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The Effects of Underestimating Background Levels of
Ozone on Human-Health Risk

The Policy Relevant Background (PRB) concentration or range of concentrations represent what EPA believes would be experienced if the United States and other countries in North America were to initiate a zero anthropogenic emissions strategy, which includes eliminating emissions associated with fertilizer. The PRB concentrations define the level below which O3 standards cannot be practicably set. In the 1996 ozone review, the EPA used 0.04 ppm in its health risk assessment evaluations as the level it expects as background for an 8-hr daily maximum concentration for clean sites. In the most current review of the ozone standard, the EPA is using a model with 2 degree by 2.5 degree spatial resolution (i.e., great uncertainty) to define ranges of concentrations for policy relevant background that are much lower than the 0.04 ppm level. At a monitoring site at Trinidad Head, California, which EPA acknowledges is a Policy Relevant Background site, numerous occurrences of hourly average concentrations greater than or equal to 0.05 ppm are measured. In a study, A.S.L. & Associates characterized the daily maximum 8-hr ozone concentrations for 3 clean sites in North America. A summary figure shows that Custer National Forest in Montana, Theodore Roosevelt National Park in North Dakota, and Yellowstone National Park in Wyoming all experienced numerous occurrences of 8-hr daily maximum concentrations greater than or equal to 0.04 ppm during each of the years monitored. The U.S. EPA, by selecting 0.04 ppm or lower as its Policy Relevant Background level, will overstate human health risk by a large amount.

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