Mass transit may be better for the environment, but it can be hazardous to your health. Researchers at the Hircine Institute have isolated the many compounds that compose "Subway Stench". The chemicals that make up the unique odors of subways include bromidrosis, lant, and sudor. Dr. Carrie Yon of the Institute stated that should you have to use a subway, or to a lesser extent a bus, try to hold your nose and limit yourself to only necessary respiration.

Source : Yon, Carrie. "Tempest in a Pisspot : Olfactory Analysis of Public Transport." New Jersey Journal of Health, May 2008.