US EPA

A group of well-trained competitive distance runners prepares to step up to the starting line of a 10-km race. It is early afternoon on a warm day. As is all too common, a haze has formed over the suburban community in which the race is to be held. The runners have finished their warm-up and are prepared to start. Some of the runners notice a slight tightness in their chest that becomes greater as they try to take a deep breath in anticipation f the starter's signal. As the race begins, the runners focus on the race ahead and for a short time they ignore their mild respiratory discomfort. However, as the pack passes the 2-km mark and the racers begin to settle into their pace, the previously affected runners again notice their respiratory discomfort as it increases. Further, some to the previously unaffected runners begin to notice mild symptoms of respiratory discomfort. As the more severely affected runners pass the 5-km mark, their respiratory discomfort has intensified the their symptoms now include throat irritation and cough. These runners begin to fall off their planned race pace as they reconsider their strategy. Some may consider dropping out of the race. These runners are pale and are unable to take a deep breath without experiencing chest discomfort and cough. Later in the race, additional runners experience moderate symptoms. Nevertheless, most of the runners with mild to moderate symptoms finish the race. At the finish line many of the runners who experienced no symptoms wonder what their fellow runners are complaining about. This hypothetical scenario illustrates what could happen if endurance events were held in an environment were air quality is poor.

Folinsbee, L. Ambient Air Pollution And Endurance Performance. Second Edition.Chapter 6, R. Shepard (ed.), Endurance in Sport. Blackwell Science, Oxford, (2000).