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

The North American Background (NAB) 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 NAB 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 surface ozone background for an 8-hr daily maximum concentration for clean sites. In the 2006 review of the ozone standard, the EPA applied a model with 2 degree by 2.5 degree spatial resolution (i.e., great uncertainty) to define ranges of concentrations for background that are much lower than the 0.04 ppm level (i.e., 0.015 to 0.035 ppm). At a monitoring site at Trinidad Head, California, which meets Policy Relevant Background criteria many times during the year, 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 background level, more than likely overestimates human health risk.

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