PHILADELPHIA – Ground-level ozone, a common form of air pollution long linked to breathing problems, is now being blamed for thousands of premature deaths each year in a new study of 95 urban areas.

Researchers found that a short-term increase in ozone pollution – the key ingredient in smog – was associated with a slightly higher death rate, even when the air was cleaner than what is required by new federal standards that took effect this year.

The finding held true across much of the country and for all age groups, though most of those who died were elderly and had a previous heart or lung illness. “It’s a problem for everyone,” said lead author Michelle L. Bell.

The study is one of several recent efforts shedding new light on threats from pollution common in the industrial world.

Last month a German study found that people sitting in a traffic jam were more likely to suffer heart attacks due to air pollution. Other research found an association between hardening of the arteries and small airborne particles from burning coal or diesel fuel.

These warnings are appearing as the amounts of ground-level ozone and most other air pollution are steadily falling. But researchers have been finding that some pollutants, including ozone, are more dangerous than previously believed.

Authors could not say for sure why ozone seems to kill people, though they noted that inflammation has been theorized as playing a key role.

The study was published in Wednesday’s Journal of the American Medical Association and was conducted by researchers from the Yale School of Forestry and Environmental Studies, where Bell is a professor, and from the Johns Hopkins Bloomberg School of Public Health.

Coincidentally, a smaller European study with similar findings was published Monday in the American Journal of Respiratory and Critical Care Medicine.

Ozone, which is generally highest in summer months, is created in the presence of sunlight. It is formed by a chemical reaction between two other forms of pollution: volatile organic compounds and oxides of nitrogen. The former comes from a wide range of sources, including paints, cars and oil refineries. Oxides of nitrogen are emitted by cars and power plants.

Ground-level ozone is not to be confused with the ozone layer in the upper atmosphere, which serves the beneficial purpose of shielding the earth from the sun’s rays.


Senior centers already urge their patrons to stay inside when the air is smoggy. Told of the new study, an official at St. Ignatius Nursing Home in West Philadelphia said now there is even more reason for seniors to avoid the outdoors on high-ozone days.

“There’s air conditioning,” said director of nurses John Sword. “The air quality inside is much different.”


The daily average for ozone nationwide is about 40 parts per billion. The level can be dramatically higher in the summer, especially in urban centers.

When ozone levels rose by 10 parts per billion during a given week, people were about half a percent more likely to die, Bell’s team found.

That would correspond to about 3,800 additional deaths nationally each year, they said.

The impact was seen even after adjusting for the effects of weather and small airborne particles, including soot, which also have been linked to premature death.

The authors said ozone likely kills even more people through long-term exposure.

The ozone-death link has been made in a few previous studies, but they were smaller and the findings were inconsistent. The new research is the most comprehensive of its kind, said Jonathan Levy, an assistant professor at the Harvard School of Public Health who was not involved with the new study.

“Something that contributes to 4,000 premature deaths annually is something to pay attention to,” Levy said.

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