WASHINGTON, March 11: Air pollution causes the blood vessels of healthy people to close up, which helps explain why high levels of pollution are linked to heart attacks and other cardiovascular problems, researchers said on Monday.

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,” Dr. Robert Brook, a specialist in the biology of blood vessels at the University of Michigan who helped lead the study, said in a statement.

The U.S. 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, as found in Mexico City, for instance, 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 in a telephone interview.

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.—Reuters