For most people, breathing polluted air means itchy eyes, a nagging cough, or a nasty smell. But for Olympic athletes competing in Beijing next year, it could mean losing a medal - unless authorities deliver on promises for a radical cleanup.
To cope, several Olympic teams are planning to spend as little time as possible in Beijing before the Games begin.
This month, Japan's softball team canceled its pre-Olympics training in Beijing, saying is was worried about pollution. Australian athletes, too, are avoiding Beijing until the last minute. Even the French judo team - who will be sparring indoors - has decided not to train in Beijing.
"The difference between these super-elite athletes is a very small line," said Dr. Todd Schlifstein, a sports medicine expert and assistant professor at New York University School of Medicine. "If they are really sensitive to the pollution in Beijing, that could inhibit their performance and have a big impact on how close they come to a medal."
Though many of the largest teams are not talking about pre-Olympic plans, nearly all are sending experts to Beijing to ready their athletes for the grimy conditions.
During a visit to Beijing this month to observe test events, International Olympic Committee President Jacques Rogge warned that some outdoor competitions might be postponed if the city's air was too polluted.
Chinese authorities plan to shut down factories and yank cars off Beijing's streets during the Olympics, but an exact timeline has not been set. Authorities have even floated the idea of giving Beijingers a one-month holiday to clean up the city. If those measures work, then arriving athletes could be welcomed by a pristine Beijing barely recognizable to residents.
Beijing's coal-burning factories, power plants and vehicles regularly spew enough fumes to shroud the city in a brown, pungent haze. Residents have grown used to the soot that coats their clothes. Some even wear surgical masks when walking outdoors to guard against the filth.
The standard pollution index, which measures the amount of dangerous substances per cubic meter, usually hits the mid-20s in London and New York. In Los Angeles, it is nearly 40. But in Beijing, it regularly tops 100.
On a normal day, Beijing's pollution is so bad it exceeds the safety benchmark set by the World Health Organization by nearly five times.
"There's a complex collection of chemicals in Beijing's air," said Dr. Michal Krzyanowski, WHO's European regional adviser on air quality and health. Sulphate, ozone and carbon are regularly inhaled by the city's residents.
The human body's cardiovascular system is very sensitive to these chemicals. Some people react particularly badly and have a hard time breathing or develop skin problems.
In serious cases, smog could even clog the arteries. While major damage takes years, a short stay in a polluted city is not risk-free - especially for competitive athletes.
"Olympic athletes who are exercising very vigorously will be taking in more air than normal people," Kryzanowski said. That means they will get a bigger dose of pollutants in their lungs than normal people.
Asthmatic athletes may find themselves reaching for their inhalers more often, as polluted environments usually set off more attacks. But even non-asthmatic athletes may be in trouble.
Because their defense mechanisms may be busy dealing with the pollution, Kryzanowski said athletes could catch minor infections or colds more easily.
Competitors who train at high altitudes might have a slight edge. Training in the mountains is common among those trying to build endurance, and could have a welcome spillover effect.
"High-altitude training helps your body as a whole become more efficient as it extracts oxygen from your hemoglobin," Schlifstein said. "If there is a high carbon dioxide concentration in the air from the pollution, this kind of training might help you compensate."
But if there are lots of other pollutants in the air, not much can be done.
"You can't train yourself to exercise better in polluted air," Kryzanowski said. "If you prolong your exposure, that will just make it worse."
Bob Larsen, an assistant coach with the U.S. Olympic track team in 2004, said he didn't think runners would be bothered by the air quality, but that other considerations might be important.
"There of course could be negative psychological reactions for unprepared athletes when they don't see clear blue sky when they arrive in Beijing," said Larsen, who coached Meb Keflezighi to a silver medal in the marathon at the Athens Games.
"For endurance athletes, I think the heat and humidity will pose a bigger challenge than the pollution," added Larsen, who visited Beijing to test conditions after the city was awarded the Games.
Experts were also worried about pollution at the 2004 Olympics in Athens. To cut noxious fumes, Greek authorities blocked car traffic surrounding the Olympic venues, and asked major factories to stop production during the Games.
To avoid the possibility that a smog-tinged Olympics could ruin Beijing's reputation, Chinese officials are cracking down on drivers and heavy industries. They have also kicked many offending factories out of the city.
Last week, Beijing wrapped up a test that pulled more than a million cars off the streets. Though government officials said the experiment slashed Beijing's air pollution index by more than 20 percent, there was still a distinct, gray haze hovering above the city.
"It's good that the government is thinking about this," said Dr. Lu Zhi, a professor of nature conservation at Peking University.
"This is not the ultimate solution," Lu added. "But it's possible it could help the athletes a little."