It's not just Olympic athletes who need to worry about the heavily polluted air in Beijing, for the dirty air may cause serious heart problems for some spectators too.
For people in certain risk groups, breathing high levels of pollution can cause heart attacks and strokes within 24 hours of exposure and increase the possibility of having blood clots in their legs on the plane home, warn two researchers in pulmonary medicine and critical care at Northwestern University's Feinberg School of Medicine.
People who are at most risk include those who already have known cardiovascular disease or risk factors for cardiovascular disease such as high blood pressure, high cholesterol, diabetes, obesity, lung disease, a current smoking habit or a family member diagnosed with heart disease before age 55.
"Being exposed to higher levels of pollution may unmask heart disease even if you've never had any symptoms," Mutlu added.
Mutlu published a study in 2007 that showed how pollution triggers heart attacks and strokes.
He found that microscopic air pollution, particles less than one-tenth of the diameter of a human hair, makes the blood thicker and sticky.
He found when lungs are inflamed by pollution, they secrete a substance, interleukin-6, which causes an increased tendency for blood 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.
Traveling long distances by car or plane is known to put people at risk for these clots.
"If you spend a few weeks in Beijing, your blood might become thicker and sticky and then when you fly 12 hours back to the U.S. that further increases your risk. If clots migrate into the lungs and cause pulmonary embolism, that can kill you," Mutlu warned.