Air Quality Index
The Air Quality Index (AQI) is a standardized index of the air quality in a given location, given in parts per billion. It measures mainly ground-level ozone and particulates (except the pollen count), but may also include sulphur dioxide, nitrogen dioxide.
The U.S. Environmental Protection Agency and the Meteorological Service of Canada differ on what AQI categories are used.
For the EPA, up to 50 on the AQI, the air is considered good (green), and up to 100 is considered moderate (yellow). Above that to 150 is unhealthy for some (orange), and to 200 is unhealthy for all (red). Severe smog or other air pollution can go to 300 (purple) or even 500 (maroon), though these are only under rare conditions.
For the MSC, good is only to 25 and moderate (fair) to 50. Above that to 100 is poor and grater than that is very poor.
The AQI can worsen (go up) due to stagnant air, often caused by an anticyclone or temperature inversion, or other lack of winds. On these days, the news media may ask the public to carpool or use public transport, or take other measures such as teleworking.