Long-term exposure to ground-level ozone significantly increases the risk of death from respiratory problems, a new study shows.
A team of scientists says the risk of dying from respiratory problems is more than 30% greater in metropolitan areas with the highest ozone concentrations than areas where ozone levels were the lowest.
Ground-level ozone, along with particulate matter, is a major component of smog. Ozone is a naturally occurring gas most prevalent high in the atmosphere, where it protects against harmful ultraviolet rays of the sun. However, ozone at ground level is produced when exhaust from cars, power plants, and other sources reacts chemically in sunlight. According to the Environmental Protection Agency, ground-level ozone can irritate breathing, decrease lung function, inflame airways, and worsen lung conditions such as asthma and emphysema.
Previous studies have shown that long-term exposure to fine particulate matter - tiny particles of dust and soot - are a major risk factor for death from heart and lung disease.
But until now, it was unclear whether ozone was linked to a higher risk of death from lung disease itself, Michael Jerrett, PhD, tells WebMD. Jerrett is an associate professor of environmental health at the University of California-Berkeley and lead author of the study.
Now that's no longer in question, he says.
"This is the first study to show that ozone, long considered a secondary pollutant, is a key cause of death," he says. "This is the first time we've been able to connect chronic exposure to ozone with the risk of death."
Ozone vs. Fine Particulate Pollution
The study, published in the March 12 issue of The New England Journal of Medicine, followed 450,000 people from 1982 to 2000 and covered 96 metropolitan regions. Over the 18-year study period, 118,777 people died.
"The study presents evidence for the first time that long-term exposure to ozone and fine-particulate pollution have separate, independent effects on mortality, and that they seem to impact different parts of the body," Jerrett tells WebMD.
The study also makes clear that controlling ozone would not only reduce deaths, but mitigate global warming, Jerrett says.
George D. Thurston, ScD, a professor of environmental medicine at New York University School of Medicine and a study co-author, says ozone tends to form in higher concentrations in suburbs and rural areas downwind of cities.
That's due in part to commuting patterns, Jerrett says. It's hard to keep counts down when governments at all levels can't control sprawl and "one-person-per-car-commuting," he says.
Cities With the Highest Ozone Levels
The risk of dying from respiratory causes rises up to 4% for every 10 parts-per-billion increase in exposure to ozone, the study says. Jerrett tells WebMD that over the 18-year study period, Riverside, Calif., had the highest daily average maximum concentration at 104 parts per billion, which "corresponded to a 50% increased risk of dying from lung disease compared to no exposure" to ozone.
He adds that in general, cities in the Northeast had lower ozone than California. New York had a 25% increased risk of lung-related death and Washington, D.C., came in at 27%, compared to 43% in Los Angeles.
The lowest ozone concentrations were recorded in San Francisco.
Charlotte-Gastonia, N.C., Chattanooga, Tenn., Raleigh, N.C., and Richmond, Va., were among the most ozone-rich areas in the South.
Limiting Exposure to Ozone Air Pollution
A clear message to the public, Jerrett says, is that exercise during the sunniest time of the day should be avoided by people who live in warm climates. He also says it's smart to keep windows closed when ozone levels are highest, and it's just not good to spend more time outside than indoors when levels of the gas are high.
Cherry Wongtrakool, MD, a pulmonologist and professor at the Emory University School of Medicine in Atlanta, says "you need particulates to have ozone, which depends on sunlight."
Plus, ozone is more seasonal, occurring at highest levels from spring to fall. She recommends commuting less and exercising, but not around busy streets in warm weather.
"Thirty percent is a very alarming number," she says.
As for governments, Jerrett says, they should take steps to reduce traffic congestion, perhaps by offering financial incentives to carpoolers, increasing high-occupancy lanes, encouraging the use of hybrid cars and maybe even by discouraging driving with higher gas taxes.
Jerrett says studies have shown that high ozone levels can cause problems quickly, and that reports on ozone levels, which can often be found in newspapers or on the Internet, should be consulted by people considering outdoor exercise.
Janice E. Nolen of the American Lung Association says the study suggests that "even low-ozone days kill people early" and that "controls have to be put on cars and power plants to deal with this."
Ozone, she says, is a huge problem around the country. Your lungs are getting repeatedly assaulted.
But the problem isn't insurmountable, Jerrett says, noting a recent New England Journal of Medicine study reporting that the average life expectancy in 51 U.S. cities had increased nearly three years between 1980 and 2000, with up to five months of that a result of decreased air pollution.
By Bill Hendrick
Reviewed by Louise Chang
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