and 5.
Inside buildings and outside buildings without solar load:
WBGT = 0.7t nw - 0.3t g
Outside buildings with solar load:
WBGT = 0.7t nw +0.2t g +0.1t a
The measurements are inputted into the equations above to obtain a WBGT value. The WBGT value is then compared to the reference values provided in the standard for the appropriate metabolic rate and state of acclimation of the worker. These reference values are provided in Table 4 of the standard. They refer to conditions where 95% of the working population can be repeatedly exposed to heat stress with no adverse health effects (ACGIH, 1989; Dukes-Dobos and Henschel, 1973). It is important to note that these reference values correspond to a given situation where the worker is physically fit, and in good health. The workers are also "normally clothed, with adequate salt and water intake and, if conditions stay within limits, are able to work effectively without exceeding a body core temperature of 38°C" (WHO, 1969; ACGIH, 1989).
Table 4. Reference values of WBGT heat stress index from ISO 7243. The values given relate to a maximum rectal temperature of 38°C
METABOLIC RATE, M
REFERENCE VALUE OF WBGT
Rate class
Related to a unit skin surface area
Total (for a mean skin surface area of 1.8m 2 ) W
Person acclimatised to heat
Person not acclimatised to heat
W/m -2
0 (RESTING)
M≤65
M≤117
65<M≤130
117<M≤234
No sensible
Sensible air
M>260
Griefahn (1994; 1997) reported finding that under conditions of thermally induced heat stress the WBGT provided a suitable predictor of heat stress. A number of limitations have been reported:
· The estimation of metabolic rate causes a high variability in reference values (Hill, 1985; Ramsey and Chai, 1983) which may be compounded by the difficulty of interpreting the results when small deviations in the reference values are observed;
· Pulket et al (1980) reported that under conditions of light work in humid environments, the WBGT correlates well with the skin temperature, but poorly with other physiological variables of heart rate, rectal temperature, and sweat loss;
· Another limitation is that the reference values are representative of the mean effect of heat, over a long period of work. It therefore does not provide a reference for those instances where workers are exposed to peak values of heat for very short periods of time (e.g. a few minutes). These exposures could be a result of either exposure to a very hot environment or a short period of intense physical activity. In these cases the reference values may not be exceeded even though the heat stress may exceed the permissible value. The highest metabolic rate value is used as the reference value, where there is uncertainty about the metabolic rate that is to be adopted;
· BS EN ISO 27423 does not provide corrections to WBGT values for different types of PPE, although the ACGIH TLVs (which are based on the WBGT index) do provide corrections for PPE.
ISO 7243 has been confirmed as a useful standard since 1984. Recently there has been a suggestion that it could be brought into line with the ACGIH developments. The limitations of the WBGT index are recognised where evaporation of sweat is restricted by impermeable clothing or high humidity. Here the rationale for the WBGT index (heavy weighting to t nw ) falls down.
I.3.10.1 ISO 7933, Hot Environments – Analytical determination and interpretation of thermal stress using calculation of Required Sweat Rate (SW req ) (BS EN 12515)
When WBGT values are exceeded, the user is directed to the required sweat rate model for a more detailed analysis of the work environment. As ISO 7933 is a major topic of this report, a detailed description is provided below.
Required Sweat Rate (SW req ) Explained
Required Sweat Rate (SW req ) is a rational/analytical index that is based on the heat balance equation. It was developed at laboratories in Strasbourg by Vogt et al (1981), evaluated by an ECSC project in industrial contexts and was finally published in 1989. For a full description of the methods (and computer program listing), the reader is referred to the Standard.
The parameters used to measure and define the thermal environment are air temperature, mean radiant temperature, humidity (in the form of partial vapour pressure) and air velocity. These are inputted into the model along with the clothing insulation value, metabolic rate and posture. The model then calculates the heat exchange between a standard person and the environment through convection and radiation, as well as respiratory heat loss. Other processes, such as sensible heat loss for cotton clothing, convection and radiation coefficients, are also modelled. Since the evaporation of sweat is the main driving force for heat loss, the model predicts the sweat rate required to facilitate the evaporation required to maintain zero heat storage. One of the main calculations therefore is the calculation of mean skin temperature. The model then interprets these calculations to provide a Duration Limited Exposure (DLE) in minutes. If the required sweat rate can be achieved, without unacceptable dehydration, heat storage or wettedness, then an eight hour exposure is acceptable. If not, then an acceptable work period (DLE) is calculated.
If heat balance is not maintained then core temperature will rise as a result of heat storage. If the required sweat rate can be achieved and does not exceed the maximum allowable water loss, then there will not be a time limit due to heat exposure in a working day. If the maximum allowable water loss value is exceeded, then a Duration Limited Exposure (DLE) is imposed, based either on the amount of water lost or the rise in core temperature. The index will then provide limiting values based on the maximum amount of allowable water loss without dehydration occurring, or the time for core temperature to reach 38°C (estimated from heat storage). These limiting values are known as Duration Limited Exposures (DLE). SW req provides a method for identifying the relative importance of the different parameters that make up human thermal environments, thereby allowing for the assessment of control measures that may be employed (see Figure 3). However, the BOHS (1996) states that the standard is "complex and difficult to use, and does not lend itself to occasional or casual use."
Air temperature (°C) Radiant temperature (°C) Wet bulb temperature (°C)
Air movement (m/s) Metabolic Rate (W.m -2 ) Mean radiant temperature (°C) Partial Vapour Pressure (kPa)
Area of body exposed
Clothing insulation (clo)
(Ar/Adu)
OUTPUTS including:
SW req
Mean skin temp
Max evaporation rate
Required evaporation rate
Required Sweat rate
State of Acclimation
ð Acclimatised and unacclimatised
Alarm & Danger Criteria
ð Predicted Sweat rate
ð Duration Limiting Exposure
Figure 3: Diagrammatic representation of the inputs, outputs and interpretation of the SW req model
ISO 7933 Equations
Required Evaporation (E req )
Therefore, with K=0, the general heat balance equation can be written as E + S = M- W- C res + E res - C - R
However if S = 0, then the E = E req and therefore the equation for the evaporative rate required to keep the body in thermal equilibrium is
E req = M- W- C res - E res - C - R
Require skin Wettedness (w req )
The required skin wettedness (w req , dimensionless) is defined as the ratio between the required evaporation rate, (E req ) and the maximum evaporative rate (E max ). E max is the maximum evaporation that can be achieved under hypothetical conditions when the skin is completely wet.
w req = E req / E max
Required Sweat Rate (SW req )
SW req = E req / r req
r req
is evaporative efficiency of sweating (dimensionless), which corresponds to the required skin wettedness.
The SW req equation, through the use of r req , takes into account the dripping of sweat from skin which does not contribute to heat loss.
NOTE: The sweat rate in watts per square metre represents the equivalent in heat of the sweat rate expressed in grams of sweat per square metre of skin surface per hour. 1W/m 2 corresponds to a flow of 1.47 g/(m 2 .h) [for standard subject (1.8 m 2 of body surface) a flow of 2.6 g/h].
Interpretation of Required Sweat Rate
Two criteria of stress and two criteria of strain form the basis for the interpretation of the values calculated by the index. These interpretations are presented for acclimatised and non-acclimatised subjects, and according to the degree of protection which is desired (warning and danger levels). Stress criteria:
a) max skin wettedness (w max );
b) max sweat rate (SW max ).
Strain criteria:
a) max heat storage (Q max , in watts hours per square metre);
b) max water loss (D max , in grams).
Abbreviated from standard:
SW req cannot exceed SW max ; w req cannot exceed w max
SW max and w max are both functions of the acclimatisation of the subject.
When the model predicts non-equilibrium of thermal balance, any heat storage or water loss is limited by the maximum values for each.
Analysis of the work situation
Predicted values are determined for the analysis of the work situation where:
w p - predicted skin wettedness E p - predicted evaporating rate SW p - predicted sweat rate
taking into account the required values ( req ) and the limit values ( max ).
If w max exceeds w req and SW max exceeds SW p , the body is in thermal equilibrium and the predicted values are.
E p
SW p = SW req
= w req = E req
The theory here is that since the predicted values do not exceed the maximum values that they are achievable and as such sufficient sweat can be evaporated to maintain thermal equilibrium. The following calculations are made when the required values exceed the maximum values and thermo equilibrium is not possible:
w p
and therefore:
SW p = E p /r p
w p E max
where r p is the evaporative efficiency of sweating corresponding to w p .
When the SW req or the SW p at the preceding step exceeds the SW max , it is necessary to determine the w p and the evaporative efficiency (r p ) such that by substitution:
= SW max r p
taking into account the relationship between w p and r p
therefore;
= w p E max
and SW p = SW max
Determination of allowable exposure time (DLE, min)
The allowable exposure times is calculated as a function of the maximum values for body heat storage (Q max ) and water loss (D max ). No time limit is placed on an 8 hour working day if E p = E req [i.e. E max value not exceed and if SW p < D max /8 (SW p is less than SW max per hour over 8 hours)]. It is necessary to calculate allowable exposure times based on either heat storage or water loss if these conditions above are not met. Heat storage (Q max ) is calculated as the difference between E req and E p resulting in an increase in body temperature and the following expressions are used to predict the DLEs based on an increase in core temperature.
DLE 1 = 60 Q max / (E req -E p )
When excessive water loss is predicted, the DLE calculation is DLE 2 = 60 D max / SW p
The shortest DLE shall be used for limiting the duration of work.
Table 5: ISO reference values for the different criteria of thermal stress and strain.
NON ACCLIMATISED ACCLIMATISED Warning Danger Warning Danger
Max Skin Wettedness
Max Sweat rate Rest:
M<65 W/m 2
SW max W/M 2
M>=65 W/m 2 SW max W/m 2
Maximum Heat Storage
Q max h/m 2
Maximum Water Loss D max .h/m 2
1.3.11 Literature review of the Validity of the ISO 7933 SW req index
The complexity of intra- and inter-individual physiological responses means that it is very difficult to predict, with accuracy, an individual's responses. The SW req aims to reduce this uncertainty by predicting for acclimatised and unacclimatised workers, by providing the alarm and danger criteria, and by limiting the exposure based on either sweat loss or core temperature increases. As such therefore, any evaluation of the SW req index should aim to investigate as many of these criteria as possible.
Practical Applications of SW req
For many years the importance of the body's ability to maintain heat balance has been understood, with Haines et al (1952) stating that "body heat balance is a physiological requirement for comfort and health." Belding and Hatch (1955) first proposed an additive index (HSI) based on the calculation of energy balance where all the environmental and personal parameters are taken into consideration in this calculation.
The practical application of this theory is difficult since there is a conflict between the engineering application of a heat stress standard and its application by human physiologists. This conflict arises as a result of the precision of the relationships between the physical stress factors of the environment and the human's physiological responses to heat. As such they identified the need for a practical engineering application, where the assumption is that a worker who maintains thermal balance with the environment does not "find the accompanying stress of serious consequence." The philosophy being that the index would provide a benchmark that allows the calculation of the required evaporative capacity over the maximal evaporative capacity, allowing the comparison of one heat exposure with another over a variety of conditions. This provides the capacity to express
heat stress in terms of evaporative requirements as an engineering, design and control tool, whereby the proposed changes to the conditions can be assessed in relation to the predicted physiological responses. This is a fundamental point, and one which seems to have been lost over the proceeding years, where the identification of heat stress was the issue and not as much the evaluation of the proposed solutions as perhaps could have been. The use of algorithms to model human thermoregulation in terms of the heat balance equation requires that suitable coefficients (such as those for evaporation and convection) need to be considered so as to describe the environmental parameters. ISO 7933 is not valid for all thermal environments; for example, the standard describes the boundary limits of its performance envelope for predicting mean skin temperature (Table 6).
Table 6: The range and unit of measure for each parameter to determine mean skin temperature
to 50.6
W.m -2
to 272
One of the major criticisms is that much of the data that led to the development of the equations were from laboratory studies that may only partly represent industrial situations. Kampmann and Pierkarski (1999) cite examples of the problems that they have identified:
· Skin temperature is described as a linear function rather than being calculated in a "self consistent manner". Therefore, comparisons with other studies need to be made to ascertain whether the assumptions that are made are adequate;
· Two variables are suitable for such a comparison:
1. The SWp can be compared with those values actually observed;
2. Those core temp rises that are determined in experimentation (Dt re ) can be compared
with the limit value .0.8°C (for Alarm Criteria) and 1°C (for Danger Criteria).
· Furthermore, they query whether the limit values are explained in such a way that they are meaningful to the reader (w max , SW max , Q max , D max .);
· There are errors in prediction of SW rate and predicting the rise in core temperature especially in humid or warm conditions.
These points provide some criteria whereby the validity of SW req can be investigated.
Kähkönen et al (1992) in their study of the applicability of ISO 7933 in Tanzania, found that even when WBGT index did not limit the work (i.e. permitted continuous heat exposure), ISO 7933 provided DLE time limits. Peters (1991) also reported this, with some 90% of cases providing the same result. Kähkönen (1993) also stated that the use of indices as a predictive tool to estimate the effects of introducing controls in advance was a useful practical application in industry. It was decided therefore to use this principle as part of both the usability and validity study of ISO 7933.
One area where the usability could be a problem was provided by Kähkönen who stated that the model should provide information to the user when a parameter outside the operating envelope of the model is inputted. The relative advantage of using a heat balance equation index over the use of
an index such as the effective temperature scale is that it combines the four environmental parameters and therefore does not obscure their respective contributions to the problem. It also allows the user to apply solution engineering and/or administrative controls to the situation and to assess what impact these individual or combined changes will have on the worker.
Cooke et al (1961) provide an important example of the necessity of being able to identify the effects of individual parameters when assessing the problems encountered in mines. They state that of the four environmental parameters, the two that can most easily be controlled by mine ventilation engineers are air velocity and air temperature. Air velocity is controlled either by the amount of air fed into the tunnels or the distribution of the air in the tunnels. Air temperature changes usually arise as a result of changes in air velocity, however they can also be changed independently. Mean radiant temperature in mines is dependent on the surface temperature of the rock, which in turn is a function of the virgin rock temperature, and the time the surface has been exposed to ventilating air. However, mean radiant temperature is not of much concern in mines because there is much evidence that the physiological responses at a mean radiant temperature of a couple of degrees above air temperature are relatively much less than those experienced when there is a 1°C increase in wet bulb temperature.
Kampmann et al (1995) who have also used coal mines as an area for applying ISO 7933 have identified what they call "considerable" problems in the practical application of the standard, when comparing those studies performed under climatic chamber conditions and those in the field. Specifically they mentioned practical difficulties with establishing limit values for those measures described by Belding as being essential:
· Maximal degree of skin wettedness - w max
· Maximal sweat rate - SW max
· Maximal permissible heat storage - Q max
· Maximal permissible degree of dehydration – D max
· and the evaluation of the thermal environment in situations where the worker was clothed was also a problem. This is especially the case where clothing is saturated with sweat.
Although they state that ISO 7933 (1989) was an improvement on the earlier 1985 version. Kampman et al (1995), identified two main areas that they felt needed to be addressed:
1. They questioned the results as being "misleading" when evaluating the combined exposure of climatic stress at a combination of climatic conditions. Specifically warm and humid environments.
2. They raise the issue that the "alarm" and "danger" limiting criteria have not been sufficiently defined.
Since the model is based on the heat balance equation, its ability to predict the heat lost due to evaporation of sweat is critical and as such, so are the effects of clothing. Parsons (1995) states that the "description and quantification of the effects of clothing, by the standard are overly-simplistic". The effects of air velocity, the pumping effect, clothing insulation etc should be considered to provide a resultant insulation and evaporative resistance of the clothing rather than using the standard basic data. Another problem is that there is no consideration for evaporative heat loss when clothing is worn, such as the wicking effect where water is evaporated from the clothing surface. Also evident from the equation is the omission of quantifying the pumping effects of air
movement within the garment when the wearer moves. Only the intrinsic clothing insulation of garments and ensembles worn are considered, with no consideration given for other material properties such as vapour impermeability (i m ). This is a recognised weakness and as such was to be addressed by the European research project BIOMED HEAT in the development of the new Predicted Heat Strain (PHS) model.
Skin Temperature
ISO 7933, estimates mean skin temperature as a function of the environment al and personal parameters (ISO 7933). An alternative for mean skin temperature is provided in the Annex C for I cl values greater than 0.6 clo, whereby the program is altered to make an approximation of t sk equal to 36°C. Parsons (1995) suggests that although this method is "crude", it may provide a sensible
alternative.
The validation stage of the present research project will investigate this.
a series of studies conducted by Kähkönen et al (1992), the mean skin temperature was adapted
the model to equal 34.5°C when the environmental conditions exceeded those for which the SW req
model is applicable. However, the standard recommends adapting the mean skin temperature to 36°C. This was acknowledged in their work but provided a practical example of the modification
of ISO 7933 mean skin temperature parameter in the assessment of field conditions. The use of this
36°C fixed skin temperature value was originally used in the SW req model and was criticised by Mairiaux et al (1986) and by both Malchaire (1986) and Hettinger et al (1987) in the final report of the ECSC project. However, the acceptance of an appropriate equation was complicated when the different data sets of different authors were compared with respect to their proposed equations. Mairiaux and Malchaire discussed the differences between the following equations proposed by Mairiaux et al (1987) and Hettinger et al respectively (both references cited by Mairiaux and Malchaire and not referenced here).
t sk = 30 + 0.138ta + 0.254Pa – 0.57Va + 0.00128M – 0.553I cl
t sk = 30.67 + 0.10t a + 0.46Pa - 0.0099M + 0.48I cl .
A main difference between these equations is the coefficient for the clothing insulation owing to the
differences in clothing worn during the studies that developed the equations. For the first equation
subjects were wearing light clothing (0.5 to 0.6 clo) and in the second they wore heavy clothing (0.7
1.0 clo). This shows the difficulty of modelling for skin temperature where one coefficient needs
be valid for all possible clothing ensembles. Work by Candas (1987) referred to by Mairiaux and
Malchaire showed that when the t sk predictions were compared to measured t sk data that the equations underestimated by a mean of 1 and 2.4°C respectively. These differences were significant above 30°C air temperature. The equation provided in the SW req model is an adapted version of Mairiaux's equation where the air temperature coefficient has been divided by three and reallocated into two coefficients, one for air and one for radiant temperature. This author questions whether the mathematical allocation of the coefficients by two thirds of 0.138 (=0.093) air temperature and one third (0.045) to radiant temperature is a valid process. No literature could be found to support this allocation, although a review of the coefficients presented in the WBGT index shows that two thirds are allocated to globe temperature and one third to air temperature. Although a direct comparison between the two models cannot be made, it provides for an interesting analysis and questions the science behind the SW req skin temperature coefficients.
t sk = 30.0 + 0.093t a + 0.045t r – 0.57Va + 0.254Pa + 0.00128M – 3.57I cl
A knowledge of thermal physiology would indicate that a simple linear regression equation could
not accurately predict skin temperature nor could such an equation be a causal model. An example
of this is the term involving clothing insulation (I cl ). It can be seen that if clothing insulation
increases, then skin temperature falls. This is contrary to what would be expected.
This point not withstanding, if air temperature equals radiant temperature, then the expected error would be between 1°C and 2.4°C (from Candas above). This error seems to be beyond what should ordinarily accepted considering the interdependence of the parameters in the SW req model and the fact that small variations in skin temperature can actually severely restrict the worker's ability to cope with heat (Goldman, 1988). Underestimating mean skin temperature results in errors in other areas of the heat balance equation (Mairiaux and Malchaire, 1988). This is illustrated by Wadsworth and Parsons (1986) who reported observed mean skin temperatures of between 37 and 38°C, which, according to Goldman (1988), is within the range in which people are at risk to heat induced collapse.
Another area of concern is the validity and applicability of the relationship between the evaporative efficiency of sweating to skin wettedness. The original equation was developed from work by Candas et al (1979) on supine subjects at rest, however work by Alber-Wallerstom and Holmer (1985) showed that there was less evaporative efficiency for subjects who were exercising upright than those in Candas' work.
This evaporation will be dependent on the ambient humidity of the environment. The evaporative capacity (E max ) is higher in a dry environment than in an environment where the humidity is high. The ratio between E req and E max will determine how much evaporation is possible and is described
as required wettedness (w req ), first proposed by Gagge (1937) and adopted independently into the Heat Stress Index (HSI) (Belding and Hatch, 1955).
Sweat Rate
In their assessment of the validity of predicted sweat rates compared to observed sweat rate,
Mairiaux and Malchaire (date) reported strong correlations. This seems at odds with other research. Kampmann et al (1992), reported from studies in coal mines that the model underestimated the
predicted sweat rate in experiments where low I cl values were observed and significantly overestimated the predicted sweat rate with high I cl values. Hanson and Graveling (1997), referring
to the work by Voss et al (date, described above), reported that if the model were used to evaluate a
warm humid environment, the observed sweat rates may be considerably higher than the maximal sweat rate (SW max ) limit of 1000 g/h. The limitation imposed by the low SW max value will predict that less sweat is produced than is observed and as such the model will predict that less heat is lost than is observed. The result of this under prediction of heat loss will be that the predicted DLEs will be lower than the observed DLEs. This was illustrated by Alder-Wallerström and Holmér
(date), who found sweat rates of 1152 ±462 g/h and 1206 ± 330g/h which were greater than the SW max values described in Table C.2 of the Standard of 520 g/h (alarm) and 650 g/h (danger). This
is further complicated by the assumed maximal degree of skin wetting (w max ), where w max may not
be exceeded (as described in Section 5.2 of the standard). This causes the SW p to be reduced. This
is not necessarily an issue under hot dry conditions where SW max will limit the SW p . However in
warm humid environments where skin wettedness exceeds the w max limit value for the criteria under which it is being applied, the limit value will result in a lower SW max and as such a lower SW p , which in turn will result in a lower predicted DLE.
Kampmann (date) referred to Voss et al (1991) stating that there were "considerable errors" when the observed data were compared to the predicted values, in both the prediction of sweat rate in humid and warm situations and the rise of core temperature. Kampmann and Pierkarski (1995) stated that a comparison between the predictive capacity of the standard with observed data from laboratory and field studies can be made by comparing the predicted sweat rate (SW p ) with the observed values for sweat loss. Mairiaux and Malchaire (date) questioned the use of a maximum skin wettedness (w max ) value of 0.85 for unacclimatised subjects. Candas (date) quotes work by Belding and Kamon (1973) which mirrored his own findings that, where skin wettedness is greater than 50%, unacclimatised workers exhibited a constant increase in core temperature and as such that exposure times should be limited or should restricted to acclimatised workers. A problem with the work reported by Mairiaux and Malchaire (ibid) is that although the slope values and corresponding correlation coefficients were presented, no analysis of the residuals were provided. This would show the magnitude of the error in the prediction of observed data. The practice of only providing
correlation coefficients as a measure of validity is flawed as it only provides an indication that there
is a relationship but not how accurate that relationship is. Their main finding was that an
exponential model of sweat rate would improve the accuracy of the predictions. As a result of this,
and work by Hettinger et al (date) the BIOMED HEAT project tried to develop such an equation.
A conservative approach is adopted by the standard to the problem of rehydration as it assumes that
no fluid replacement to the tissues is available to replace that fluid lost as a result of sweating.
Alder-Wallerström and Holmér (date) reported that their studies showed that ISO 7933 predicted a danger DLE at 29 minutes based on an excessive rise in body temperature (rise to 38°C), while an observed alarm category rise (to 37.8°C) in temperature was not observed within the experiment lasting 60 minutes. There has also been research that has shown that the index provides a conservative prediction of the physiological responses of workers due to an over prediction of the effects of the thermal environment (McNeill and Parsons 1999; Peters, 1995). Wadsworth and Parsons (1986) on the other hand, found that the SW req model under-predicted the effects of the thermal environment when comparing the DLE with observed core temperature. One explanation may be that the model that Wadsworth and Parsons (1986) and Mairiaux and Malchaire (1986) studied was the original ISO DIS 7933 (1983) which had a fixed mean skin temperature of 36°C. Therefore any direct comparisons between this earlier work and later work investigating ISO 7933 (1989) would not be possible. Although Wadsworth and Parsons (1986) did alter the skin temperature by ±1°C and they reported that this did not have an effect on the predictive accuracy of the model. There were also other changes to the 1983 version (ISO DIS 7733). These included a significant change to the qualification of the vapour permeability properties of clothing.
Limit Values
Kampmann and Pierkarski (date) highlight the importance of the effects of the limiting values on the permissible exposure times. The fact that the ISO 7933 SW req is based upon a rational model of human heat balance means that any limiting reference values that may be used to calculate the predicted values are not independent of each other and, as such, if they are incorrect they may influence predictions other than those which they are limiting. This they state, may "confuse" the results of the predictions. The interdependence of the limit values means that if the predicted values are not accurate, then inaccurate results may be achieved. However this is not a result of the interdependence of the limit values, but rather a result of the reference values such as the D max being inaccurate. Kampmann et al (1995) cite as an example that, during chamber trials, the observed sweat rate was higher than the predicted SW max . The result, due to the equation, is that the heat storage is unrealistically high, due to the dependence of the core temperature heat loss on the sweat rate. However, the problem is not with the interdependence of the limit values but rather with the accuracy and validity of the predictions. If an index is to provide limiting values which are representative of the input values of an equation, then there is no alternative for the values to be but dependent. Perhaps what should be questioned is the validity of the interdependence the index models (i.e. do small variations in metabolic rate have a representative effect on DLEs etc.).
Parsons (1995) quotes Kampmann et al (1992) to provide possible reasons for the poor performance of ISO 7933 under warm humid conditions:
a) The interdependence between core temperature and sweat rate in the model is not a valid representation of the thermo-physiological effects.
b) Both the predicted sweat rates and the maximum sweat rates on which they are based are exceeded by observed sweat rates in field and laboratory studies.
c) The previous point leads to a poor prediction of heat storage and therefore an over prediction of the core temperature increases, thereby under-predicting the DLE time limit.
d) The effects of drinking are not taken into account and they should be.
e) Effects of clothing on thermoregulation are not representative of observed effects.
f) Calculation of the mean skin temperature is over simplistic.
g) W max should not be a limiting criteria under warm-humid conditions.
This poor performance of ISO 7933 and the resultant uncertainty is in itself a risk and therefore, it seems, has not been extensively used.
1.3.12 BIOMED HEAT and the Predicted Heat Strain (PHS) Model
Introduction to the BIOMED HEAT project
Eight European laboratories collaborated on a project concerned with the assessment of hot working environments and the improvement of the predictive capabilities of ISO 7933. The laboratories were located in Belgium (1), Germany (3), Italy (1), Netherlands (1), Sweden (1) and Loughborough in the UK. The projects were led by the following scientists: Malchaire (Co ordinator), Griefahn, Gebhardt, Kampmann, Alfano, Havenith, Holmér and Parsons respectively. The three year programme (1996-99) involved two phases. Phase I was concerned with the development and collation of knowledge in areas required to improve the method described in ISO
7933. These included heat exchange through clothing, prediction of mean skin temperature,
conditions with high humidity, the evaporative efficiency of sweating, criteria for estimating exposure times, and a measuring strategy. Phase II provided a validation stage and the development of a new model along with a philosophy of application. The results have been published in a series of eight papers (Malchaire et al, 1999 – 2000).
Malchaire (1999) provided the following list of the criticisms of ISO 7933:
1. Skin temperature prediction;
2. The effect of clothing on convection, radiation and evaporation;
3. The combined effects of clothing and movement (the pumping effect);
4. The link of metabolic rate and the increase in core temperature;
5. The accuracy of the prediction of sweat rate in very humid conditions;
6. The limiting criteria (i.e. the SW max , E max , Q max , D max ), and specifically the Alarm and the
Danger criteria level;
7. The maximum water loss allowed to predict dehydration levels.
As a result of the BIOMED HEAT collaboration and past research into the validity of ISO 7933, a number of changes were to be made to the predictive algorithms. Additionally a major shift in philosophy was observed, whereby the DLE would no longer predict for "alarm" and "danger" criteria but for the mean subject (Kampmann, Malchaire and Piette, 1999). Figure 4 uses the same process as used in Figure 3 (SW req ) to describe visually some of the aspects of the new PHS model.
State of Acclimation Duration Air temperature Radiant temperature Partial Vapour Pressure Air movement Metabolic Rate (W) Clothing insulation Emissivity of Protective Clothing Permeability Index of clothing (im) Fraction of body area protected Posture (sitting or standing)
DLE (based on)
D in tsk over time D in tco over time D in sweat over time
ð Sweat Rate - (7.5% & 5% water loss)
ð Core temperature time to 38°C
Figure 4: Diagrammatic representation of how the PHS model may be used to evaluate possible control options
(Note: The underlined input values are new to the PHS model and were not inputted into the ISO SW req model
The Predicted Heat Strain (PHS) model
Through the collaboration of the eight partners, a database was generated of 1113 experiments (one subject per experiment) with minute by minute data of environmental conditions and physiological responses. It consisted of 747 laboratory and 366 field experiments. The laboratory experiments were divided into 2 subsets with 369 (subset 1) and 378 (subset 2) experiments. Subset 1 was used to generate the PHS model and subset 2 to validate it. Male subjects made up 1020 of the experiments and 452 of the subjects were acclimatised. More than 50% of the laboratory data consisted of experiments on nude or semi-nude subjects (clo<0.5) and 95% of the field experiments had clothed subjects. From the data used, the following ranges for each parameter were generated for which the model would be validated.
Table 7: Range of data for each input parameter for which the PHS model has been validated (from Piette and Malchaire, 1999)
RANGES OF VALIDITY OF THE PHS MODEL
t a
Pa (kPa) t r -t a (°C) va (m.s M (W) I cl (clo)
One of the major criticisms of the SW req was that it was only validated for clo<0.6 and did not take PPE into account. This table shows that PHS will only be valid for clo<1, which again seems to not
be valid for most of the PPE that may be worn. NIOSH (1986) stated that most PPE worn in industry had an insulation value of about 2 clo.
BIOMED HEAT Validation of PHS
Data considerations
The BIOMED HEAT project evaluated the validity of the PHS model when compared to observed data. Statistical weight was afforded to each experiment dependent on duration, thereby negating the effects that time may have had. The following criteria were used to produce the Table below:
1. Mean sweat rate over the whole experiment was used.
2. One rectal temperature per hour (randomly selected) was used.
Some of the results of the BIOMED HEAT analysis (from both the statistics and the scatter plots) are presented below.
Table 8: Regression between observed and predicted temperatures and sweat rates (Piette and Malchaire, 1999)
SUBSET 1&2
Sweat Rate (g/h)
Observed (mean±sd)
415 ±159
432±183
Predicted (mean±sd)
Alpha (CI95%) Observed – Predicted (mean±sd)
-33.2±110.4
Rectal Temperature
37.44±0.47
0.979±1.020
-0.01±0.40
Figure 5: BIOMED HEAT – Observed and predicted sweat rates (with 95% CI limits) in the 672 laboratory experiments (taken from Piette and Malchaire, 1999)
Figure 6: BIOMED HEAT – Observed and predicted rectal temperatures (with 95% CI limits) in the 672 laboratory experiments (taken from Piette and Malchaire, 1999)
Figure 7: BIOMED HEAT – Observed and predicted sweat rates (with 95% CI limits) in the 237 field experiments (taken from Piette and Malchaire, 1999)
Figure 8: BIOMED HEAT – Observed and predicted rectal temperatures (with 95%CI limits) in the 237 field experiments (taken from Piette and Malchaire, 1999)
Piette and Malchaire (1999) attribute the difference in the appearance of the scatter plots between the laboratory and field trials to the "precision of the climatic and physiological measurements"
being lower in the field resulting in their observation that "the correlations between observed and predicted values are lower and the 95% confidence intervals are larger." However, if the statistics in Table 8 are viewed, this difference is actually quite small and could be attributed to the larger sample size in the laboratory set (subset 1 & 2). A further comparison of the mean ± standard deviation of the difference between the observed and predicted values shows that here too, the two data sets perform similarly.
The mean sweat rate over the whole experiment is a reasonable criterion to use as it allows for the estimation of sweat rate in grams per hour. The difference between the 95% CI for the sweat rates from both sets of data however is relatively small since the units are in grams per hour.
The validity study is based on the accuracy with which the PHS model can predict random rectal temperatures to within 95% CI that are considered safe. This equation was developed by extrapolating predictions from a selection of one rectal temperature per hour. This seems to be an inappropriate validation criterion as it raises a number of issues for which the validity of the model will not only be dependent but which users would judge its success; the model predicts safe exposure duration (min). The use of a random rectal temperature that is neither time-dependent nor limit criteria dependent (i.e. t re =38°C) does not allow for the comparison of the most critical output from the user's point of view: TIME. How the t re deviations around the regression line of the relationship between randomly, non-time-dependent selected data, will translate into deviations of time is not quantified.
Another inaccuracy may be that the validators have established 95% CI (see scatter plots), for individual data points and not the mean. Since the purpose of the PHS model is to predict the mean DLE and therefore the duration to a mean ∆t re to 38°C, any CI limits should be established for the scatter around the mean value and not around individual values? Although the data sets had been weighted to negate the effects of time, the actual time for ∆t re to 38°C should have been investigated and compared to predicted data.
The fact that the t re slope does not have a time coefficient means that there is no way of establishing what this effect would be in terms of time when points at or near the CI limits are observed. Kampmann et al (1999) and Piette and Malchaire (1999) presented graphs showing how the DLEs for each model changed with different input values (two parameters while all the others are held constant), from which they concluded that the PHS model provided more "realistic results" than the ISO model. Yet no direct statistical comparison of the ISO and PHS DLEs was made. No statistical analysis was performed by BIOMED HEAT to compare the predicted and observed DLEs for the increase in core temperature. This, combined with the inappropriate use of the ∆t re and not the DLEs as the validation criterion, means that the validity of the PHS model to predict safe work times has not yet been established.
The following conclusions can be drawn from this literature review:
1. There has been extensive research into providing a valid heat stress index and a number of indices have been produced and are used;
2. There is a system of international standards that can contribute to the assessment of hot environments;
3. ISO 7933 (1989) provides a comprehensive heat stress index (SW req ) and method of assessment of hot environments. However, there is growing evidence that it has limited validity and is not usable;
4. A BIOMED European research project has updated the methods used in ISO 7933 and integrated the results into a new index – the Predicted Heat Strain (PHS);
5. The PHS index has been validated by those who developed it but further validation is required;
6. The validity and usability of both ISO 7933 (SW req ) and PHS models need to be established as part of a systematic approach to developing a heat stress assessment method for British industry.
Chapter 2 is concerned with the validity of the existing and proposed standards for heat stress assessment (SWreq and PHS respectively) and contains Sections 2.2 to 2.12 of the report. The validity is determined by comparing the thermal strain of people exposed to a range of hot conditions in a climatic chamber with the thermal strain predicted by the standards.
Sections 2.2 to 2.6 describe the experimental design, rationale and aims of the studies. Sections 2.7 to 2.9 describe the three studies that were involved. They covered 10 experiments and 77 subject exposures. Study 1 (Experiments 1 to 6) involved subjects in boiler suits performing stepping tasks in hot conditions, Study 2 (Experiments 7 and 8) involved subjects wearing relatively impermeable clothing and performing stepping tasks. Study 3 was not designed directly for this project but allowed a validity assessment as full data were available and it was performed in the same laboratory as Studies 1 and 2. It was an investigation into simulated Indian tea picking for which subjects wore traditional ethnic dress. Over all 10 experiments, air temperatures ranged from 35 to 45 o C, radiant temperature was similar to air temperature, there was low air velocity and humidity ranged from 3.3 to 4.5 kPa ? To ? % rh). Exposure times ranged from 45 to 90 minutes.
During the experiments, internal body temperature (aural), skin temperatures, sweat rates and metabolic rates were measured. Section 2.10 describes the results. An important finding is that both models are highly sensitive to metabolic heat production. This was either measured (by indirect calorimetry) or estimated using a number of models. A comparison is made of measured and estimated values for the same conditions. Subsequent analysis of validity is conducted for both values of metabolic rate (estimated and measured). Comparisons were made between predicted and observed values for Duration Limited Exposures (allowable exposure times based upon physiological criteria) and sweat rates. Section 2.11 discusses the outcomes and consequences of the validity study. Section 2.12 concludes that neither the SW req nor PHS models were valid predictors when protective clothing is worn. It was found that subjects generally produced more sweat than that predicted by the PHS and

redpajama_set_name: RedPajamaCommonCrawl