Haile Gebrselassie is so spooked by Beijing's smog and sultry summer weather that he pulled out of the Olympic marathon. Doctors say the world-record holder is right to worry.

Gebreselassie suffers from a form of asthma triggered by strenuous exercise and exacerbated by pollution. Athletes with exercise-induced asthma will be vulnerable in Beijing, where the air is filled with smoke from coal-fired power plants, dust kicked up by construction crews and car exhaust.

"If the pollution was bad enough, it could lead to him being hospitalized," said John Balmes, a doctor who studies the effects of pollution on the lungs at the University of California, San Francisco. Gebrselassie "is correct in worrying about how he would feel."

As many as 25 percent of Olympic athletes suffer from asthma, which causes airways to swell and produce mucus, reducing oxygen supplies to straining muscles. A mild attack cuts lung function by about 10 percent, while a severe case can reduce it by more than 50 percent, said Karen Holzer, an asthma specialist with the Australian Olympic team.

Gebrselassie, 34, said last week he plans to compete only in the 10,000 meters in Beijing because running the marathon would reduce his chances of appearing in the 2012 London Olympics. Marathon runners who overexert themselves can suffer long-term damage and shorten their careers, said his agent, Jos Hermens.

"If it's extreme conditions, then it's a big, big danger," said Hermens, whose Ethiopian client won gold medals in the 10,000 meters at the 1996 and 2000 games.

The long-term effects on asthmatics of exertion in a highly polluted environment aren't known, Balmes said.

Beijing plans to spend $17 billion to improve air quality before the games, scheduled for Aug. 8-24. The anti-pollution drive helped increase the number of "blue sky days" to 246 last year from 100 in 1998, according to the government.

"You can be assured of clean air in August," Beijing Vice Mayor Ji Lin said last week.

Beijing's air pollution index was measured at 343, or heavily polluted, for the 24 hours through noon yesterday, according to the city. The World Health Organization recommends a maximum level of 50 for the index, with includes sulfur dioxide, nitrogen dioxide and suspended particulates.

The International Olympic Committee said March 17 that poor air quality may put some athletes at risk during the games. The IOC is drawing up contingency plans in case pollution prevents some events from taking place as planned, Arne Ljungqvist, chairman of the IOC's medical commission, said in a statement.

The U.S. plans to test athletes for asthma in Beijing prior to the games. Because people react differently to varying levels of pollution, some competitors may not know they have exercise- induced asthma until they start training in the Chinese capital, U.S. team physician Randy Wilber said.

"My concern is that an athlete who has perfectly normally functioning lungs in Colorado Springs will have significant problems in Beijing's air pollution," Wilber said, referring to the U.S. Olympic Committee's training center.

That may prevent some athletes from taking drugs that ease swelling of the airways. Competitors must prove they are asthmatic before the games to be allowed to take medications banned by international sports federations, including the commonly prescribed albuterol.

"There's a lot of paperwork," Holzer said. "You've got to have everything in place before you go."

China's economic growth of more than 10 percent a year has filled Beijing's air with particulate matter from building sites, carbon monoxide from vehicle emissions and oxides of nitrogen and sulfur from power plants and factories.

Carbon monoxide hinders athletes because it competes with oxygen at the binding sites of hemoglobin, which carries oxygen in blood from the lungs to the body. The other pollutants inflame airways and irritate sinuses and eyes.

The biggest threat to athletes may be ground-level ozone, produced when oxygen reacts with nitrogen dioxide in sunlight and high temperatures. Ozone causes a burning sensation in the lungs and increases phlegm production, Balmes said.

"It's a mild burn unless you're a marathon runner out there breathing very hard for a couple of hours," said the chief of occupational and environmental medicine at San Francisco General Hospital and a marathon runner.

Heat and humidity are a bigger concern than pollution for endurance athletes, said Dr. John Brotherhood, who is advising the Australian Olympic Committee on heat problems in Beijing.

Athletes cool themselves by sweating, a process that's impaired when high humidity prevents perspiration from evaporating. Relative humidity in Beijing in August may climb to 95 percent and morning temperatures can top 30 degrees Celsius (86 degrees Fahrenheit).

"The worst scenario is that somebody might get heat stroke and their temperature might get dangerously high," Brotherhood said. "But usually what happens is that people get profoundly fatigued or they just feel that have got to stop or slow down."

Gebreselassie's dream is to run the marathon at the 2012 games, and he won't jeopardize that by tearing through the streets of Beijing for 26.2 miles, Hermens said.

"It's not just the pollution but the heat and humidity," he said. "The marathon's just too dangerous for him."