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
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
"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
The data and statistical models used to complete the
study are available on the Health and Air Pollution
Surveillance System Web site at
www.ihapss.jhsph.edu. 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.