No athlete pulled out of the 2002 Winter Olympic Games in Utah because of bad air quality. The government didn't order vehicles removed from the road. Factories didn't close and construction work didn't stop.

That's all happening in Beijing, where the city's deputy director of its Environmental Protection Bureau, Du Shaozhong, felt compelled to guarantee "good air quality" for the Summer Games, which begin Friday. China has been taking extraordinary steps to clean up its air, shutting down traffic and moving factories out of urban centers.

But worries, like the pollution itself, linger.

The biggest short-term impacts on air quality are emissions from sources like vehicles and factories and how stagnant weather conditions are, said C. Arden Pope, Brigham Young University epidemiologist, professor of economics and a world-renowned expert on the health effects of pollution. Chinese officials are trying to limit emissions and undoubtedly hope the weather will do its part. But it's hard to predict, Pope said.

It is not known what, if any, long-term effects short-term exposure to possibly severe pollution will have on healthy athletes, said Dr. Richard Kanner, a pulmonologist at University Hospital. "It's not ethical to do a study getting healthy young athletes to run through severe pollution," he said. Studies in polluted chambers aren't comparable.

Athletes who perform at outdoor venues will experience the worst of whatever's in the air, said Dr. Max Testa, exercise and pulmonary specialist at The Orthopedic Specialty Hospital. And because of the exertion required, they'll breathe through their mouths, losing the nose's filtering ability.

Adults at rest pump 12 liters of air in and out each minute. Athletes in competition breathe 200 liters in and out in a minute, he said. But while all the athletes in a given event will process about the same volume of air, they won't be equally disadvantaged by pollution. "Some are more sensitive to pollution than others, and you can't train for polluted air," Testa said.

He doubts that short-term exposure from the Games will turn into long-term damage. "The only long-term effect may be they don't win a medal," he said. "It definitely could impact performance."

If he were coaching the athletes, he said, he'd tell them to "get there at the last minute and use a mask before competition. Breathe unfiltered air just for the race."

Some of the symptoms related to pollution include chronic irritation, coughing, wheezing and itching eyes, he said.