Air Quality Index
The Air Quality Index (AQI) is a standardized indicator of the air quality in a given location.  It measures mainly ground-level ozone and particulates (except the pollen count), but may also include sulfur dioxide, and nitrogen dioxide. Various agencies around the world measure such indices, though definitions may change between places.
The United States Environmental Protection Agency (EPA) and the Meteorological Service of Canada (MSC) differ on what AQI structure and health classification is used:
Health classifications used by the EPA:
- 0-50  Good is usually green
- 51-100 Moderate is usually yellow
- 101-150 Unhealthy for sensitive groups is usually orange
- 151-200 Unhealthy is usually red
- 201-300 Very unhealthy is usually purple
- 301-500 Hazardous is usually maroon
The EPA's AQI 100 corresponds to 0.08 ppm ozone, and to other levels for other pollutants. Source: EPA
The AQI standards in Canada are relatively more stringent. The current health classifications used by the Meteorological Service of Canada (MSC) are as follows:
- 0-25*: Good (green)
- 2650: Fair (yellow)
- 51-100: Poor (orange)
- 101+: Very poor (red)
In Ontario, 31 is the upper limit for good and 32 the lower limit for moderate. Zero to 15 is classified as very good, and is given the color blue.
In June 2007, the EPA proposed a slight possible tightening of the pollution standards associated with smog after an independent EPA scientific board said that the standard needs to be substantially reduced and that there is no scientific justification for retaining the current, weaker standard. 
In light of the new scientific findings, one should expect adjustments in the AQI such that pollution currently denoted as "moderate" will in the future be recognized as "unhealthy."
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.

# Other Indices

## Hong Kong
The Air Pollution Index (API) levels for Hong Kong are related to the measured concentrations of ambient respirable suspended particulate (RSP), sulfur dioxide (SO2), carbon monoxide (CO), ozone (O3) and nitrogen dioxide (NO2) over a 24-hour period based on the potential health effects of air pollutants.
An API level at or below 100 means that the pollutant levels are in the satisfactory range over 24 hour period and pose no acute or immediate health effects. However, air pollution consistently at "High" levels (API of 51 to 100) in a year may mean that the annual Hong Kong "Air Quality Objectives" for protecting long-term health effects could be violated. Therefore, chronic health effects may be observed if one is persistently exposed to an API of 51 to 100 for a long time.
"Very High" levels (API in excess of 100) means that levels of one or more pollutant(s) is/are in the unhealthy range. The Hong Kong Environmental Protection Department provides advice to the public regarding precautionary actions to take for such levels.

## Malaysia
The air quality in Malaysia is reported as the API or Air Pollution Index. Four of the index's pollutant components (i.e., carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide) are reported in ppmv  but PM10 particulate matter is reported in g/m3.
Unlike the American AQI, the index number can exceed 500. Above 500, a state of emergency is declared in the reporting area. Usually, this means that non-essential government services are suspended, and all ports in the affected area closed. There may also be a prohibition on private sector commercial and industrial activities in the reporting area excluding the food sector.
For more information on the API reading please go here 

## Singapore
Singapore uses the Pollutant Standards Index to report on its air quality. 

## United Kingdom
The Met Office of the United Kingdom (UK) issues air quality forecasts wherein the level of pollution is described either as an index (ranging from 1 to 10) or as a banding (low, moderate, high or very high). These levels are based on the health effects of each pollutant.
The forecast is produced for a number of different pollutants and their typical health effects are shown in the following table.

## United States
The United States Environment Protection Agency (USEPA) developed the Pollutant Standards Index (PSI) to provide accurate, timely and easily understandable information about daily levels of air pollution.
It is no longer in use, having been replaced by the AQI, which is more sensitive. For example, particulate matter with an aerodynamic diameter less than 2.5 micrometres (PM2.5) is a sub index, replacing the less sensitive PM10 component of the PSI.

# Notes
In the context of this article about air quality:
- ppmv = parts per million by volume = volume of pollutant gas per million volumes of ambient air
- PM10 = particulate matter smaller than 10 m in diameter
- g/m3 = micrograms per cubic metre of ambient air
- m = micrometre
The air quality in the United States has improved dramatically over 23 years.

# Air quality by country or region
- British Columbia
- Hong Kong