PM2.5 (fine particulate matter)

Particulate matter 2.5 micrometers in diameter or smaller

PM2.5 is airborne particles 2.5 micrometers or smaller, fine enough to reach the deep lung and, at the smallest sizes, pass into the bloodstream.

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 is heterogeneous: combustion soot, road dust, cooking aerosol, secondary sulfates and nitrates, so 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.