A set of Olympic rings are seen standing in front of apartments for athletes during a media opportunity at the Olympic and Paralympic athlete's village in London, Thursday, July 12, 2012. (AP Photo/Matt Dunham)
It's just days before the Olympic opening ceremonies and some allergists are warning that London's air pollution could cause breathing problems for some athletes.
One specialist says the amount of nitrogen dioxide in London is comparable to the level in Beijing before the Chinese government banned half the cars there in preparation for the 2008 Games.
Doctor William Silvers warns that athletes with asthma or other exercise-induced respiratory problems could have trouble.
He says marathon runners are particularly at risk.