Environmental Protection Agency staff scientists are recommending tightening the nation’s ground-level (tropospheric) ozone standards. In a final paper submitted as part of the current review of national air quality standards, the staff concluded that the current primary ozone standard is not adequate to protect public health.
EPA staff made this recommendation based on an expanded body of epidemiological and human effects studies that show significant ozone health effects occur even in areas with ozone levels below the current standard. The paper recommends a range of levels for the EPA administrator to consider in setting the primary ozone standard, extending from below 0.080 ppm down to 0.060 ppm.
The previous draft of the staff paper identified options that included retaining the current standard of 0.084 ppm, along with a range of alternative levels down to 0.064 (the lowest level analyzed), with a focus on a level of 0.07 ppm.
The final staff paper also recommends specifying the level of the standard to three decimal places. Ozone air quality measurements have advanced sufficiently to now reflect that level of precision.
The staff also recommends a different secondary standard to protect against ozone damage to public welfare, including damage to plants, concluding that even when the current primary standard is attained, significant environmental effects continue to occur.
Instead, the paper suggests a standard based on a cumulative, weighted total of 12-hour (8 am – 8 pm) exposures over a 3-month period within the growing season to adjust for the differences in the way plants respond to ozone exposure as compared to humans. The recommended range runs from 21 parts per million-hours to 7 parts per million-hours.
The assessments, conclusions and recommendations included in the staff paper are staff judgments. They do not represent agency decisions on the ozone standards. EPA will propose action on the ozone standards by June 20, 2007 and take final action by March 12, 2008.
Ground-level ozone is formed by reactions of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) emitted from mobile (e.g., cars, trucks) and stationary (e.g., power plants) sources. These reactions are most likely to produce high levels of ambient ozone during periods of high temperature and high solar radiation during the summer months.
The Clean Air Act requires EPA to periodically review its air quality standards to ensure they continue to protect health and the environment, and to update the standards if necessary. EPA last updated the standards for ozone in 1997.
In December 2006 in a unanimous decision, a Federal court of appeals struck down a 2004 EPA ruling that had loosened the existing 8-hour ozone standard. The court ruled that the EPA violated the Clean Air Act by relaxing those limits to levels in excess of 0.09 ppm. (Earlier post.)