PM2.5 (fine particulate matter)
Particulate matter 2.5 micrometers in diameter or smaller
PM2.5 is airborne particles 2.5 micrometers or smaller, the fine soot and smoke small enough to reach the deep lung and, at the smallest sizes, pass into the bloodstream.
What does PM2.5 (fine particulate matter) mean?
PM2.5 is the mass of suspended particles 2.5 micrometers across or smaller, reported in micrograms per cubic meter. The 2.5 micrometer line is where biology changes. Larger particles get caught by the nose and the airway's mucus-and-cilia defenses, but particles below this size slip past them, settle in the alveoli where the lung trades oxygen for carbon dioxide, and the smallest fraction crosses into the bloodstream to reach organs including the heart and brain. The mix varies, from combustion soot to road dust, cooking aerosol, and sulfates and nitrates formed in the air. Two homes at the same reading can carry different risk. Outdoor air across most of the country sits above the WHO health-based guideline of 5 micrograms per cubic meter, the federal limit is nearly twice that, and the research shows no level low enough to take the risk to zero. Your home is the part of that exposure you can act on.
Sources cited for PM2.5 (fine particulate matter)
Every number on this PM2.5 (fine particulate matter) page traces to one of the 3 sources below: 1 government or regulatory.
Government & regulatory
Institutional & standards
- WHO 2021 Global Air Quality Guidelines, annual PM2.5 5 µg/m³ (verified via Huang et al., Nature Aging 2025)
- IARC Monograph 109, Outdoor Air Pollution, 2013 (particulate matter classified Group 1)
Related air science
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