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April 4, 2026

What Is PM2.5? Health Effects, Sources, and How to Reduce Exposure

PM2.5 is the most studied air pollutant in human health research. Long-term exposure shortens life expectancy more than HIV, malaria, and tuberculosis combined.

What PM2.5 actually is

PM2.5 stands for particulate matter 2.5 micrometers or smaller. For scale: a human hair is 70 micrometers across. PM2.5 is 30x thinner. These particles are small enough to:

  1. Bypass the nose and throat's filtering
  2. Reach deep into the lungs (alveoli)
  3. Cross into the bloodstream

That last part is why PM2.5 is so damaging - it's not a lung problem, it's a whole-body problem.

Where PM2.5 comes from

Globally, the biggest sources:

  • Vehicle exhaust (especially diesel)
  • Coal and wood burning for power and heat
  • Industrial emissions - factories, refineries, cement plants
  • Wildfires (rising fast as climates warm)
  • Agricultural burning of crop stubble (huge in South Asia, Southeast Asia)
  • Construction and road dust

Indoor sources matter too: gas stoves, candles, frying food, vacuum cleaners without HEPA filters.

Health effects (what the science says)

WHO classifies outdoor PM2.5 as a Group 1 carcinogen - same category as tobacco smoke and asbestos. Documented effects:

  • Heart disease: PM2.5 doubles risk of heart attack
  • Stroke: 17-30% higher risk in high-exposure areas
  • Lung cancer: 9% increased risk per 10 μg/m³ chronic exposure
  • Asthma: triggers attacks; new-onset asthma in kids
  • Premature death: estimated 4-7 million deaths/year globally
  • Cognitive decline: emerging links to dementia and reduced IQ in children
  • Pregnancy outcomes: low birth weight, preterm birth

Safe levels

  • WHO 2021 guideline (annual mean): 5 μg/m³
  • WHO 2021 guideline (24-hour): 15 μg/m³
  • US EPA standard (annual): 9 μg/m³
  • EU standard (annual): 25 μg/m³ (under revision)

For reference, Delhi's annual average typically runs 80-110 μg/m³ - 16-22x the WHO guideline. London sits around 11 μg/m³.

7 ways to reduce your exposure today

  1. Check the AQI before going outside for exercise. Skip outdoor cardio when AQI > 150. Live AQI for your city.
  2. Wear an N95 or KN95 mask when AQI exceeds 100. Cloth masks don't filter PM2.5.
  3. Run an air purifier with a true HEPA filter in your bedroom. PM2.5 indoors typically tracks 50-70% of outdoor levels in unfiltered homes.
  4. Close windows during pollution peaks. Use mechanical ventilation with HEPA filtration if possible.
  5. Switch to electric or induction cooking. Gas stoves can spike indoor PM2.5 above 100 μg/m³ during cooking.
  6. Avoid wood-burning fireplaces - they're efficient PM2.5 emitters.
  7. Get email alerts when your city's AQI crosses your threshold: free alerts here.

Bottom line

PM2.5 is the single most impactful air pollutant on human health. You can measurably reduce your exposure with cheap, simple actions. Track your city's PM2.5 live on atmos.today.

Sources: WHO Air Quality Guidelines 2021, US EPA Integrated Science Assessment, IARC Monograph 109. Reviewed: 2026-05-02.