Healthy, young volunteers with no history of heart disease showed unfavorable changes in their heart function after just two hours of exercising while being exposed to ozone, report researchers in the journal Circulation. The changes included surges in markers of inflammation, as well as drops in levels of enzymes that break down clots in the blood vessels — alterations that may explain the link between exposure to air pollution and heart risk.
The study is among the first to document the physiological changes caused by exposure to ozone, a major pollutant formed when volatile organic compounds from industrial waste or car exhaust reacts with sunlight. Previous studies have linked exposure to ozone to heart problems, but had not quantified the precise effect of the pollutant on biological markers of heart and lung function.
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In the study, 23 young participants participated in two hours of intermittent exercise in a lab while being exposed first to “clean” air, and then to air containing 0.3 parts per million of ozone, which is higher than the amount found in average U.S. cities but about the peak level calculated for heavily polluted cities like Beijing, China and Mexico City. (However, the level is equivalent to the amount of ozone someone in an average American city would be exposed to over the course of seven to eight hours.) Scientists then compared readings on various biological markers of heart and lung function between the two sessions, to get a sense of ozone’s impact. Under the ozone conditions, the participants experienced a nearly 99% jump in levels of interleukin-8, an marker for inflammation in the blood vessels. They also showed a 42% drop in plasminogen levels, which lowers the body’s ability to break up blood clots.
The study recorded the participants’ readings for only 24 hours after the experiment, and the changes were temporary and reversible: once the volunteers stopped breathing the heavy concentration of ozone, their measurements returned to normal levels.
But the results show that ozone can have potentially harmful, and even deadly effects on the heart, say the authors. “This study provides a plausible explanation for the link between acute ozone exposure and death,” Robert Devlin, a senior scientist at the U.S. Environmental Protection Agency’s National Health and Environmental Effects Research Laboratory and the study’s lead author, said in a statement.
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“The results complement what are suspicions are, and help us to apply what we know about pollution risk to populations we think are at especially high risk,” says Dr. Tracy Stevens, a cardiologist at St. Luke’s Mid America Heart Institute and a spokesperson with the American Heart Association. The findings highlight the dangers of pollution to people who might already have unstable plaques in their heart vessels, she says, since ozone can trigger a surge in inflammatory markers that drive these plaques to rupture, causing a heart attack.
While Stevens says the risks of pollution in aggravating inflammation are known, having data that quantifies the risk as the current study does may help more people to appreciate and address ways to reduce inflammation. The American Heart Association recommends that people with heart disease avoid going out on high-ozone days, and lower their risk of exposure to heavily polluted air, including cigarette smoke.
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Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.