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The Air Quality Index (AQI) (also known as the Air Pollution Index (API) or Pollutant Standard Index (PSI)) is a number used by government agencies to characterize the quality of the air at a given location. As the AQI increases, an increasingly large percentage of the population is likely to experience increasingly severe adverse health effects.

To compute the AQI requires an air pollutant concentration from a monitor or model. The function used to convert from air pollutant concentration to AQI varies by pollutant, and is different in different countries.

In many countries, air quality index values are divided into ranges, and each range is assiged a descriptor and a color code. Standardized public health advisories are associated with each AQI range. An agency might also encourage members of the public to take public transportation or work from home when AQI levels are high.

Many countries monitor ground-level ozone, particulate matter (PM10), sulfur dioxide (S02), carbon monoxide (CO) and nitrogen dioxide (NO2) and calculate air quality indices for these pollutants. Most other air contaminants do not have an associated AQI.

Canada
Environment Canada, the national environmental protection agency of Canada, uses Air Quality Health Index (AQHI) categories ranging from 1 to 10+ and each category has an assigned color code (see adjacent table) that enables members of the general public to easily identify their health risks as indicated in published air quality forecasts.

As shown in the adjacent table:


 * The three AQHI levels of 1, 2 and 3 are all in the low risk category.
 * The three AQHI levels of 4, 5 and 6 are all in the moderate risk category.
 * The four AQHI levels of 7, 8, 9 and 10 are all in the high risk category.
 * The AQHI level of 10+ is the very high risk category.

As of 2009, many of the Canadian provinces, if not all, have adopted the AQHI categories implemented by Environment Canada.

Hong Kong
The Hong Kong Environmental Protection Department (Hong Kong EPD) has developed an Air Pollution Index (API) based upon the measured concentrations of ambient particulate matter (PM10), sulfur dioxide, carbon monoxide, ozone and nitrogen dioxide over a 24-hour period.

Hong Kong's Air Pollution Index (AQHI) scale ranges from 0 to 500 corresponding to adverse health effects that range from low to severe as shown in the adjacent chart:


 * An API 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.


 * Persistent high API values (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.


 * API values in excess of 100 (very high) mean that levels of one or more pollutant(s) is/are in the unhealthy range. The Hong Kong EPD provides advice to the public regarding precautionary actions to take for such levels.

China
China's Ministry of Environmental Protection (SEPA) is responsible for measuring the level of air pollution in China. As of August 2008, SEPA monitors daily pollution level in its major cities. The Air Pollution Index (API) level is based on the concentration sulfur dioxide, nitrogen dioxide, particulate matter (PM10), carbon monoxide, and ozone measured at monitoring stations in each city.

The adjacent table presents China's national API scale, which is not color-coded.

API Mechanics
An individual score is assigned to the level of each pollutant and the final API is the highest of those 5 scores. The final API value is calculated each day.

The scale for each pollutant is non-linear, as is the final API score. Thus, an API of 100 does not mean twice the pollution of API at 50, nor does it mean twice as harmful.

Beijing's API
China's capitol city, Beijing, has its own API scale, which was developed by the Beijing Municipal Environmental Protection Bureau and differs somewhat from the national API scale as shown in the adjacent table.

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 μg/m³.

Unlike the U.S. EPA's 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 http://www.doe.gov.my/index.php?option=com_content&task=view&id=188&Itemid=370&lang=en

Mexico
The air quality in Mexico is reported in IMECAs. The IMECA is calculated using the measurements of average times of the chemicals ozone, sulphur dioxide, nitrogen dioxide, carbon monoxide and particulate matter (PM 10).

Singapore
Singapore uses the Pollutant Standards Index to report on its air quality.  The PSI chart below is grouped by index values and descriptors, according to the National Environment Agency.

United Kingdom
AEA Technology issue air quality forecasts for the UK on behalf of DEFRA 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 Air Quality Index (AQI) ranges used by the U.S. Environmental Protection Agency and their corresponding health effect categories and color codes are provided in the adjacent table. If multiple pollutants are measured at a monitoring site, then the largest or "dominant" AQI value is reported for the location.

The U.S. EPA has developed conversion calculators, available online, for the conversion of AQI values to concentration values and for the reverse conversion of concentrations to AQI values.

A national map of the United States containing daily AQI forecasts across the nation, developed jointly by the U.S. EPA and NOAA is also available online.

The Clean Air Act of 1990 requires the U.S. EPA to review its National Ambient Air Quality Standards every five years to reflect evolving health effects information. The Air Quality Index is adjusted periodically to reflect these changes.

Air pollutant concentration measurement units
In the United States, the concentrations of the air pollutants involved in the AQI are usually expressed as:
 * Ozone and sulfur dioxides: ppbv = parts per billion (10 9) by volume = volume of pollutant gas per billion volumes of ambient air
 * Carbon monoxide: ppmv = parts per million (10 6) by volume = volume of pollutant gas per million volumes of ambient air
 * PM10, defined as particulate matter having an aerodynamic diameter of 10 μm (micrometer) or less: ug/m³ = micrograms of particulate matter per cubic metre of ambient air
 * PM2.5, defined as particulate matter having an aerodynamic diameter of 2.5 μm (micrometer) or less: ug/m³ = micrograms of particulate matter per cubic metre of ambient air