In the developed world, the greatest source of particulate matter is from diesel engines.

CNN: Air pollution kills, but deaths can be prevented

The diesel engine exhaust doubled the background particulate matter levels found outdoors at its peak.

BBC: Image of a smoke filled room

This will allow truckers to reduce emissions significantly (diesel emissions include nitrous oxides, particulate matter, and other air toxics and they are a major source of pollution in some areas).

FORBES: Acceleration of the Natural Gas Highway

That dominance is threatened by tightening emissions regulations governing nitrogen oxides and particulate matter--emissions that plague diesel engines in particular.

FORBES: EPA, Inc.

Public hybrid buses have been tested in New York to some success, according to the EDTA, which says the buses emit 90 percent less particulate matter and 30 percent fewer greenhouse gases than regular diesel buses.

CNN: All About: hybrid transportation