Atmospheric Environment Vol. 28, No. 19, pp.
In the United States of America, the Environmental Protection Agency (U.S. EPA) is presently assessing whether to change its primary (human health) and secondary (welfare) standards for surface ozone. However, the current effort to define the forms and levels of standards to protect human health and vegetation has identified deficiencies in the exposure regimes that have been used in both human health and vegetation experiments. Many of the human health effects and vegetation experiments have used combinations of hourly average concentrations that do not occur under ambient conditions (e.g. square wave). As a result, researchers are attempting to use exposure summary statistics that, when applied as standards, may provide different levels of protection because they do not necessarily capture the important components of the exposure regime.
For human health considerations, preliminary research results indicate that (1) concentration may be as important or more important than duration and ventilation rate and (2) the higher hourly average concentrations be more important than the lower values. In several studies using combinations of hourly average concentrations that do not occur under ambient conditions, effects were noted in subjects exposed for 6.6 h to constant ozone concentrations and time durations which are similar to or divergent from the U.S. National Ambient Air Quality standard of 0.12 ppm for 1 h. Applying the results of these studies, it has been proposed by some scientists that the daily maximum 8-h average concentration should be included as a primary standard. The WHO guidelines for ozone include both a 1-h and an 8-h guideline. The daily maximum is determined by averaging the hourly average concentrations over a continuous 8-h period.
If research results continue to support the observation that the higher hourly concentrations are more important than the lower values within the 8-h envelope, then the adoption of the daily maximum 8-h average concentration as a federal standard may lead to a situation in which different health effects are observed at two sites that experience the same daily maximum 8-h average concentration, but different distributions of hourly average concentrations. This result implies that the daily maximum 8-h average concentration exposure index may provide different levels of protection because the index fails to capture the variety of the combination of the hourly average concentrations.
For vegetation, experiments have shown that when relating exposure with crop yield loss and biomass reduction, the higher concentrations are more important than the lower values. The 3-month maximum SUM06 exposure index has received considerable attention for relating ozone exposure with effects. The SUM06 exposure index is calculated by summing the absolute value of the hourly average concentrations equal to above 0.06 ppm. Although similar in form to the AOT40 exposure index presently under consideration in Europe for determining the critical level of ozone, the SUM06 metric is different. The AOT40 index is calculated by subtracting 0.04 ppm from each hourly average concentration above 0.04 ppm and then summing the residual. >p>The U.S. EPA has estimated that a 3-month SUM06 value of 26.4 ppm-h results in 50% for the National Crop Loss Assessment Network (NCLAN) crops experiencing a predicted 10% yield reduction. In the case of the NCLAN program, most of the exposure regimes used in the experiments contained numerous hourly average concentrations above 0.10 ppm. Few of the treatments used in the NCLAN experiments contained exposure regimes near levels typically experienced under ambient conditions. Many monitoring sites in the United States of America that exceed the 26.4 ppm-h value over a 3-month period do not necessarily experience frequent occurrences of hourly average concentrations >0.10 ppm. Thus, if the 3-month SUM06 index were adopted as a standard to protect vegetation, it is possible that many areas in the United States of America will be inappropriately designated nonattainment status.
Although there are serious deficiencies in the current database, it is still possible to consider alternative standards to protect human health and vegetation. A human health exposure index that accumulates and differentially weights the hourly average concentrations over an 8-h period may be more biologically appropriate than a simple arithmetic average concentration. For vegetation, the use of a 3-month SUM06 index to protect vegetation, whose value is based on NCLAN exposure regimes will require a multiple parameter approach. The additional parameter should be combined with the 3-month SUM06 index to guarantee that those monitoring sites in the United States of America that exceed a 3-month cumulative SUM06 value of 26.4 ppm-h also will experience numerous hourly average concentrations >0.10 ppm.
Because of the deficiencies in the exposure regimes used in most human health and vegetation studies to date, it is important that a major effort be implemented in these research areas to apply realistic exposure-response relationships. Exposure regimes that mimic conditions that occur under ambient levels must be used. In addition to the enhanced ozone treatments used in the experiments, care must be taken to use control treatments that are also experienced under normal conditions. Additional focus should be on the identification of exposure indices that adequately capture the uniqueness of the combinations of hourly average concentrations used in the exposure regimes. Atmospheric scientists should develop ambient-type exposure regimes that can be applied to human health or vegetation experiments. These regimes will assist in relating exposure to response so that adequate protection from ozone exposures can be achieved.
The type of information developed will ensure that averaging times give the proper amount of emphasis to changes in dose rate (i.e.. concentration changes) in the different types of geographical areas (e.g., Houston, etc.) affected by elevated ozone. The information can also be used to improve: laboratory health effects studies, interpretation of field data on affected populations, and model simulations required to ensure that control strategies will reduce the overall burden due to ozone.
We are entering a new era for the study of ozone issues; both human health and vegetation are apparently significantly affected at levels that require additional insight concerning the exposure regimes that are responsible for eliciting effects. In addition, strategies needed to reduce the magnitude of the ozone problem are becoming more complex. For example, some developing nations are following the same path as the developed countries by introducing cars into the cities and suburban areas around the world. A significant increase in tropospheric ozone can result from automobile emissions. As atmospheric scientists, it is important to assist in the establishment of the appropriate basis for considering the likelihood or frequency of significant exposures that occur over a variety of locations and meteorological conditions.
A. S. Lefohn, Ph.D.
P. J. Lioy, Ph.D.