They said their study fits in with other research that shows air pollution can cause not only breathing problems but heart problems. "These findings suggest a possible reason why the rate of heart attacks and other cardiovascular events increases with exposure to air pollution for people with known heart and blood vessel disease," said Dr. Robert Brook, a specialist in the biology of blood vessels at the University of Michigan who helped lead the study.

The Environmental Protection Agency estimates that air pollution contributed to 60,000 heart-related deaths in 1996.

Brook said the experiment involved fairly high levels of pollution, such as those found in Mexico City or on bad days in Los Angeles. But he said the harmful pollution could not be seen or smelled, and people would not feel the effects. "You don't even know. You can't tell that you are inhaling it. You can breathe in these rather high levels of air pollution and be mostly unaware," Brook said.

Brook and his brother, Dr. Jeffrey Brook of the University of Toronto, tested 25 healthy volunteers with an average age of 35. They sat in a chamber and air was pumped in — sometimes filtered and sometimes containing ozone and fine particulate matter. "These come from the combustion of normal fossil fuel," Brook said. Cars, power plants, iron smelters, and other industry all create ozone and fine particulate pollution.

TINY BITS OF METAL

The tiny particles of carbon and other material have even smaller bits of iron, manganese, and zinc clinging to them. They are inhaled deep into the lungs, and some studies suggest they may be absorbed directly into the bloodstream. Brook said the body's immune system may mistake these particles for bacterial or viral invaders and attack. As white blood cells move in, they release inflammatory chemicals called cytokines that cause the blood vessels to constrict.

These bits of metal may also damage healthy cells. After two hours of breathing the polluted air, the blood vessels of the volunteers constricted between 2 percent and 4 percent on average, Brook and his team reported in this week's issue of the journal Circulation. Their vessels did not constrict when they breathed clean, filtered air.

The researchers used ultrasound to measure the diameter of the brachial artery, which runs from the shoulder to the elbow.

"Although the degree of constriction in and of itself is unlikely to produce significant problems in healthy individuals, such a constriction could conceivably trigger cardiac events in those individuals who have or are at risk for heart disease," Brook said.

He said his study fit in well with one published last week in the Journal of the American Medical Association. In it, a team at Brigham Young University in Provo, Utah, found that long-term exposure to air pollution increases the risk of death from lung cancer, heart attack, stroke, asthma, pneumonia, emphysema, and chronic bronchitis. "We are hoping that this line of research will add some strength to well-known association studies," Brook said. "Now we can say, 'Gee, there is a clear linkage here between bad air and cardiopulmonary events.'"