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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.

Air Quality Indices by country

Canada's AQHI Air Quality Health Index (AQI) Health Risk Category Color Code 1 – 3 Low 4 – 6 Moderate 7 – 10 High 10+ Very High

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's API Air Pollution Index (API) Health Effect Category Color Code 0 – 25 Low   26 – 50 Medium   51 – 100 High   101 – 200 Very High   201 – 500 Severe

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's National API Air Pollution Index (API) Air Quality Level Air Quality Category 0 – 50 I Excellent 51 – 100 II Good 101 – 200 III Slightly polluted 201 – 300 IV Moderately polluted 301+ V Heavily polluted Beijing's API 0 – 50   Good 51 – 100   Moderate 101 – 150   Unhealthy for sensitive groups 151 – 200   Unhealthy 201 – 250   Very unhealthy 251 – 500   Hazardous

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 pollutant concentrations are obtained quite differently. Sulfur dioxide, nitrogen dioxide and PM₁₀ concentrations are obtained as daily averages. Carbon monoxide and ozone are more harmful and are obtained as an hourly averages. The final API value is calculated as a daily average.

The scale for each pollutant is non-linear, as is the final daily API value. Thus, an API value of 100 does not mean it is twice the pollution of API at 50, nor does it mean it is 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's API Air Pollution Index (API) Air Quality Category 0 – 50 Good 51 – 100 Moderate 101 – 200 Unhealthy 201 – 300 Very Unhealthy 301+ Hazardous

Malaysia

The air quality in Malaysia is described in terms of an Air Pollutant Index (API). The API is an indicator of air quality and was developed based on scientific assessment to indicate in an easily understood manner, the presence of pollutants and its impact on health. The API system of Malaysia closely follows the similar system developed by the U.S. Environmental Protection Agency (U.S. EPA).

Monitoring stations measure the concentration of five major pollutants in the ambient air: PM₁₀, sulfur dioxide, nitrogen dioxide, carbon monoxide and ozone. These concentrations are measured continuously on an hourly basis. The hourly value is then averaged over a 24-hour period for PM1₁₀ and sulfur dioxide and an 8-hour period for carbon monoxide. The ozone and nitrogen dioxide are read hourly. An hourly index is then calculated for each pollutant. The highest hourly index value is then taken as the API for the hour.

When the API exceeds 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.

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. [1] The PSI chart below is grouped by index values and descriptors, according to the National Environment Agency.[2]

PSI	Descriptor	General Health Effects
0 - 50	Good	None
51 - 100	Moderate	Few or none for the general population
101 - 200	Unhealthy	Mild aggravation of symptoms among susceptible persons i.e. those with underlying conditions such as chronic heart or lung ailments; transient symptoms of irritation e.g. eye irritation, sneezing or coughing in some of the healthy population.
201 - 300	Very Unhealthy	Moderate aggravation of symptoms and decreased tolerance in persons with heart or lung disease; more widespread symptoms of transient irritation in the healthy population.
301 - 400	Hazardous	Early onset of certain diseases in addition to significant aggravation of symptoms in susceptible persons; and decreased exercise tolerance in healthy persons.
Above 400	Hazardous	PSI levels above 400 may be life-threatening to ill and elderly persons. Healthy people may experience adverse symptoms that affect normal activity.

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.

Index	Banding	Health Effect
1 - 3	Low	Effects are unlikely to be noticed even by individuals who know they are sensitive to air pollutants.
4-6	Moderate	Mild effects, unlikely to require action, may be noticed amongst sensitive individuals.
7-9	High	Significant effects may be noticed by sensitive individuals and action to avoid or reduce these effects may be needed (e.g. reducing exposure by spending less time in polluted areas outdoors). Asthmatics will find that their 'reliever' inhaler is likely to reverse the effects on the lung.
10	Very High	The effects on sensitive individuals described for 'High' levels of pollution may worsen.

The forecast is produced for a number of different pollutants and their typical health effects are shown in the following table.

Pollutant	Health Effects at High Level
Nitrogen dioxide Ozone Sulphur dioxide	These gases irritate the airways of the lungs, increasing the symptoms of those suffering from lung diseases.
Particulates	Fine particles can be carried deep into the lungs where they can cause inflammation and a worsening of heart and lung diseases

United States' AQI Air Quality Index (AQI) Air Quality Category Color Code 0 – 50 Good   51 – 100 Moderate   101 – 150 Unhealthy for Sensitive Groups   151 – 200 Unhealthy   201 – 300 Very Unhealthy   301 – 500 Hazardous

United States

The Air Quality Index (AQI) ranges used by the U.S. Environmental Protection Agency (U.S. EPA) 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
• PM_2.5, 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

References

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See also

• Air pollution

• Atmospheric dispersion modeling
• Emission standard
• European emission standards
• Smog
• U.S. National Ambient Air Quality Standards

External links

• AQI at airnow.gov - cross-agency U.S. Government site
• New Mexico Air Quality and API data - Example of how New Mexico Environment Department publishes their Air Quality and API data.
• AQI at Meteorological Service of Canada
• The UK Air Quality Archive
• The pollution index of the UK Met Office
• API at JAS (Malaysian Department of Environment)
• API at Hong Kong - Environmental Protection Department of the Government of the Hong Kong Special Administrative Region
• San Francisco Bay Area Spare-the-Air - AQI explanation
• Malaysia Air Pollution Index
• AQI in Thailand provinces and in Bangkok
• The American Lung Association declares EPA standards fall short.