Michael Jerrett of the University of California, Berkeley, and colleagues studied nearly 500,000 people across the United States for 18 years and found that ozone played no role in heart disease deaths once air pollution consisting of very tiny particles was taken into account.

But ozone, at ground level a corrosive form of oxygen that is the primary constituent of smog, was the key player in respiratory fatalities.

"We now know that controlling ozone is not only beneficial for mitigating global warming, but that it could also have near-term benefits in the reduction of deaths from respiratory causes," Jerrett said in a statement.

Doctors have long known that ozone is hazardous. Short-term exposure aggravates asthma symptoms and causes breathing problems. Ozone alerts are common in much of the United States during hot summer days.

The study, published in Thursday's New England Journal of Medicine, shows that long-term exposure increased mortality, said Jerrett.

"This is the first time we've been able to connect chronic exposure to ozone, one of the most widespread pollutants in the world, with the risk of death," he said.