Air Quality (AQI): How It Works
The Air Quality Index converts several pollutant measurements into one number on a coloured scale. It is genuinely useful for daily decisions — and it hides more than it shows, because the index reports only the single worst pollutant at that moment.
The scale
| AQI | Category | What it means in practice |
|---|---|---|
| 0–50 | Good | No precautions needed |
| 51–100 | Moderate | Unusually sensitive people may notice effects |
| 101–150 | Unhealthy for sensitive groups | Children, older adults and people with heart or lung conditions should limit prolonged exertion outdoors |
| 151–200 | Unhealthy | Everyone may be affected; reduce prolonged outdoor exertion |
| 201–300 | Very unhealthy | Avoid outdoor exertion; keep windows closed |
| 301+ | Hazardous | Stay indoors with filtration where possible |
The pollutants
| Pollutant | Main sources |
|---|---|
| PM2.5 — fine particles | Combustion, vehicles, wildfires, cooking |
| PM10 — coarse particles | Dust, construction, pollen |
| Ozone (O₃) | Formed in sunlight from vehicle and industrial emissions |
| Nitrogen dioxide (NO₂) | Traffic, particularly diesel |
| Sulphur dioxide (SO₂) | Coal and heavy industry |
| Carbon monoxide (CO) | Incomplete combustion |
PM2.5 is the one that matters most for health. At under 2.5 micrometres these particles pass deep into the lungs and enter the bloodstream, and they are consistently associated with cardiovascular and respiratory harm.
Why the headline number hides things
The reported AQI is the highest sub-index across all measured pollutants. An AQI of 80 could mean moderate particulates and clean air otherwise, or one pollutant near a threshold with the rest excellent. It tells you the worst thing, not the overall picture.
The scale is also non-linear by design: an AQI of 150 is not twice as bad as 75, and each range corresponds to different concentration widths for different pollutants. It is a communication device, not a measurement.
National scales differ
The same air produces different numbers in different countries, because breakpoints are set nationally. Indian, Chinese, European and US indices all use different thresholds — so an AQI of 100 in one country does not describe the same air as 100 in another. Comparing numbers across borders requires knowing which scale is being used.
The World Health Organization's guideline values are considerably stricter than most national standards, which is why air rated 'moderate' locally can still exceed WHO guidance substantially.
What to do at each level
- Above 100 — sensitive groups should shorten or move outdoor exercise indoors.
- Above 150 — everyone should reduce prolonged outdoor exertion. Close windows.
- Above 200 — stay indoors; run an air purifier with a HEPA filter if available.
- Masks — only a well-fitted N95 or FFP2 filters fine particles. Cloth and surgical masks do not.
Timing and location
Ozone peaks in the afternoon, when sunlight drives its formation, so exercising early is better on high-ozone days. Particulates often peak in the morning and evening with traffic and, in some regions, with heating or crop burning. Readings also vary sharply over short distances — a monitor several kilometres away, or beside a major road, may not describe your street at all.
General information, not medical advice. If you have asthma, COPD or a heart condition, follow the plan your doctor has given you for poor air quality days.