Airborne pollutants can be classified into three major categories.

1. Particulate Matter

• Suspended particles with a diameter < 100 micrometers
  • Geological dust, emission dust (smog), water-soluble salts (photochemical reaction products, PM2.5)

2. Gaseous Pollutants

• VOCs: Volatile organic compounds, solvents, formaldehyde
• Irritant gases: Ozone, nitrogen oxides

3. Microorganisms

• Microbial aerosol carriers: Sneezing droplets
• Reproductive bodies: Spores