Sept. 14 (HealthDay News) -- Breathing polluted air for even two hours can boost blood pressure, potentially raising the risk of cardiovascular disease in those exposed to smog, a new study suggests.

Although the increase may not mean much for healthy people, "this small increase may actually be able to a trigger a heart attack or stroke," study author Dr. Robert D. Brook, an assistant professor of medicine at the University of Michigan, said in a university news release.

An estimated nearly one in three Americans suffer from high blood pressure, meaning the heart is straining to push blood through the circulatory system.

In the study, which appears in a recent issue of Hypertension, researchers tested 83 people as they breathed levels of air pollution similar to those in an urban city near a roadway.

"We looked at their blood vessels and then their responses before and after breathing high levels of air pollution," study co-author Robert Bard, a University of Michigan clinic research coordinator, said in a news release.

The air pollution caused diastolic pressure -- the lower number in a blood pressure reading -- to rise within two hours. Blood vessels were impaired for as long as 24 hours.

Tests showed that microscopic particles in the air, rather than ozone gases, caused the rise in blood pressure and impaired blood vessel function.

"If air pollution levels are forecasted to be high, those with heart disease, diabetes or lung disease should avoid unnecessary outdoor activity," Brook said.

SOURCE: University of Michigan, news release, September 2009