What is PM?
Particle pollution is a mixture of microscopic solids
and liquid droplets suspended in air. This pollution, also known as particulate
matter, is made up of a number of components, including acids (such as
nitrates and sulfates), organic chemicals, metals, soil or dust particles,
and allergens (such as fragments of pollen or mold spores). Unlike summertime
ozone, particle pollution can occur year-round, and is linked to serious
health problems, even at concentrations found in many cities.
Particles come in a wide range of sizes. Fine particles are less
than 2.5 micrometers in diameter. How small is that? About 1/30th the diameter
of the average human hair – so small, you’d need an electron
microscope to see them.
Some fine particles can be emitted directly (think of smoke from a woodstove).
But most are formed secondarily from complex atmospheric reactions of gases
such as NOx and sulfur dioxide (SO2), that are emitted from power plants,
industries, cars, buses and trucks. Fine particles are also found in smoke. More about how smoke from fires can affect your health.
Inhalable coarse particles are larger than 2.5 and up to 10 micrometers
in diameter. Sources of coarse particles include crushing or grinding operations
and dust stirred up by vehicles traveling on roads.
See the EPA:
Particulate Matter Web site for more information.