The world's fastest marathon runner has dropped out of this year's Olympic marathon in China because he says there's too much pollution in the air.
Ethiopian athlete Haile Gebrselassie has asthma and says the city of Beijing is so polluted that running 26 miles in the smog would be bad for his health.
Now Olympic bosses are worried the thick haze of pollution that hangs over Beijing could affect other athletes.
Some teams are trying to cope with the smog by wearing masks while they train.
Factories are reducing their emissions
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China is also doing its bit to try to minimise air pollution in the run up to the games in August.
Factories have been ordered to cut the amount of smoke they pump into the atmosphere and some cars have been banned from the roads.
Olympic bosses have said that if pollution levels are too high during the Games, then endurance events could be delayed for a few hours or even moved to another day.
Meanwhile, Gebrselassie says he is still hoping to take part in the Olympics, by trying to qualify for the 10,000m race.