"We know how to train for heat and humidity, but not a lot of research has been done on running in the polluted atmosphere we think we'll find in Beijing," Kyle O'Brien, an American marathoner who ran at the track and field world championship in Osaka, said today.

Therein, lies the problem.

Air pollution levels in China's capital are nearly five times higher than the World Health Organization's recommended safety level. A mix of major pollutants such as ozone, nitrogen dioxide and particulate matter are common. Sulfate and carbon also float regularly in the air.

It's not that officials haven't noticed.

China is spending billions to close dirty factories and build new subways, and in an experiment for the Olympics, the city earlier this month pulled 1 million private vehicles a day off the streets. The congestion eased, but a gray haze remained.

Sounding an alarm recently, International Olympic Committee president Jacques Rogge warned that choking smog might force some endurance events to be postponed.

During last week's world championship in Japan, the stifling heat regularly hit the mid-90s with humidity in the 60 to 70 percent range.

Add the mix of pollutants Beijing might offer, and athletes, particularly those in the endurance events, foresee problems.

Like O'Brien, teammate Mike Morgan ran Saturday in Osaka, and he, too, was about 10 minutes off his personal best.

"The heat and humidity knock you out, and you have to adjust for it," he said. "I assume pollution will have more of the same effect, and Beijing is going to be a different story with the city surrounded by pollution."

Some research suggests training at altitude might offer an edge, building endurance which might help counteract the effects of pollution.

Perhaps, but others are concerned the dirty air might cost them a legitimate shot at a medal.

"It is difficult for a distance runner to compete there," said Tirunesh Dibaba, who won her second straight 10,000-meter world title in Osaka. "It could possibly affect me. I might even consider running just one event because of the pollution."

How to train for Beijing certainly will get more discussion in the coming months. For now, it's mostly speculation or guesswork.

Some teams have promised to stay away until the last moment. The most prominent so far is Australia, which is planning to send a medical staff of 80 to deal with heat, humidity and pollution, about one for every six athletes.

"It will be hot and humid and the pollution is high," said Dr. Juan Manuel Alonso, chairman of the IAAF medical and anti-doping commission. "It wouldn't be good to compete in that, but I'm confident the air quality will be fine."

Obviously, endurance athletes take in more air and feel the most threatened, but even sprinters are worried.

David Gillick, the Irish 400-meter record holder, said he'd stay away as long as possible at a camp Ireland plans to set up in Japan.

"There's no scientific proof if you go in and spend two weeks there it will be beneficial," he said. "Or maybe it will work against us. Will we get sick? Will we just keel over with it? I think a lot of people are going to be stuck between a rock and a hard place. It's not going to be ideal, but what do you do?"

"I'd like to stay away," Gillick added. "Just come in two days beforehand and go for it. We're in sprint events. I think it's a different story for endurance athletes."

Masaaki Sugita, an exercise physiologist with Japan's athletics governing body, was in Beijing earlier this month. He said on some days the air was better than expected, but smog and haze still blanketed the city. At one test event he said he saw archers wearing masks.

If there is a solution, Sugita suggests it rests with China.