Relatively low levels of ozone (<120micrograms/m3) can affect lung function in endurance athletes, making it more difficult to bring large volumes of air into their lungs (Respiratory Effects of Low-Level Photochemical Air Pollution in Amateur Cyclists,” American Journal od Resp. Crit. Care Medicine, vol. 150, pp.962-996, 1994). As a result, exercise scientist have searched for years to find ways to minimize ozone-related respiratory problems in athletes.

via Running Research News.