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The newspaper of The Johns Hopkins University December 6, 2004 | Vol. 34 No. 14
Changes in Ground-Level Ozone Linked to Increased Mortality

By Tim Parsons
Bloomberg School of Public Health

In a recent study of 95 U.S. cities, changes in ground-level ozone were found to have been significantly associated with an increase in deaths. The risk of death was similar for adults of all ages and slightly higher for people with respiratory or cardiovascular problems. The increase in deaths occurred at ozone levels below the Environmental Protection Agency clean air standards. The study, conducted by researchers at the Johns Hopkins Bloomberg School of Public Health and the Yale University School of Forestry and Environmental Studies, appears in the Nov. 17 edition of the Journal of the American Medical Association.

Ground-level ozone is a pollutant in the Earth's lower atmosphere that is formed when emissions from cars, power plants and other sources react chemically to sunlight. Stratospheric ozone, which is higher in the atmosphere, is the "ozone layer" that protects the Earth from ultraviolet radiation.

"This is one of the largest ozone pollution studies ever conducted," said lead author Michelle Bell, who was previously at Johns Hopkins and is now an assistant professor at Yale.

The ozone study was part of the ongoing National Morbidity Mortality and Air Pollution Study at the Bloomberg School of Public Health, which routinely assesses health effects of air pollution on a national scale. To determine the association between ozone and mortality, the researchers looked at the total number of noninjury-related deaths and cardiovascular and respiratory mortality in the 95 largest U.S. communities from 1987 to 2000. Air pollution data were supplied by the EPA. Mortality data were supplied by the National Center of Health Statistics. The researchers accounted for variables such as weather, particulate matter pollution and seasonality, which could impact mortality rates.

The researchers found that an increase of 10 parts per billion in weekly ozone levels was associated with a 0.52 percent daily increase in deaths the following week. The rate of daily cardiovascular and respiratory deaths increased 0.64 percent with each 10 ppb increase of weekly ozone. The average daily ozone level for the cities surveyed was 26 ppb. The EPA's maximum for ground-level ozone over an eight-hour period is 80 ppb. The researchers calculated that a 10 ppb reduction in daily ozone, which is roughly 35 percent of the average daily ozone level, could save nearly 4,000 lives throughout the 95 urban communities included in the study.

Francesca Dominici, senior author of the study and associate professor in the Department of Biostatistics at the Bloomberg School, said, "Our study shows that ground-level ozone is a national problem, which is not limited to a small number of cities or one region. Everyone needs to be aware of the potential health risks of ozone pollution." The data and statistical models used to complete the study are available on the Health and Air Pollution Surveillance System Web site at The site is maintained by the Johns Hopkins Bloomberg School of Public Health and is sponsored by the Health Effects Institute.

Other study authors are Aidan McDermott, Scott L. Zeger and Jonathan M. Samet, all of Johns Hopkins.

Funding was provided by grants from the U.S. Environmental Protection Agency, the National Institute for Environmental Health Sciences, NIEHS Center for Urban Environmental Health and the Health Effects Institute.


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