Particulate matter (PM) is a mixture of solid particles and liquid droplets in the air. The size of PM is directly linked to potential health problems. EPA has set federal standards for PM10 (PM that is 10 microns or less in diameter) and PM2.5 (PM that 2.5 microns or less in diameter. Health research has also documented health effects of ultrafine particles, or particles that are 0.1 microns or smaller.
The chemical composition of PM is also a factor in the type and severity of health impacts. PM can be emitted directly into the atmosphere, or it can form in the atmosphere through the photochemical reactions of precursors. There are several PM species, or chemical compounds, including organic carbon, elemental carbon, geologic material, trace metals, secondary organic aerosols, ammonium nitrate, and ammonium sulfate. More information is available on EPA’s website.