A study sponsored by the Environmental Protection Agency and the Centers for Disease Control and Prevention has found that air even at the E.P.A.'s current acceptable level of ozone — 80 parts per billion — can bring on a significantly increased risk of premature death.
Ozone, the major component of smog, is a molecule consisting of three oxygen atoms bound together. It can cause lung damage when inhaled. By applying statistical models to air pollution, weather and mortality for 98 American cities over a 14-year period, the researchers determined that an increase of 10 parts per billion in ozone concentrations measured day to day causes a 0.3 percent increase in early mortality.
The study will be published in April in the print edition of Environmental Health Perspectives and is now online at the journal's Web site.
Michelle L. Bell, the lead author on the study, said that in a city the size of New York a 0.3 percent increase in mortality was equivalent to an additional 2,000 deaths a year.
Even very low levels of ozone concentration are dangerous, noted Dr. Bell, an assistant professor in the School of Forestry and Environmental Studies at Yale.
"We found strong evidence that if there is any safe level for ozone's impact on mortality, it is at very low concentrations, nearing natural background levels," she said. "This means that any reduction in ozone would benefit public health, even in areas that currently meet regulatory standards."
The E.P.A. is reviewing the scientific information on ozone to decide whether to revise the standards set in 1997, Dr. Bell added. "One hundred million people live in areas that exceed the current E.P.A. acceptable level," she said.