PM2.5 (fine particulate matter)

PM2.5 is airborne particles 2.5 micrometers across or smaller. They come mostly from combustion: vehicle exhaust, power plants, wood burning, and industry, plus secondary formation from gases like sulfur dioxide and nitrogen oxides. Because they are so small, they reach deep into your lungs (the alveoli) and can cross into your bloodstream. There they trigger oxidative stress, blood-vessel dysfunction, and inflammatory signals. People carrying the APOE ε4 allele may face extra brain risk, as the particles cross the blood-brain barrier. Dose-response work by Pope, Burnett, and colleagues in large US cohorts shows a continuous link between long-term PM2.5 and death from all causes, heart disease, and lung cancer, with no apparent safe threshold. The WHO's 2021 Air Quality Guidelines tightened the recommended annual limit to 5 µg/m³. Most European and North American cities still exceed it.

Last reviewed: June 22, 2026

This definition is educational and is not medical advice, a diagnosis, or treatment. Talk to a doctor about any health decisions. Read our full medical disclaimer