This is an excerpt from Advanced Environmental Exercise Physiology by Stephen S. Cheung.
Exercising in Urban Environments
Analogous to the epidemiological studies demonstrating that elderly persons and those with existing cardiovascular conditions are the most at risk from heat waves (Yip et al. 2008), research suggests that air pollution may affect different subsets of the population more than others. From this perspective, athletes training and competing outdoors may be much more susceptible to air pollutants and have a much higher effective dose than the general population or athletes primarily training and competing indoors. Besides the higher ventilation rates during exercise, the first obvious difference is in the time spent outdoors by athletes, which often also coincides with the times of peak pollution levels. In urban settings, walkers, runners, and cyclists may also be exercising near major roadways, where dosage of pollutants may be much higher than average reported values (Sharman et al. 2004).
In urban environments, it appears essential that both recreational and elite athletes be properly educated about protecting themselves from the worst effects of high pollutants while exercising. As it is difficult to actually change the air that we breathe, most of the advice comes down to commonsense methods to reduce exposure. Chief among these are to exercise in quieter settings away from heavy traffic and busy roadways. If this is not possible, then exercise should be performed during nonpeak periods of traffic flow. For example, analysis of pollutants in Toronto, Canada demonstrated that different pollutants peak at different times throughout the day (Campbell et al. 2005). O3, particulates, and SO2 peaked during midday, while CO and NO2 peaked during the morning rush hour. Overall, pollution levels were lowest prior to 0700 h and after 2000 h.
Lessons From the Olympic Games in Beijing
The hosting of the 2008 Summer Olympics in Beijing, one of the world's most polluted megacities, instigated a thrust within the sport science community to understand the potential effects of air pollution on exercise. The pollution levels for ozone, carbon dioxide, sulfur dioxide, nitrogen oxides, and particulate matter are very close to or exceed the standards set for long-term health in the general population by the U.S. Environmental Protection Agency (Qi et al. 2007), and the effects of pollution are potentially exacerbated by the high heat and humidity typical during August.