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.

How Big is Particle Pollution?
Drawing illustrates the relative size of particle pollution.  Five, 10 micrometer particles fit across one human hair.  Four, 2.5 micrometer particles fit across one 10 micrometer particle.
           
Enlarge or print this figure  

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.