In the researchers' lab, the men used a stationary bike for 15 minutes, rested for 15 minutes, biked for another 15 minutes, and rested for 15 more minutes while their hearts were closely monitored.
The men took the test once while breathing clean, filtered air and again on another day while breathing air that contained diesel fumes from a car. They had blood tests six hours after exposure to the filtered air or the diesel exhaust.
When the men exercised in the polluted air, their hearts were more stressed and produced less of a clot-busting chemical called tPA (tissue plasminogen activator).
The men didn't report any symptoms during or after the tests, and it's not clear if the findings apply to people without heart disease. Also, air pollution is a jumble of chemicals, not just diesel exhaust, and the researchers don't yet know which chemicals drove the study's results.
Still, the study may help explain why air pollution is associated with cardiovascular events such as heart attacks, write the researchers, who included Nicholas Mills, MD, of the Centre for Cardiovascular Science at Scotland's University of Edinburgh.
An editorial published with the study urges people not to get the wrong idea.
The issue isn't about exercising -- it's about air pollution, according to editorialist Murray Mittleman, MD, DrPH, of the Cardiovascular Epidemiology Research Unit at Boston's Beth Israel Deaconess Medical Center.
"Considering the unequivocal benefit of habitual exercise ... the risk benefit ratio may be optimized if people exercise away from traffic when possible," writes Mittleman.