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.
Detailed Explanation
PM2.5 refers to fine inhalable particles with diameters that are generally 2.5 micrometers and smaller — about 30 times smaller than the width of a human hair. These particles are so small that they bypass the body's natural defenses, penetrating deep into the lungs and even crossing into the bloodstream. PM2.5 is widely considered the most dangerous common air pollutant due to its strong association with cardiovascular disease, respiratory illness, lung cancer, and premature death. Major sources include vehicle exhaust, power plant emissions, industrial processes, wildfires, and residential wood burning. In the western United States, wildfire smoke has become the dominant source of PM2.5, with smoke events pushing AQI readings well into the hazardous range. The EPA's annual standard for PM2.5 was tightened in 2024 from 12 micrograms per cubic meter to 9 micrograms per cubic meter, reflecting growing evidence of health effects at lower concentrations. AirHistory data shows PM2.5 is the dominant pollutant in roughly 40% of monitored US cities, particularly in areas affected by wildfire smoke or heavy traffic.
Related Terms
Particulate Matter (PM)
A mixture of solid particles and liquid droplets suspended in the air, ranging in size from visible dust to microscopic particles.
Wildfire Smoke
Airborne particulate matter and gases produced by wildfires, now the leading source of hazardous air quality events in the western United States.
Criteria Pollutants
The six common air pollutants regulated by the EPA under the Clean Air Act: ozone, PM2.5, PM10, CO, NO2, and SO2.
Air Quality Index (AQI)
A standardized EPA scale from 0 to 500 that communicates daily air quality and associated health risks.
Frequently Asked Questions
Airborne particles smaller than 2.5 micrometers in diameter — small enough to penetrate deep into the lungs and enter the bloodstream.
PM2.5 refers to fine inhalable particles with diameters that are generally 2.5 micrometers and smaller — about 30 times smaller than the width of a human hair. These particles are so small that they bypass the body's natural defenses, penetrating deep into the lungs and even crossing into the bloodstream. PM2.5 is widely considered the most dangerous common air pollutant due to its strong association with cardiovascular disease, respiratory illness, lung cancer, and premature death.