Particulate Matter (PM)
A mixture of solid particles and liquid droplets suspended in the air, ranging in size from visible dust to microscopic particles.
Detailed Explanation
Particulate matter is the general term for a mixture of solid particles and liquid droplets found in the air. PM varies widely in size, shape, and chemical composition. The EPA regulates two size categories: PM10 (inhalable particles with diameters of 10 micrometers or less) and PM2.5 (fine inhalable particles with diameters of 2.5 micrometers or less). The size distinction is critical because it determines how deeply particles penetrate the respiratory system. PM10 deposits in the nose, throat, and upper airways. PM2.5 penetrates into the deepest parts of the lungs (the alveoli) and can cross into the bloodstream, reaching the heart, brain, and other organs. The sources of particulate matter are diverse: combustion (vehicles, power plants, wildfires, cooking), mechanical processes (construction, mining, agriculture), natural sources (windblown dust, sea salt, volcanic emissions), and atmospheric reactions that create secondary particles from gaseous precursors. The health effects of particulate matter exposure are among the most thoroughly documented in environmental health science. Short-term exposure aggravates respiratory and cardiovascular disease. Long-term exposure is associated with reduced lung function, chronic bronchitis, lung cancer, and premature death. The WHO estimates that outdoor particulate matter pollution causes over 4 million premature deaths worldwide annually. AirHistory tracks both PM2.5 and PM10 as part of each city's pollutant breakdown data.
Related Terms
PM2.5 (Fine Particulate Matter)
Airborne particles smaller than 2.5 micrometers in diameter — small enough to penetrate deep into the lungs and enter the bloodstream.
PM10 (Coarse Particulate Matter)
Inhalable particles with diameters between 2.5 and 10 micrometers, typically from dust, construction, and agricultural activities.
Criteria Pollutants
The six common air pollutants regulated by the EPA under the Clean Air Act: ozone, PM2.5, PM10, CO, NO2, and SO2.
Wildfire Smoke
Airborne particulate matter and gases produced by wildfires, now the leading source of hazardous air quality events in the western United States.
Frequently Asked Questions
A mixture of solid particles and liquid droplets suspended in the air, ranging in size from visible dust to microscopic particles.
Particulate matter is the general term for a mixture of solid particles and liquid droplets found in the air. PM varies widely in size, shape, and chemical composition. The EPA regulates two size categories: PM10 (inhalable particles with diameters of 10 micrometers or less) and PM2.5 (fine inhalable particles with diameters of 2.5 micrometers or less).