Smog clogs arteries: pollution does lasting harm to blood vessels.

There's a new reason to worry about air pollution. Known for many years to harm the lungs, air pollution also damages the circulatory system, a study now suggests.

A reexamination of data collected for various health care trials in the Los Angeles area indicates that the more air pollution there is around a person's home, the thicker the walls of his or her carotid artery become. Thickened artery walls are a leading risk factor for heart attacks and strokes.

Nino Kunzli of the University of Southern California in Los Angeles and his collaborators carried out the study, scheduled for an upcoming Environmental Health Perspectives.

Animal studies have previously shown that air pollution--specifically small dust particles less than 2.5 micrometers across--irritates the lungs and provokes inflammation of the blood vessels. Over time, inflammation of the arteries leads to the thickening and hardening of the artery walls, or atherosclerosis.

C. Arden Pope III of Brigham Young University in Provo, Utah, says that constant exposure to air pollution probably keeps people's arteries slightly inflamed. In the Jan. 9 Circulation, Pope and his colleagues reported that long-term exposure to air pollution increases a person's risk of dying of a heart attack or stroke. The authors suggested, but couldn't prove at the time, that the air pollution was causing atherosclerosis.

The Los Angeles study has now demonstrated the specific link between air pollution and atherosclerosis, Pope says.

Kunzli and his colleagues used instrument recordings around Los Angeles and land-use data from the area to estimate the amount of air pollution at the home of each of the 798 people in the study.

Atherosclerosis progresses on a time-scale of years or decades, so it's likely that the differences seen in the study took a long time to develop. "But whether this is half a year, a year, 10 years, or a lifetime--that's difficult to answer," says Kunzli.

An important question to investigate now, Pope says, is whether the damage can be reversed. "If we clean the communities up, or if I move to a clean community, will I recover?" he asks. He suspects a reversal is possible because scientists have observed such an effect in studies of atherosclerosis caused by smoking.

To further describe the link between people's exposure to air pollution and cardiovascular conditions, the Environmental Protection Agency is now funding a study in which University of Washington researchers will track 8,700 people for 10 years.