Friday, September 30, 2005

Cyclist exposure to air pollution

Roadies and mountain bikers typically ride away from heavy traffic, but commuting cyclists often ride right in the thick of heavy automotive traffic. If you commute by bike, what is your exposure to air pollutants compared to driving or riding a bus, train, or subway?

According to this study in London, subway users have about three times the exposure to particulate pollutants than other commutes, while cyclists have slightly less exposure than car drivers. This Amsterdam study suggests, however, that cyclists may inhale more pollutants than car drivers because cyclists breathe more than drivers during the commute.

After a reader inquiry, Umbra Fisk examines the question. According to Umbra, studies show that car and public transit occupants actually have a higher level of exposure to air pollutants than cyclists and pedestrians do. This is because the fresh air intake on cars are at the front of the car -- immediately behind the exhaust pipe of the car ahead. Cyclists, on the other hand, tend to ride to the side of traffic with the 'air intake' two or three feet above that of cars. According to Umbra, "The nasties are densest at the middle of the traffic zone, and less intense on the edges." Because pollutants dissipate in a volume of space, I expect an inverse cube relationship between distance from the emission source and level of exposure. In other words, if you double the distance, you reduce the exposure level to one eighth.

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