PM2.5 (Fine particulate matter)
PM2.5 is particulate matter 2.5 micrometers or smaller in diameter. Small enough to penetrate deep into the lungs and cross into the bloodstream.
Reviewed by Hayden Williams. Last reviewed 2026-05-01. Unit: μg/m³.
PM2.5 is the most-studied air pollutant in human health research. A human hair is roughly 70 micrometers across, so PM2.5 is about 30 times thinner. Particles of this size bypass the nose and throat's natural filtering, reach the alveoli, and pass through the lung lining into the blood. Once in circulation, PM2.5 contributes to systemic inflammation that affects the cardiovascular system, brain, and other organs - not only the lungs.
Health effects
PM2.5 is classified by the WHO as a Group 1 carcinogen, the same category as tobacco smoke and asbestos. Long-term exposure shortens life expectancy and is linked to heart disease, stroke, lung cancer, asthma, COPD, premature birth, and cognitive decline.
Documented effects from peer-reviewed literature: 17-30% increased stroke risk in high-exposure areas, 9% increased lung cancer risk per 10 μg/m³ chronic exposure, doubled heart-attack risk during pollution episodes, measurable reductions in childhood IQ, and an estimated 4-7 million deaths globally per year. The 2013 IARC Monograph 109 classified outdoor air pollution and PM2.5 specifically as carcinogenic to humans.
Vulnerable groups
Children (developing lungs), elderly (reduced cardiopulmonary reserve), pregnant women (linked to low birth weight and preterm birth), people with asthma / COPD / heart disease, and outdoor workers face the highest documented risk. Children inhale roughly 2x more air per kg of body weight than adults, magnifying exposure.
Common sources
- Vehicle exhaust (especially diesel and old gasoline engines)
- Coal-fired power plants and industrial boilers
- Wood and biomass burning (residential heating, cooking, agricultural)
- Wildfires (rapidly increasing as climates warm)
- Construction and road dust
- Secondary PM2.5 formed in the atmosphere from SO2, NOx, and VOC precursors
- Indoor sources: gas stoves, candles, frying, vacuums without HEPA filtration
Regional context
PM2.5 burden is concentrated in South Asia (India, Pakistan, Bangladesh annual averages routinely 50-110 μg/m³), parts of East Asia (Northern China, Mongolia in winter), and Sub-Saharan Africa during dust and biomass-burning seasons. Most of Europe, Australia, and northern North America sit in the 5-15 μg/m³ range with episodic spikes from wildfires.
Regulatory thresholds
How PM2.5 is measured
Reference-grade ground sensors use beta-attenuation or tapered element oscillating microbalance (TEOM) instruments at regulatory monitoring stations. Low-cost optical sensors (Plantower, Sensirion) provide much denser coverage but require calibration. Satellite-derived PM2.5 (from MODIS, VIIRS, TROPOMI aerosol optical depth combined with chemical transport models) gives global coverage at coarser resolution.
How to reduce your exposure
- Skip outdoor exercise when AQI exceeds 100 (sensitive groups) or 150 (everyone)
- Wear a properly fitted N95 or KN95 outdoors during peaks - cloth and surgical masks do not filter PM2.5
- Run a True HEPA air purifier in bedrooms and main living areas (CADR matched to room size)
- Close windows during pollution episodes; use mechanical ventilation with HEPA filtration
- Switch to electric or induction cooking - gas stoves can spike indoor PM2.5 above 100 μg/m³
- Avoid wood-burning fireplaces - efficient PM2.5 emitters
Trends
China has reduced major-city PM2.5 by roughly 40% since 2013 through coal phase-outs, vehicle restrictions, and industrial relocation. The European Union and the United States have seen multi-decade declines under the Clean Air Act and Air Quality Directives. Wildfire-driven PM2.5 is rising in western North America, southern Europe, and Australia. South Asian PM2.5 has plateaued at extremely high levels.
Cities where PM2.5 matters most
Live readings for cities historically affected by elevated PM2.5:
Frequently asked
What is a safe level of PM2.5?
WHO 2021 recommends an annual average below 5 μg/m³ and a 24-hour average below 15 μg/m³. Levels above 35 μg/m³ are unhealthy for sensitive groups; above 55 μg/m³ everyone should reduce outdoor activity.
How does PM2.5 affect health?
PM2.5 penetrates deep into the lungs and crosses into the bloodstream. Long-term exposure raises risk of heart attack, stroke, lung cancer, COPD, premature birth, and cognitive decline. WHO classifies it as a Group 1 carcinogen.
Where does PM2.5 come from?
Combustion. Vehicle exhaust, coal/wood/biomass burning, industry, wildfires, and agricultural burning are top sources. Secondary PM2.5 also forms in the atmosphere from precursor gases like SO2 and NOx.
Is PM2.5 indoors different from outdoors?
Indoor PM2.5 is typically 50-70% of outdoor levels in unfiltered homes, but cooking, candles, and tobacco can spike indoor levels above outdoor ones. HEPA purifiers can cut indoor PM2.5 by 80%+.
How is PM2.5 measured?
Reference monitors use beta-attenuation or TEOM instruments. Low-cost optical sensors (Plantower, Sensirion) are widely deployed for community networks. Satellites (MODIS, VIIRS, TROPOMI) estimate it indirectly via aerosol optical depth.
Why is South Asia's PM2.5 so high?
Vehicle exhaust, coal and biomass burning, agricultural stubble burning in autumn, brick kilns, and Indo-Gangetic Plain geography that traps pollution under winter inversions. Annual averages in Delhi, Lahore, and Dhaka run 16-22x the WHO guideline.
Sources + further reading
- WHO 2021 Global Air Quality Guidelines
- US EPA Integrated Science Assessment for Particulate Matter (2019)
- IARC Monograph 109: Outdoor Air Pollution
- Health Effects Institute - State of Global Air
Track PM2.5 live across 217 cities on atmos.today. See methodology for how readings are sourced and calculated.