The sources for this information vary in credibility, but most of it comes
directly from published studies or other reputable sources like the
Berkeley Wellness letter.

1.  Exercise will extend your life by about the amount of time you spend
doing it.  So if you spend an hour on your bike, you've added an hour to
your life.  

2.  Drivers of cars are exposed to up to eighteen times more pollution
than "ambient air", approximately 300 feet from the road.  Cyclists share
the road with cars, but they do not trap pollutants, and they take air in
at a much higher position than cars (assuming a diamond frame) so...

3.  Cyclists breathe approximately 1/2 as much pollution than cars (this
appears to be _per breath_).

4.  Over _time_, a cyclist will breathe much more than a sedentary driver,
since the cyclist is using more than twice as much air.  Athletes appear
to be very sensitive to foul air.

5.  In general, cycling takes longer than driving, so the bike commuter
may be exposed to pollution for longer periods of time.

6. A UK study found that cyclists had 1/2 the blood level of CO that
drivers did after traveling along a ten mile stretch of congested road. 

7. CO blood levels may be less of a problem than inhaled particulates,
which are much harder to measure.  Masks make breathing difficult if they
are properly sealed, and are ineffective if they are not sealed.

As a result, the health advantages of commuting by bike depend on several
key factors:

1.  Would you exercise anyway? That is, would you drive to the gym and
ride a stationary bike in relatively clean air if you weren't commuting in
traffic.

2.  How hard do you ride? The harder you ride, the more air -- and
therefore pollution -- you take in.  But then the better the training
effect will be, so if you don't do any other exercise, this is a wash.

3.  How long is your drive compared to your ride? If it takes
significantly more time to ride, you may be exposed to more pollution.

4.  What kind of car? An open air Jeep would take in and trap less
pollution than a sedan.

The health effects of exercise far outweigh any additional health dangers
from pollution.  If you would exercise anyway, though, commuting may not
in your best interest.  If you commute on low volume side streets, or on
sidepaths, pollution might not get you, but other hazards might.

Here is a rationalization for those of us who want to believe that
cyclists get less pollution than motorists:

One thing I've noticed about my commute: when I drive, I am _always_
surrounded by traffic.  All us cars meet at the light and move from light
to light more or less together.  When I ride my bike, I meet cars at
lights, but I don't spend a lot of time around them when they're rushing
past me to get to the next light.  The vast majority of time is spent
between packs of cars, without much motor traffic.  Since I'm not around
cars very much I can believe:
- I am breathing more garbage than a motorist when I'm in traffic 
- I am breathing less garbage than a motorist when I'm not in traffic 
- I am not in traffic far more often 

Therefore: I am probably getting less pollution on the bike than in my car!

Contributors:
[I did not retain the mail address of contributors who posted to the group
without a sig; also, I may have missed some posts that weren't emailed to
me]

drobinso@mendel.une.edu.au
ilana@niwot.scd.ucar.EDU
burrows@bcu.ubc.ca 
jlbell@presto.eecs.umich.edu
jones@greg.cs.usu.edu
banders@netcom.com
curt@cynic.portal.ca
doosh@netcom.com
"td"
"Robert"
ae505@yfn.ysu.edu
cs4601ah@coral.cs.unm.edu
tbd@dfw.net
timlee@netcom.com

Some parts © 2009 Advameg, Inc.