What Is PM2.5?
PM stands for particulate matter. The number refers to the size: PM2.5 particles are 2.5 micrometers or smaller in diameter — about 30 times smaller than a human hair. Because they're so fine, they penetrate deep into the lungs and can enter the bloodstream.
PM10 (particles up to 10 micrometers) is also measured, but PM2.5 is the more health-relevant metric. Your nose and throat filter out most PM10; PM2.5 bypasses those defenses.
PM2.5 Concentration: What the Numbers Mean
Concentrations are measured in micrograms per cubic meter (μg/m³). The US EPA AQI categories:
| PM2.5 (μg/m³) | AQI Range | Category | Action |
|---|---|---|---|
| 0 – 12 | 0 – 50 | Good | No restrictions |
| 12.1 – 35.4 | 51 – 100 | Moderate | Sensitive groups should limit prolonged outdoor exertion |
| 35.5 – 55.4 | 101 – 150 | Unhealthy for Sensitive Groups | Sensitive groups avoid prolonged outdoor activity |
| 55.5 – 150.4 | 151 – 200 | Unhealthy | Everyone reduce outdoor exertion; sensitive groups stay indoors |
| 150.5 – 250.4 | 201 – 300 | Very Unhealthy | Avoid outdoor activity; wear N95 if going out |
| 250.5+ | 301 – 500 | Hazardous | Stay indoors, seal windows; N95 if outdoor unavoidable |
PM2.5 vs. PM10: Which Matters More?
| PM2.5 | PM10 | |
|---|---|---|
| Particle size | ≤ 2.5 μm | ≤ 10 μm |
| Primary sources | Combustion (vehicles, fires, industry) | Dust, pollen, construction |
| Lung penetration | Deep — alveoli and bloodstream | Moderate — upper airways |
| Health impact | Higher for cardiovascular/respiratory disease | Moderate — mainly respiratory irritation |
| Filtered by N95 mask | Yes (≥95% efficiency) | Yes |
Which Mask Actually Helps?
N95 (US Standard)
Filters ≥95% of airborne particles ≥0.3 μm when fitted correctly. This includes PM2.5. The key word is "fitted" — a loose N95 provides far less protection. Perform a seal check by cupping your hands over the mask and exhaling; air should not escape around the edges.
KN95 / KF94
KN95 is the Chinese standard (≥95% filtration, equivalent in spec to N95). KF94 is the Korean standard (≥94% filtration, boat-shaped design that creates an air gap from the face). Both provide adequate PM2.5 protection. Quality varies by manufacturer — buy from reputable sources.
Surgical masks / cloth masks
These are not effective against PM2.5. Surgical masks filter larger particles and droplets but have poor edge sealing. Cloth masks offer minimal particulate protection. For PM2.5, only respirators with a proper seal help.
Who Is Most at Risk?
- Children and infants: Lungs still developing; higher breathing rates per body weight mean greater exposure
- Elderly adults: Reduced lung function and more pre-existing cardiovascular conditions
- People with asthma, COPD, or heart disease: PM2.5 triggers flare-ups and cardiac events at lower concentrations
- Pregnant women: Associated with low birth weight and preterm birth at elevated PM2.5 levels
- Outdoor workers: Extended exposure time multiplies cumulative dose
Indoor Air Quality During High PM2.5 Days
Indoor PM2.5 concentrations are typically 50–70% of outdoor levels with windows closed. On hazardous days:
- Keep windows closed and use air conditioning with a clean filter
- Use a HEPA air purifier in the main living area; HEPA filters capture PM2.5 effectively
- Avoid activities that generate indoor particles (smoking, burning candles, cooking without ventilation)
- Check your HVAC filter — change if overdue, as a dirty filter can recirculate particles