The study, sponsored by the Environmental Protection Agency and the Centers for Disease Control, found that if a safe level for ozone exists, it is only at very low or natural levels and far below current U.S. and international regulations. A 10 part-per-billion increase in the average of the two previous days' ozone levels is associated with a 0.30 percent increase in mortality.

The current study builds on research published in November 2004 in the Journal of the American Medical Association, which was the first national study of ozone and mortality.

"This study investigates whether there is a threshold level below which ozone does not affect mortality. Our findings show that even if all 98 counties in our study met the current ozone standard every day, there would still be a significant link between ozone and premature mortality," said Michelle Bell, lead investigator on the study and assistant professor of environmental health at the Yale School of Forestry & Environmental Studies. "This indicates that further reductions in ozone pollution would benefit public health, even in areas that meet regulatory requirements."

Researchers found that even for days that currently meet the EPA limit for an acceptable level of ozone--80 parts per billion for an eight-hour period--there was still an increased risk of death from the pollutant.

An effort is now under way by the EPA to consider whether more stringent standards for ozone are needed. The agency is mandated to set regulations for ozone under the Clean Air Act. Ozone, a gas that occurs naturally in the upper atmosphere, is created in the lower atmosphere when vehicle and industrial emissions react with sunlight. Levels typically rise when sunlight and heat are highest in the summer.

"Over 100 million people in the United States live in areas that exceed the National Ambient Air Quality Standard for ozone. Elevated concentrations of ozone are also a growing concern for rapidly developing nations with rising levels of ozone from expanding transportation networks," said Francesca Dominici, co-author of the study and associate professor of biostatistics at Johns Hopkins.

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Yale School of Forestry & Environmental Studies