The Automobile Pollutant Exposure Theory of Bicycling Avoidance

Published 2008-04-19

I have a few pet issues. Some people would call these pet peeves, but pretty much the only pet peeve I have is “beliefs that have not been clearly thought through.” One of my pet issues is an excuse for not bicycling I used to hear quite a bit. (For some reason people offer this less frequently — to me, anyway — as an excuse.) The excuse is something along the lines of “I’d ride my bike more often but I don’t want to expose myself to all those automobile pollutants.”

Which is kind of silly for two reasons:

  1. The research doesn’t support it
  2. It doesn’t make any damn sense

First, the facts. Most people have a skewed sense of both risk and exposure re: automobile pollutants. My reading (some references below) indicates that:

  • "Gas-phase" (i.e. non particulate) pollutants are at least as high inside a car as outside.
  • Fresh air controls or air conditioning have no effect on pollutant levels inside a car.
  • Levels of particulate pollutants are higher outside cars, but the effect drops dramatically away from a high-traffic road.
  • Cars produce the most pollutants when they’re idling or operating under light load — in other words, at rush hour, or in a parking lot.
  • The more time you spend in and around cars, especially at rush hour, the greater your exposure.
  • Children living within a third of a mile from a major freeway are more likely to develop asthma and other diseases, and have less-developed lungs.
  • Airborne pollutants are lower in neighborhoods with higher residential density and mixed land use (exactly the kind encouraged by a city with lots of cyclists.)

Second, let’s think this through. Consider some other source of air pollution. A steel mill, for example. Where would you find more airborne pollutants emitted by a steel mill: near the steel mill, or inside the steel mill? Generally speaking, how would steel mill pollutants vary as a function of distance from the source? As a function of the duration of exposure to the source?

The explicit premise of the automobile pollutant exposure theory of bicycling avoidance is “automobile pollutants are bad.” The implicit premise is “being near running automobiles is bad.” From these premises we can reason that:

  • The greater the distance from running automobiles, the better
  • The fewer running automobiles you’re near at once, the better
  • The less time, in total, that you’re exposed to running automobilies, the better

Given the premises of the automobile pollutant exposure theory of bicycling avoidance, if your lifestyle necessitates spending lots of time in or near cars (that is, driving everywhere), then you're actually maximizing your exposure to automotive pollutants. Moreover, communities that encourage frequent motoring hit you twice: first, because you spend all your time inside a car; and second, because the town has more freeways, more heavy traffic, more large intersections, more parking lots — in other words, more (and more frequent) exposure to automobiles in situations where they produce the most pollutants.

Finally, and this is the emotional appeal, imagine a scene. Imagine riding your bike down a quiet street, or a bike path, a good distance away from a major highway. Imagine taking a deep breath. What does the air smell like? Does it smell like fresh-cut lawns? Pine trees? Dead leaves? Wet pavement? Bacon and eggs from a nearby kitchen? Coffee roasting in the coffee shop? (That’s what my commute smells like.)


(Prompted by a post at the Bicycle Transportation Alliance Blog.)