People who go to Beijing to watch the summer Olympics could face heart attacks and strokes because of the city's polluted air, researchers from Northwestern University said.

The researchers said in a news release that the effects can show up within 24 hours of exposure.

People with risk factors such as high blood pressure, high cholesterol, diabetes, obesity or smoking are most vulnerable.

"If the air quality is bad, you are more likely to have serious heart disease related events," said Dr. Gokhan Mutlu. "Being exposed to higher levels of pollution may unmask heart disease even if you've never had any symptoms."

Mutlu published a paper in 2007 that said that pollution thickens the blood and makes it more likely to clot. A more recent study has shown that people who live in polluted areas are more likely to have blood clots in their legs.

Dr. Scott Budinger said people can lower their risks in China. Men over 40 should take an aspirin, he said. The benefits of aspirin are less certain for women, he said it probably wouldn't hurt for them to take one, too.

He also suggested that people stay indoors during heavy traffic periods.

The release said that the poor air can also cause blood clots when people fly overseas afterward.