The symptoms of hand-arm vibration syndrome (HAVS) are widely known but the precise relationship between dose (how much vibration) and response (with vascular, neurosensory and musculoskeletal elements) is not.
In a 2015 paper in the journal Ergonomics, Professor Michael Griffin of Southampton University, who sits on BSI and ISO committees for vibration, wrote: "The complex and time-varying nature of human exposures to vibration and shock, the complexity of the different disorders and uncertainty as to the mechanisms of injury and the factors influencing injury have prevented the definition of dose-response relationships well proven by scientific study."
Despite that lack of "well proven" dose-response relationships, the European Physical Agents (Vibration) Directive (2002/44/EC) sets an action value (EAV) for vibration exposure of 2.5 m/s2 A(8) -- A(8) being the average exposure over an eight-hour shift -- and an exposure limit value of 5 m/s2 A(8), calculated using the standard BS EN ISO 5349-1:2001 Mechanical vibration. Measurement and evaluation of human exposure to hand-transmitted vibration.
But the directive is also clear that "the risks arising from exposure to mechanical vibration shall be eliminated at their source or reduced to a minimum", which UK legislation interprets as reducing it so far as is reasonably practicable (SFAIRP). Staying below the EAV is not a sufficient defence if there are reasonably practicable steps you can take to eliminate or reduce vibration. The Health and Safety Executive (HSE) topic inspection pack for hand-arm vibration issued to inspectors suggests that evidence of any exposure above 1 m/s2 A(8) should prompt them to ask for evidence that the hierarchy of controls has been applied.
Time and motion
Two sources of information are needed to assess daily exposure, A(8):
the total time each operator is "in contact with the equipment while it is operating"; and
the "probable magnitude of vibration" of each tool.
There are inaccuracies in both sources -- and vendors providing technology solutions for each problem.
Observing each worker on a "typical" day is resource intensive, and employees' retrospective logs are often inaccurate. As an alternative, tool-timers aim to provide a more reliable pattern of tool usage, while still using a preset vibration magnitude. Vibration specialist HAVi supplies a timer called HAVi watch. "Before each job the operator synchronises their watch with the tool they are going to use," says technical director Andrew Mee, "and it records the trigger time to calculate the cumulative exposure during the day."
Reactec's tool, HAVwear, can also operate in tool-timer mode. The company's chief executive Jacqui McLaughlin says: "Through the use of tool tags with a preset vibration level chosen by the dutyholder, HAVwear will determine HAV daily exposure points in compliance with HSE guidance." Both HAVi and Reactec provide software that enables managers to monitor vibration exposure over time and to identify at-risk people as well as possibly problematic tools and jobs.
The HSE was cautiously accepting of tool timers in an information sheet of frequently-asked questions on vibration monitoring it issued in 2017 (bit.ly/HSE_HAVS): "There is nothing wrong with using these devices. It's just that there is not generally a need for one to be issued to every worker or attached to every tool ... continual monitoring and recording of vibration exposure is not a requirement of the regulations, nor does HSE advise that it is a sensible thing to do on a routine basis."
Obtaining reliable and accurate field data ... is a very time-consuming and boring pastime
Dr Chris Nelson, a former HSE specialist inspector focusing on noise and vibration and project leader for the ISO 5349-1 standard, can see some value in tool timers, but also has concerns. "Body-worn timing devices may improve the accuracy of the assessment of trigger time, which can be used with a representative vibration magnitude to calculate daily exposure. However, I don't see these being worn all the time, perhaps just during the risk assessment process and for research purposes."
Susan Hewitt, a senior scientist specialising in noise and vibration at the HSE's Health and Safety Laboratory (HSL) for more than 30 years, believes tool-timers can be beneficial if used "in an educated manner -- provided they inform the elimination and reduction of vibration risk, and it doesn't reduce the money that could be better spent on reducing exposure".
Under UK regulations, dutyholders must refer to information on the "probable magnitude" of the vibration but this is only measured "if necessary". However, HSE guidance (L140) includes examples of the huge variations in measured typical, best and worst-case vibration magnitudes. Data provided by manufacturers, or a single point-in-time measurement of vibration might not therefore be very "probable". You can have professionals measure each piece of equipment, but Peter Wilson, technical director of the Industrial Noise and Vibration Centre (INVC) says: "Obtaining reliable and accurate field data requires costly instrumentation, a high level of practical training, considerable organisation (of both tools and operators) and is a very time-consuming and boring pastime."
Mee and Wilson suggest using databases of field measurements to provide probable magnitudes. HAVi provides a database at www.thetooladvisor.com that anyone can access, and Wilson has one for INVC customers, advocating this method as "by far the simplest, quickest and most cost-effective approach to HAV risk assessment and management".
Mike Jones, founder of another measurement specialist, Curotec, is not convinced by the accuracy of vibration databases: "The problem with a database of other people's tool measurements is that the condition of the tools, the nature of the work, and the way people do the job vary so much."
Jones believes Curotec has overcome this problem with Q2, a device that attaches to the body of the vibrating tool to "measure actual vibration emissions in real time, on the tool". He adds: "We calibrate every tool for the client so that the tool-mounted position gives an accurate reading of the vibration going into the hand."
As with the tool-timers, exposure can be summed across tools, with warnings provided to the operator and to their supervisor if levels start to approach preset limits -- which Jones recommends should be below the EAV and set by occupational health specialists.
Griffin points out that dynamics such as grip force and hand posture, which are not factored into ISO 5349-1, also have an impact on the risk from vibration exposure. These cannot be accounted for by on-the-tool measurements.
HAVSco director David Mawdsley argues that ISO 5349-1 was "based on the technologies that were current at the time" and that "with better and smaller accelerometers, chips and batteries there is a better way of meeting the intent of the directive -- to protect the workforce. The sensible alternative is to measure the actual vibration level transferred to the hand." Mawdsley believes that his HAVSense product, a device worn between the fingers, does this.
Paul Rubens, general manager at Svantek claims his SV 103 is the "first personal HAV dosimeter on the market". Rubens points to the HSE FAQ statement that, "Handheld mounts are not generally recommended, but may be used with care such that you ensure there is good and continuous contact with the vibrating surface." He explains: "The SV 103 uses a contact force sensor that ensures that accelerometer readings are only taken when the palm and sensor are in firm contact with the vibrating tool."
Look, no hands
Wilson believes that measurement anywhere other than on the tool introduces too many other variables. "With on-the-tool measurement, the vibration transducer is hard-mounted to the handle using a tight cable tie or other mechanically rigid fastening," he says. "This method minimises the number of variables in the measurement, and is the basis for the technique specified in BS EN ISO 5349," (see Figure 1).
By contrast, Wilson says of handheld devices, "If the vibration transducer is gripped between hand and handle a substantial additional variable is introduced in the form of soft, highly-damped springs between the transducer and the vibration source. Higher frequencies are progressively filtered out by the additional springs. Handle shape and grip strength both vary the measured vibration amplitude. Consequently, the results are much more variable, and are dependent on the dominant frequency of the tool vibration," (see Figure 2).
Mawdsley dismisses this concern about close-coupling: "We're less interested in what the tool is doing than in what the fingers are experiencing. If the grip is looser or firmer, the vibration experienced will be different, and that is what a device held between the fingers and the tool can sense."
Though Wilson thinks the SV 103 might be better than a simple accelerometer in other devices "because at least they measure force", he is worried about employers relying on the measurements they give.
"The data used to set statutory exposure limits were based on values obtained from transducers that were hard-mounted to tool handles," he says. "Consequently, if you get a different value using new measurement methods compared with a hard-mounted transducer, your measurement is wrong, by definition."
To overcome the difficulties of holding a device between the fingers or on the palm, Reactec has combined the tool-timer function of its HAVwear with monitoring vibration exposure on the wrist. Those with doubts about the accuracy of measuring with a device in the hand are even more critical of measuring on the wrist.
Mee explains. "With a metal beam, say, you can measure the vibration at any point. Because we understand the material properties and the behaviour of a simple beam, we can calculate the vibration at other points on the beam. But the human body isn't like that. The distance between where you wear a watch and where you hold a tool and where I wear a watch and where I hold a tool, will be different. We will have different amounts of muscle and fat. One of us might wear the strap tighter. There are too many variables."
Wilson has further concerns. "With a wrist-mounted device the transducer is decoupled from the vibration source (see Figure 3), isolating it from the vibrating tool. Mainly low-frequency vibration is transmitted -- much of the rest is filtered out. The measured value may bear little relationship to the vibration in the handle or the energy passing into the fingers and hand. The main problem is the vibration transmitted into the fingers, not into the wrist or the palm of the hand," (see Figure 3).
McLaughlin counters these objections by explaining how HAVwear's software transforms the measurement at the wrist into one of the vibration exposure to the fingers. "We compared measurements taken with our wrist-mounted device with those taken using reference instruments configured in line with ISO standard measurement practices. From this, we developed an algorithm to transform the wrist measurement into a representative reading of the vibration received by the fingers. The variation in transmissibility through the hand will be far less than the variation that occurs using point-in-time measurement on the tool."
Everyone interviewed for this article agreed that different users could experience different levels of vibration from the same tool. In theory, if you could measure the exposure directly, that would overcome all the user variables. Jones agrees. In theory, "It would be better if you could measure vibration into the fingers directly, and I have looked at whether it can be done reliably. It can't."
Before he became an HSE inspector, Nelson looked at the possibility of hand-mounted vibration measurement devices as part of his PhD. "It wasn't just the technology holding the idea back. It was the physics -- a hand-mounted device will not transmit vibration to the accelerometer equally across the relevant frequency range."
Wilson doesn't say never -- but definitely not yet. "It might be possible to devise better measurement techniques, but extensive work would be required to correlate any new vibration values with operator risk -- it would take years."
Hewitt is optimistic that "better exposure response relationships might be found. I'd never want to stifle innovation, but any new approach to managing a hazard must be evidence-based."
One argument against body-mounted devices is that their measurements are often different (often greatly) from tool-mounted readings. Rubens draws a different conclusion from this: "They're right -- the vibration is different but that's exactly why the measurement needs to be made on the hand so that the measurement is of the dose received rather than just the vibration generated."
Though Reactec points to research from the Institute of Medicine and Kindai University in Japan, and Svantek to a study by the Polish National Research Institute as evidence of the effectiveness of their approaches, until there is enough independent evidence to show that on-the-body measurements provide a better dose-response relationship, most people will continue to play safe, and follow the HSE approach.
If vibration could be measured accurately and continuously, the question remains: should we be investing in measuring rather than in fixing the problems? Griffin sends a warning: "Measurement, evaluation and assessment do not control risks: they are only useful if they lead to a better decision than would otherwise have been possible."
In a paper for the UK's 53rd annual conference on Human Responses to Vibration in September, Chris Steel and Paul Pitts of the HSE made it clear that "assessment should be a relatively short process which allows dutyholders to establish the level of risk and move their focus on to control procedures".
Most of the vendors agree with this approach to using their products but Jones does see a place for continuous monitoring. "Provided an organisation is doing everything to reduce risk SFAIRP, although continuous collection of data might not be necessary to defend a criminal case, it could be an essential tool in civil litigation."
But Nelson, who after ten years as an HSE specialist inspector is now an expert witness in noise and vibration legal cases, warns of the dangers of indiscriminate information collection. "You are in a worse situation," he points out, "if you have collected information that shows you should have known there was a problem, and you can't show you were doing something to control the risk."
HSE specialist inspector Steel reinforces the same point, referring to the dangers of "guilty knowledge" in his conference paper.
Nelson and Hewitt say that measurement devices might be useful as part of research projects, and Steel and Pitts agree that "where individuals with diagnosed HAVS are required or desire to continue working with vibrating tools ... there are credible arguments regarding the need for monitoring". Even then, Steel and Pitts would prefer employers to change work practices and arrangements than work people to their limits.
The regulator's focus is on reducing risk from vibration SFAIRP, and vibration exposure assessment is one part of that. Hewitt is clear on why managing to the EAV is not the answer. "Given we don't know what a safe level of exposure is over a typical working life in excess of 40 years, the focus has to be on reducing vibration risk SFAIRP."
Nelson agrees. "The EAV is not stated to be a 'safe' level. Employers should concentrate on the elimination or reduction of vibration exposure and risk, rather than on attempts to quantify it more precisely."
However, the technology is evolving, so if despite your best efforts to reduce the risk SFAIRP you still have concerns, the technology -- and the HSE's approach to the technology -- is worth following.