THURSDAY, June 16, 2005 (HealthDay News) -- Three independent ozone studies all appear to confirm what scientists and environmentalists have long suspected: that higher atmospheric ozone levels are dangerous and perhaps life-threatening.
The three studies, published in the July issue of Epidemiology, were funded by the U.S. Environmental Protection Agency (EPA) in an attempt to quantify the risk of mortality when ambient ozone levels increase. Ambient ozone is caused by emissions from car exhaust and industrial pollution.
"All three of the individual studies are indicating the same thing, which is that the higher the level of ozone, the higher the level of mortality," said the lead author of one of the studies, Michelle Bell, an assistant professor of environmental studies at Yale University.
For every 10 parts per billion (ppb) increase in daily ozone level, the total death rate for that day and the two following days increased by 0.87 percent, according to Bell's meta-analysis of 39 studies that looked at mortality effects after short-term exposure to ambient ozone. The other two studies showed similar results.
These findings are particularly important, Bell said, because the EPA is now studying whether to change the 1997 recommendations that are the basis of today's ozone alerts, and this data provides definitive proof of the dangers of ozone, proof that wasn't available eight years ago.
"In 1996, it was known that ozone was bad, but now it has been linked to mortality, which makes the data extremely important," Bell said.
George Thurston, an associate professor of environmental medicine at the New York University School of Medicine, said there have been other studies that suggested this link, but that the size and scope of these studies is important.
"This is more broad-based evidence that shows these associations more comprehensively," he said.
The three studies were meta-analyses, which are statistical reviews of previous research data. By combining results of many studies -- 153 in all -- the researchers were able to identify patterns that weren't apparent in individual studies.
In findings similar to those of Bell and her Johns Hopkins University colleague Francesca Dominic, Jonathan I. Levy and his fellow scientists at the Harvard School of Public Health found an 0.86 percent increase in death rates per 10 ppb increase in ozone levels.
The third study, by Kazuhiko Ito and colleagues at New York University, suggested a smaller overall effect of ozone on death rates, but confirmed that the main effect occurred during the warm months.
The chemical reactions that produce ozone, Bell said, are extremely dependent on warm temperatures, which is why ozone is not as much of a problem in cooler climates.
Also reported in the studies were that mortality rates were lower in cities where air conditioning was prevalent during hot weather.
"Air conditioners are not just a comfort issue, but a health issue," Thurston said.
John Bachmann, the EPA's associate director of science/policy in the agency's air quality and standards division, said the consistent and significant findings in all three studies linking higher ozone levels with higher mortality rates suggests there is a strong relationship between the two. He said the findings should help the agency in its assessment of the cost/benefit ratio of air quality regulations concerning ozone.
"We know that the benefits of lower ozone levels include decreased hospital admissions and fewer illnesses, but we wanted to resolve the question of whether we should be counting mortality in assessing the benefits of reducing ozone levels," he said.
"Now, these studies will be included in the scientific review, and the results will play some role in the review of the standards for ozone levels," he said, adding that it was "way too premature" to say whether these studies will result in tougher ozone standards. The agency now recommends ground ozone levels not exceed 80 ppb.
For more on ozone pollution, head to the EPA.