TUESDAY, Nov. 16, 2004 (HealthDayNews) -- Increases in air ozone levels may contribute to the deaths of thousands of Americans each year, new nationwide statistics suggest.
The study, funded by the U.S. Environmental Protection Agency, compared death rates and daily atmospheric ozone levels in 95 U.S. cities over 14 years. It found that increases in daily ozone were associated with concurrent increases in deaths due to cardiovascular, respiratory and other causes.
"I think this study really nails down, for the first time, that ozone air pollution can cause increased risk of death in the general population," said George Thurston, an associate professor of environmental medicine at the New York University School of Medicine, and co-director of the university's EPA Particulate Matter Research Center.
The study, led by Michelle Bell, of Yale University, appears in the Nov. 17 issue of the Journal of the American Medical Association.
Ozone is a toxic gas containing three atoms of oxygen, not the usual two. A natural layer of ozone exists high in the stratosphere, where it helps protect life on Earth from the sun's dangerous ultraviolet rays.
Ozone is much less friendly to humans when inhaled at ground level, however. Previous studies have linked inhaled ozone to cardiovascular and pulmonary illness -- "anything from respiratory symptoms, such as coughing and wheezing, through to hospital admission," Bell said.
Unfortunately, manmade pollutants continue to raise ozone levels in the air people breathe. "Cars and smokestacks produce gases that then undergo a chemical transformation, becoming ozone in the air," Bell explained.
Until now, there has been no large-scale study confirming a link between high ozone and an increase in death rates.
In their study, Bell and her team at Yale collected 1987-2000 U.S. government mortality data for 95 major urban centers, totaling about 40 percent of the U.S. population. They then compared day-by-day changes in urban death rates to concurrent fluctuations in local ozone levels, based on EPA monitoring data.
"We found that ozone is associated with mortality -- mortality was higher on days when the previous week's ozone layers were higher," Bell said.
Pooling the data to calculate a national average, Bell's team concluded that death rates from all causes rose by 0.52 percent on days when ozone counts over the past week had risen by 10 parts per billion.
That ozone-related increase in risk was even higher -- 0.64 percent -- when the researchers focused solely on deaths from cardiovascular and respiratory events, such as heart attack or asthma.
Thurston, who has worked with the EPA on ozone monitoring in the past, stressed that ozone may be even deadlier than is suggested by this study.
"It's actually a rather conservative study," he said. "For one thing, it only considers acute death -- it doesn't consider that long-term exposure to ozone may have a cumulative effect over and above that sudden, acute effect."
The findings also suggest that current EPA standards for acceptable levels of ozone in the air may need reviewing, Bell said.
"Right now, we don't know is there's a 'safe' threshold level," she said. "In our study, we looked at a subset of days that met an approximation of the EPA standard. And looking at those days, we still found an association between ozone and [increased] mortality -- and those days actually met the current standard."
According to Thurston, over the next four or five years, the EPA's Clean Air Scientific Advisory Committee will meet to examine the latest data on ozone and decide whether changes to current regulations are needed.
"This study will be a very important publication in that re-evaluation," he said.
"If we were able to reduce ozone by just 10 parts per billion -- about a one-third reduction, which is reasonable given current technology -- we would save about 4,000 lives each year," Bell said.
In the meantime, there are things every American can do to improve the quality of the air they breathe while reducing their exposure to ozone.
"Anything we can do on an individual basis to lower ozone would be useful, whether it be through energy conservation or changing transportation patterns -- for example, taking the bus to work, or carpooling," Bell said.
"The second thing we can do, as individuals, is reduce our exposure to ozone," Bell added. "Ozone levels are always higher outdoors than indoors, so on very hot days with very high ozone, staying inside, or at least not exercising outdoors, can reduce your risk."
To learn about the air quality in your region, visit the Environmental Protection Agency.