What is "particulate matter" (PM)?

What is "particulate matter" (PM)?

Terry Tamminen (Author & Environmental Policy Advisor) gives expert video advice on: Is there a correlation between smog and human disease? and more...

Particulate matter, or PM as it's called, is fine soot. I'm sure everyone has driven, at some point, behind a big old yellow school bus, and as the school bus goes up the hill, we see this big cloud of black soot coming out of the diesel engine tailpipe. Well, that's particulate matter, the fine particles, which actually is diesel fuel that hasn't combusted in the engine and is being spewed out of the engine in the form of this fine smoke or soot. Particulate matter is measured in terms of its size; as PM 2.5, meaning 2.5 microns or larger, and then PM 10, which is 10 microns or larger. Just to give you a little background on that, a human hair is about 35 microns in diameter.