The basic idea
AQI (Air Quality Index) compresses several pollutants — fine particles, ozone, nitrogen dioxide, and others — into a single number from 0 to 500+. The higher the number, the more polluted the air, and the greater the health risk from breathing it.
The scale, level by level
| AQI | Level | What it means |
|---|---|---|
| 0–50 | Good | Air quality is satisfactory, poses little or no risk. |
| 51–100 | Moderate | Acceptable, but unusually sensitive people may notice mild effects. |
| 101–150 | Sensitive groups | Sensitive groups (asthma, heart/lung conditions, children, elderly) may experience health effects. |
| 151–200 | Unhealthy | Everyone may begin to experience health effects; sensitive groups more seriously. |
| 201–300 | Very unhealthy | Health alert — everyone may experience more serious effects. |
| 301+ | Hazardous | Emergency conditions — the entire population is likely affected. |
What's actually being measured
PM2.5 & PM10
Fine particulate matter from smoke, vehicle exhaust, and dust — usually the main driver of the AQI reading.
Ozone (O₃)
Forms from sunlight reacting with pollutants — often highest on hot, sunny afternoons.
Nitrogen dioxide (NO₂)
Mostly from traffic and combustion — tends to peak during rush hour.
Sulfur dioxide & CO
Linked to industrial activity and vehicle emissions, tracked in most monitoring networks.
Where the data comes from
AirActionToday uses live readings from official monitoring stations aggregated by the World Air Quality Index project, which combines thousands of government and research stations worldwide.
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