It’s perfectly safe to exercise in the polluted air of Beijing, the site of the upcoming Olympic Games, say International Olympic Commission officials. It’s perfectly safe, that is, as long as you don’t have to breathe deeply for more than an hour. “They have scientifically proved there is no risk for the wide majority of sports,” IOC chief inspector Hein Verbruggen tells But “for endurance events that include minimum one-hour continuous physical efforts at high level—urban road cycling, mountain biking, marathon running, marathon swimming, the triathlon, and road walking—the IOC Medical Commission’s findings indicated that there may be some risk.” Beijing’s air is among the worst in the world, due to a lack of pollution-control laws, heavy industrial activity, and automobiles. Should pollution be particularly bad at any time during the Games, the IOC said, it may delay events. The reassurances haven’t satisfied many of the athletes. “My main fear,” says U.S. marathon runner Brian Sell, “is that I go run this marathon and develop some long-term complication and it hampers me in future marathons.”