Dr. Robert A. Kloner of the Heart Institute of the Good Samaritan Hospital and Keck School of Medicine, University of Southern California, both in Los Angeles, said when pollutants are inhaled, they trigger an increase in "reactive oxygen species" -- superoxiding molecules that damage cells, cause inflammation in the lungs and spark the cascade of harmful effects in the heart and cardiovascular system.

Recent research suggests that ultrafine air pollutants, such as those coming from car exhaust, may pass into the blood stream and damage the heart and blood vessels directly. Hearts directly exposed to ultrafine air pollutants show an immediate decrease in both coronary blood flow and pumping function, as well as a tendency to develop arrhythmias, studies at the Heart Institute found.

"We used to think air pollution was a problem that primarily affects the lungs. We now know it is also bad for the heart," Kloner said in a statement.

The findings are published in the Journal of the American College of Cardiology.