The Air Quality Index (AQI)
is a
standardized
index of the air
quality in a given location. It measures mainly
ground-level ozone and
particulates (except the
pollen count), but may also include
sulphur dioxide, and
nitrogen dioxide. Various agencies around the world measure such
indices, though definitions may change between places.
The U.S. Environmental Protection Agency and the Meteorological Service of Canada differ on what AQI structure and health classification is used:
Health classifications used by the EPA:
- 0-50: Good (green)
- 51-100: Moderate (yellow)
- 101-150: Unhealthy for sensitive groups (orange)
- 151-200: Unhealthy (red)
- 201-300: Very unhealthy (purple)
- 301-500: Hazardous (maroon)
Health classifications used by the MSC:
- 0-25*: Good (green)
- 26*-50: Moderate (yellow)
- 51-100: Poor (orange/red)
- 101+: Very poor (purple)
In Ontario, 31 is the upper limit for good and 32 the lower limit for moderate.
The AQI can worsen (go up) due to lack of dilution with fresh air. Stagnant air, often caused by an anticyclone or temperature inversion, or other lack of winds lets air pollution remain in a local area. On these days, the news media may ask the public to carpool or use public transport, or take other air pollution prevention measures such as teleworking.