Long-term ground ozone exposure increases risk of dying from lung disease

Long-term exposure to elevated levels of ground ozone - a major constituent of smog - significantly raises the risk of dying from lung disease, according to a new nationwide study of cities that evaluated the impact of ozone on respiratory health over an 18-year period.

The study found that the risk of dying from respiratory disease is more than 30 percent greater in metropolitan areas with the highest ozone concentrations than in those with the lowest ozone concentrations.

Over the last decade, several nationwide studies have shown that long-term exposure to tiny particles of dust and soot in air pollution is a risk factor for death from heart and lung disease. However, it was unclear whether long-term exposure to ozone, a widespread pollutant in summertime haze, was linked to a higher risk of dying from lung disease itself.

The new study, published in the March 12 issue of the New England Journal of Medicine, is the first nationwide population study on the long-term impact of ozone on human health, and the first to separate ozone's effects from those of fine particulate matter, the tiny particles of pollutants emitted by factories, cars, and power plants.

"Many studies have shown that a high-ozone day leads to an increase in risk of acute health effects the next day, for example, asthma attacks and heart attacks," says George D. Thurston, Sc.D. who directed the air pollution exposure assessment part of the study. "What this study says is that to protect the public's health, we can't just reduce the peaks, we must also reduce long-term, cumulative exposure." Dr. Thurston is a professor in the Department of Environmental Medicine at NYU School of Medicine, a part of NYU Langone Medical Center.

Ozone in the upper atmosphere protects against harmful ultraviolet (UV) radiation. At ground level, ozone, or O3, forms when nitrogen dioxide from tailpipes, coal-fired power plants and other industries collides with oxygen in the presence of sunlight. Considered a secondary pollutant because it takes time to form, ozone tends to be higher in concentration in suburbs and rural areas downwind of cities. Fine particulate matter, a primary pollutant, is more prevalent at its source, in the inner city, along roadways and in industrial areas.

In concert with rising death rates from respiratory disease, "background levels of ozone have at least doubled since pre-industrial-revolution times," says Michael Jerrett, Ph.D., associate professor, Division of Environmental Health Sciences, at the University of California, Berkeley, and the lead author of the new study.

The study analyzed data on some 450,000 people who were followed from 1982 to 2000 as part of an American Cancer Society study. Over that period 118,777 people in the study died. The data, which included cause of death, were linked to air pollution levels in 96 cities using advanced statistical modeling to control for individual risk factors, such as age, smoking status, body mass, and diet, as well as for regional differences among the study populations.

By statistically controlling for the other major component of smog - fine particulate matter, particles smaller than 2.5 microns - the researchers were able to tease out the cardiovascular impact of the pollutants and still see ozone's effects on respiratory health.

Ozone data collected between 1977 and 2000 showed that California had both the city with the highest and the city with the lowest concentration of ozone pollution in the country. The researchers estimate that the risk of dying from respiratory causes rises 4 percent for every 10 parts-per-billion increase in exposure to ozone. Based on that result, Dr. Thurston says the city with the highest mean daily maximum ozone concentration over the 18-year period of the study, was Riverside (104 ppb). This long-term cumulative exposure corresponded to roughly a 50 percent increased risk of dying from lung disease compared to no exposure to the pollutant. Los Angeles ran a close second, with an estimated 43 percent increased risk.

Northeast cities were generally lower in ozone than California. In Washington, DC, and New York City, for example, the study results indicate a 27 and 25 percent increased risk of respiratory death, as a result of their respective long-term ozone exposures, says Dr. Thurston. The estimated increased risk from cumulative exposure in New York occurs even though New Yorkers breathe air that is nearly in compliance with the EPA's present short-term ozone standard of 75 ppb, he says.

The lowest ozone concentration was seen in San Francisco (33 ppb long-term average daily maximum), which had an associated 14 percent increase in risk. San Francisco has low levels of ozone pollution because fog regularly blankets the city, which prevents the necessary photochemical reaction from occurring, says Dr. Jerrett. In addition, Dr. Thurston points out that the Los Angeles area, which has high levels, is located in a basin, which prevents the rapid dispersal and dilution of air pollution that occurs in San Francisco.

The EPA provides a list of counties in the United States, their present ozone concentrations, and their compliance status with regard to the current short-term ozone standard at the following URL: http://epa.gov/air/ozonepollution/pdfs/2008_03_design_values_2004_2006.pdf.

The present EPA air quality standards do not protect against the long-term cumulative effects of ozone exposures, but only address the health effects of short-term daily peaks in ozone exposure, says Dr. Thurston. Currently, the Environmental Protection Agency's standard for short-term (8-hour) ozone exposure is 75 parts per billion, which exceeds the 60 ppb recommended by the EPA's own scientific advisory group, the American Lung Association and more than a dozen other public health organizations. The EPA will be reviewing its ozone standard in the coming year.

"How do we lower the burden of disease?" queries Dr. Thurston. "Do we look only at only those affected by the highest days, or do we look at everyone's exposure over the entire year? Since we all share the same air, paying attention to cumulative exposure shifts the whole exposure distribution for us all, and that's where the health payoff is. A small reduction in everybody's year-round risk benefits us all."

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