A study of six U.S. metropolitan areas including New York City, Los Angeles and Baltimore has found that long-term exposure to air pollution can lead to the same likelihood of developing lung disease as a person who smokes a pack of cigarettes a day for 29 years.

Environmental health researchers found that ground level ozone pollution was strongly linked to an increased risk of emphysema, a lung disease often caused by smoking, and a decline in lung health, according to a study released this week.

“Ground level ozone is a powerful oxidizing agent and common air pollutant worldwide,” the study says, noting that toxicologic studies have shown ozone causes persistent lung inflammation and contributes to chronic lower respiratory disease, including emphysema.

For the study, researchers tracked more than 7,000 participants in Baltimore; Chicago; Los Angeles; New York City; St. Paul, Minnesota; and Winston-Salem, North Carolina, between 2000 and 2018. They found that the likelihood of developing emphysema increased on average by more than half a percent every 10 years.

Researchers said concentrations of air pollutants such as ozone, nitrogen oxide and black carbon are associated with greater risk of emphysema.

“These findings may offer one explanation for why emphysema is found in some people who have never smoked,” said James Kiley, director of the National Heart, Lung, and Blood Institute’s Division of Lung Diseases. “The study’s results, duration and timing offer insight into the long-term effects of air pollution on the U.S. population.”

While some air pollutant levels decreased over the years, long-term concentrations of ground level ozone in the six cities did not, leading the researchers to call for more effective strategies to reduce ozone.

The researchers hailed from different universities, including the University of Washington, the University of Buffalo, Columbia University and Boston University School of Public Health, among others. The research was backed by federal funding, including money from the National Institutes of Health.

The study was published Tuesday in the Journal of the American Medical Association.

John Balmes, a professor of environmental health sciences at the University of California, Berkeley, said he was surprised by the findings.

“The levels of exposure are generally less than from tobacco smoke — active smoking is what I’m talking about. That’s what’s stunning about these results,” he said.

Air pollution has been linked to reduced lung function and can worsen asthma and chronic obstructive pulmonary disease, Dr. Balmes said.

What is less clear is whether air pollution can cause asthma and chronic inflammatory lung disease such as emphysema. Dr. Balmes said the study seems to suggest that air pollutants could cause emphysema.

He said society needs to reduce ozone levels, especially because of climate change. Motor vehicle emissions that are cooked on sunny afternoons are primary contributors to higher ozone levels, he said.

He recommended that individuals who suffer from lung disease reduce exposure by staying inside on days with high levels of ozone, which doesn’t penetrate well indoors.

Emphysema is an incurable disease in which the lungs’ lining becomes damaged beyond repair, making it difficult to breathe.

More than 11 million people in the U.S. have been diagnosed with chronic inflammatory lung disease, including about 3 million diagnosed with emphysema.

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