Pollution, previously linked to poor respiratory health, has now been linked to raised blood pressure.

German researchers looked at 5,000 people and found long-term exposure increased blood pressure, even when other key factors were considered.

The team, which has presented its work to the American Thoracic Society, says efforts should be made to reduce exposure to pollution.

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.