What is the Air Quality Index (AQI)?
Quick Answer: The Air Quality Index (AQI) is a 0 to 500 scale that reports how clean or polluted the air is, with higher numbers meaning greater pollution and health risk.
The AQI is calculated from several common pollutants: fine particles (PM2.5), coarse particles (PM10), ground-level ozone, nitrogen dioxide (NO2), sulphur dioxide (SO2), and carbon monoxide (CO). Each pollutant is measured and converted to an index value, and the highest single value among them sets the overall AQI that is reported, so the worst pollutant defines the number you see.
Results map to colour-coded categories ranging from Good (green) through Moderate, Unhealthy, and Very Unhealthy up to Hazardous (maroon). Checking the AQI before outdoor activity is especially helpful for sensitive groups, including children, older adults, and people with asthma or heart conditions, who can limit exposure when levels rise.