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Science, Uncertainty, and EPA's
New Ozone Standards

Allen S. Lefohn, Ph.D.

On July 19, 1997 EPA made its final decision on the promulgation of new National Ambient Air Quality Standards (NAAQS) for ozone (O3) and particulate matter. As a result of an accelerated review schedule caused by an American Lung Association lawsuit, in December 1996, EPA formally announced revisions to ozone and particulate matter standards. New forms of the primary (human health) and secondary (vegetation) standards for O3 were proposed.

During its review, the EPA found that several of the key experiments, published in the late 1980s and early 1990s, showed that individuals exposed to extended daily periods of O3 at levels below the current 1-hour standard exhibited health effects. The EPA believed it was necessary to change the form of the primary standard because additional protection, not offered by the existing 1-hour standard, was needed for children and other at-risk populations. Thus, the EPA proposed that the form of the primary standard be changed from the current 1-hour daily maximum to a daily maximum 8-hour average concentration at a level of 0.08 ppm. The EPA proposed taking the third highest 8-hour daily maximum concentration for each of three years and then taking the average of these three concentrations.

Because O3 is phytotoxic to plant species and can produce acute foliar injury responses, reduced crop yield and biomass production, and shifts in competitive advantages of vegetation species in mixed populations, the EPA wanted to establish a secondary standard to protect vegetation. The EPA believed that the previous 1-hour O3 standard was not sufficiently protective of crops and forests because O3 effects are cumulative and not necessarily related to the one-time maximum peak, on which the current 1-hour standard focuses. Two alternative forms of a secondary standard were proposed. One would be based on a level identical to the primary standard; the other on a new seasonal accumulating-type standard. In the end, EPA adapted only one standard: the 3-year average of the 4th highest 8-hour daily maximum ozone concentration.

Although the EPA devoted considerable time and effort to reviewing and summarizing the relevant science concerning human health effects and vegetation in the peer-reviewed literature, there are still areas of uncertainty associated with the data that form the scientific basis of the recommendations for both standards. These uncertainties have ramifications for human health and vegetation, and may influence whether geographic areas reach attainment.

The paper published in the June 1997 issue of ES&T addresses the following:

  • How closely did the controlled human health experiments performed in the laboratory mimic exposures experienced in the real world?
  • Can the human health standard be attained?
  • What is the realistic range of natural background O3 concentrations that occur under ambient conditions?
  • Did the EPA overestimate its human health risk assessment by using too low a value for natural background?
  • How closely did the controlled vegetation experiments mimic exposures experienced in the real world?
  • Is the form of the proposed secondary standard adequate?
  • Is there an alternative form of the secondary standard that would be more appropriate?
  • In which directions should future human health and vegetation research be focused?

While scientific research has played an important role in the discussions concerning both the primary and secondary standards, uncertainty does exist. At the very least, the ramifications of these uncertainties need to be both understood and considered when ultimately defining and selecting new standards. There has been considerable debate in the public arena, as well as within the Executive Branch and Congress, concerning the uncertainty associated with the scientific database and the attainability of the proposed standards. The experiments that form the basis for the primary and secondary O3 standards are not perfect. Only time will tell how adequately science was able to address the public's desire for clean air.

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