PM2.5 — Fine Particulate Matter
PM2.5 refers to atmospheric particulate matter with a diameter of less than 2.5 micrometers. These tiny particles are among the most harmful air pollutants because they can penetrate deep into the lungs and enter the bloodstream.
What is PM2.5?
PM2.5 particles are about 30 times smaller than a human hair. This extremely small size allows them to bypass the nose and throat's natural filtering mechanisms and travel deep into the respiratory tract — reaching the alveoli in the lungs and, in some cases, passing directly into the bloodstream.
PM2.5 is measured in micrograms per cubic meter (µg/m³). A reading of 0–12 µg/m³ is considered good by US EPA standards; above 35.4 µg/m³ (24-hour average) is classified as unhealthy for sensitive groups.
Sources of PM2.5
- Traffic — Vehicle exhaust and brake/tire wear are major urban sources
- Industry — Factories, power plants, and smelters emit significant PM2.5
- Wildfires — Smoke from wildfires can travel thousands of miles
- Agriculture — Crop burning and livestock operations
- Residential burning — Wood stoves, fireplaces, coal heating
- Secondary formation — Chemical reactions in the atmosphere between gas-phase pollutants (SO₂, NOx, ammonia) form new particles
PM2.5 Health Effects
Short-term exposure
- • Coughing and wheezing
- • Shortness of breath
- • Irritation of eyes, nose, throat
- • Aggravated asthma attacks
- • Irregular heartbeat
Long-term exposure
- • Reduced lung function
- • Heart disease
- • Lung cancer
- • Premature death
- • Impaired cognitive development in children
PM2.5 Safe Levels
| Standard | Annual Mean | 24-hour Mean |
|---|---|---|
| WHO 2021 Guidelines | 5 µg/m³ | 15 µg/m³ |
| US EPA Standard | 9 µg/m³ | 35 µg/m³ |
| EU Standard | 10 µg/m³ | 25 µg/m³ |
Many cities in South Asia and China regularly exceed WHO guidelines by 5–20x.
PM2.5 to AQI Conversion
| PM2.5 (µg/m³, 24h) | AQI Range | Category |
|---|---|---|
| 0.0–9.0 | 0–50 | Good |
| 9.1–35.4 | 51–100 | Moderate |
| 35.5–55.4 | 101–150 | Unhealthy for Sensitive Groups |
| 55.5–125.4 | 151–200 | Unhealthy |
| 125.5–225.4 | 201–300 | Very Unhealthy |
| 225.5+ | 301–500 | Hazardous |
How to Protect Yourself
- Check AQI before outdoor activities — aim for below 100
- Wear N95/KN95 masks when PM2.5 AQI is above 150
- Keep windows closed on high-pollution days; use air conditioning
- Use HEPA air purifiers indoors — they remove 99.97% of particles
- Avoid exercising near busy roads, especially during rush hour
- Exercise indoors when outdoor AQI is above 100