Yet another reason to commute by bike!
http://www.livescience.com/environme...pollution.html
On average, taxi passengers were exposed to more than 100,000 ultrafine particle counts per cubic centimeter. Bus travelers were exposed to just under 100,000 and people in cars about 40,000.
Pedestrians and bicyclists, meanwhile, were exposed to counts of just 5,000 and 8,000, respectively.
Ultrafine particles are so small that large amounts can be inhaled in a single breath, after which they can settle in the lungs and damage cells.