Dr. Gokhan Mutlu of Northwestern's Feinberg School and Northwestern Memorial Hospital said that for people in certain risk groups, breathing high levels of pollution can cause heart attacks and strokes within 24 hours of exposure.

Those vulnerable include those with cardiovascular disease or risk factors for cardiovascular disease such as high blood pressure, high cholesterol, diabetes, obesity, lung disease or a current smoking habit.

"Being exposed to higher levels of pollution may unmask heart disease even if you've never had any symptoms," Mutlu said in a statement.

Mutlu published research last year that showed microscopic air pollution -- particles less than one-tenth of the diameter of a human hair -- makes the blood thicker and sticky -- when lungs are inflamed by pollution, they secrete a substance, interleukin-6, which causes an increased tendency for blood to clot.

"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 United States that further increases your risk," Mutlu said. "If clots migrate into the lungs and cause pulmonary embolism, that can kill you."