By Janet Raloff
Ozone pollution appears to cause potentially dangerous changes in the heart at levels that might be encountered in the world’s most polluted cities. Scientists have uncovered signs of inflammation and heart rhythm disturbances in 23 healthy young volunteers who briefly inhaled elevated levels of ozone, the primary irritant in urban smog.
The alterations, reported online June 25 in Circulation, go a long way toward explaining population data that have started linking ozone to an elevated risk of death from heart attacks and stroke (SN: 12/11/04, p. 372).
Many air pollution scientists, “including me, have in the past thought associations with ozone [and disease outside the lung] were really associations with particles or some other pollutant,” says Douglas Dockery of the Harvard School of Public Health in Boston. But he says the new study now directly shows ozone is causing acute — and even chronic — risk for heart attacks.
For two hours on separate days, toxicologist Robert Devlin of the Environmental Protection Agency in Research Triangle Park, N.C., and his colleagues exposed men and women to clean air or to air containing 0.3 parts per million ozone. On the high-ozone day, volunteers inhaled the same cumulative dose that they would have received over eight hours in a place that exceeded the U.S. federal limit of 0.075 parts per million for that length of time.
Ozone reaches such levels in Los Angeles and Houston. And heavily polluted cities such as Beijing and Mexico City have experienced peak hourly ozone concentrations approaching the raw level used in the experiment.
A growing body of data indicates inflammation underlies the progression of cardiovascular disease. In the new study, blood levels of several inflammatory agents increased after ozone exposure — sometimes more than doubling —throughout a period that lasted more than a day. This “caught us by surprise,” Devlin says, and “we think it’s one of the more important and significant findings.”
The high ozone exposure also triggered subtle changes in heart rate variability. Although small, this points to an increased risk of arrhythmias, notes EPA cardiologist Wayne E. Cascio. Ozone also altered levels of several proteins involved in blood clotting.
Usually, various biochemical players in the body’s clotting network “keep each other fairly mellow,” says cardiologist Tracy Stevens of St. Luke’s Mid America Heart Institute in Kansas City, Mo. But in this study, she notes, ozone exposure induced changes to the clotting system that riled things up in ways “that would make one more prone to clot.”
Most impressive, she says, “were all those changes in markers of inflammation.” In healthy young adults, such changes shouldn’t cause harm, Stevens says. But in the elderly and persons with diabetes or heart disease, these immune system players could inflame the plaque that had been accumulating in arteries, provoking it to rupture.
When this happens, Stevens explains, the body turns on clotting in its attempt to heal what it reads as a wound. “It’s these clots that obstruct blood flow and trigger a sudden crisis,” she says — such as a heart attack.
