Short-term exposure to smog at existing levels can trigger premature deaths, especially among the aged and already sick, according to a new report done for the U.S. Environmental Protection Agency.

The National Research Council report, released today, is particularly important for Allegheny County, which has a high percentage of older residents and hasn't met federal standards for smog-causing, ground-level ozone for more than 15 years.

"Allegheny County has an older population, so it has many more susceptible people and many more compromised people who could be affected," said Dr. Evelyn Talbot, an epidemiologist in the University of Pittsburgh's Graduate School of Public Health and a member of the research council committee that worked on the report.

Evidence of a relationship between exposure to smog for 24 hours or less and premature mortality has been mounting in recent years, but interpretations of that evidence have differed. The National Research Council review of that research is the first to confirm it is strong enough that the EPA should include premature death from ozone exposure as a factor in setting future standards.

Dr. Talbot said studies that tracked the number of emergency room visits and hospital admissions on days when ozone levels were high show an increase in the number of deaths on those days and for periods of up to five days later.

"If there's a pre-existing disease, a compromised individual, when ozone is high, for every 10 parts per billion increase in the ozone level we see a 3 [percent] to 5 percent increase in premature mortality," she said. "That's not a large number, but when you consider that 130 million people live in areas where the ozone is high, you do see a significant effect."

Dr. Talbot said the Graduate School of Public Health recently started work on a study funded by the Centers for Disease Control and Prevention targeting how ozone affects Pittsburgh residents.

Health, environmental and science groups criticized the EPA last month when it issued new federal smog standards that were tighter than previous standards but did not follow the recommendation of its own scientific advisory committee which had concluded even stricter standards were needed to protect human health.

That committee recommended setting the standard at between 60 and 70 parts per billion, but the EPA, after consulting with the White House, adopted a 75 parts per billion standard, reducing it from 80 parts per billion.

Ozone in the upper atmosphere protects the skin by shielding it from harmful ultraviolet rays. But ground-level ozone, the key component of unhealthy smog, forms when volatile organic compounds and nitrogen oxides emitted by power plants, motor vehicles and industries are baked in the atmosphere during hot weather.

High ozone levels exacerbate existing asthma conditions and breathing problems, especially in the young and elderly populations, and for individuals who work or exercise regularly outdoors.

Last year, Allegheny County had eight ozone readings from 87 to 99 parts per billion on seven days, pushing the county into non-attainment of the old standard. From 2002 through 2005, the old ozone standard was exceeded 68 times. If the new standard of 75 parts per billion had been in effect, it would have been exceeded on 96 days.

Allegheny, Armstrong, Beaver, Washington and Westmoreland counties and five Philadelphia area counties in Eastern Pennsylvania are not expected to meet the new ozone standard.

"This study certainly quantifies what people have suspected, that breathing higher ozone levels is not good for your health and that there are effects from not only long-term exposure, but also acute short-term exposure," said Dr. Bruce Dixon, director of the Allegheny County Health Department.

Dr. Dixon said local controls on power plants and industries have an effect on ozone levels. But because much of the county's bad air is transported on prevailing winds from coal-burning power plants and industry in the Ohio River Valley to the west, any efforts to further reduce ozone levels must be regionally focused.

"Even if we did everything we could we'd still have several days with high ozone readings each summer," he said.

The EPA estimates that 345 counties nationwide will not meet the new standard. In July 2007, the EPA proposed tightening the national ozone standards to between 70 and 55 parts per billion, measured over eight hours, to "provide increased protection for children and other at-risk populations."

Emissions from those coal-burning power plants in the Ohio River Valley produce more than one-third of the pollutants measured by county air monitors.

If a county is in non-attainment of the ozone standards, the EPA can impose limits on new industries and road building there.