Key facts
- PM2.5 particles are 2.5 micrometres or smaller.
- PM2.5 can be emitted directly or form in the atmosphere from precursor gases.
- WHO’s 2021 guideline is 5 µg/m³ annual mean and 15 µg/m³ 24-hour mean.
- Risk depends on concentration, exposure duration, particle composition and personal susceptibility.
What is PM2.5?
PM2.5 is not a single chemical. It is a mixture of microscopic solid particles and liquid droplets small enough to remain suspended in air. The mixture can include soot, smoke, metals, organic compounds, sulfates, nitrates and other material.
A typical human hair is many times wider than a PM2.5 particle. Their small size allows fine particles to penetrate more deeply into the respiratory system than larger particles.
Common sources of PM2.5
- Vehicle exhaust and other combustion emissions
- Coal, diesel, biomass and waste burning
- Power plants and industrial processes
- Wildfire and agricultural smoke
- Atmospheric reactions involving sulfur dioxide, nitrogen oxides and other gases
Health effects
EPA and WHO describe particulate pollution as a cardiovascular and respiratory concern. Studies associate exposure with aggravated asthma, reduced lung function, respiratory symptoms, heart effects and premature death, particularly among susceptible populations.
Children, older adults and people with existing heart or lung disease can be more vulnerable. Exposure is not determined by the outdoor reading alone; time spent outdoors, activity level, indoor infiltration and protective measures also matter.
WHO PM2.5 guideline values
| Averaging period | WHO 2021 guideline |
|---|---|
| Annual mean | 5 µg/m³ |
| 24-hour mean | 15 µg/m³ |
Frequently asked questions
Is PM2.5 visible?
Individual PM2.5 particles are too small to see, although high concentrations can contribute to visible haze or smoke.
Is there a completely safe PM2.5 level?
WHO guidelines are health-based targets, but health evidence does not support treating pollution below one threshold as risk-free for every person.
Does an N95 respirator reduce PM2.5 exposure?
A genuine, well-fitted particulate respirator can reduce inhalation of airborne particles, but effectiveness depends strongly on fit, correct use and condition.
Sources
This reference was prepared from the following primary public-health and regulatory sources:
- WHO Global Air Quality GuidelinesWorld Health Organization
- Particulate Matter (PM) BasicsUnited States Environmental Protection Agency
- Health and Environmental Effects of Particulate MatterUnited States Environmental Protection Agency