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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.

Most air contaminants do not have an associated AQI. Many countries monitor ground-level ozone, particulates, sulfur dioxide, carbon monoxide and nitrogen dioxide and calculate air quality indices for these pollutants.

Air Quality Indices by country

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.

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 respirable suspended particulate (RSP), sulfur dioxide (SO₂), carbon monoxide (CO), ozone (O₃) and nitrogen dioxide (NO₂) 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 State Environment Protection Agency (SEPA) is responsible for measuring the level of air pollution in China. As of 28 August 2008, SEPA monitors daily pollution level in 86 of its major cities. The Air Pollution Index (API) level is based on the level of 5 atmospheric pollutants, namely sulfur dioxide (SO₂), nitrogen dioxide (NO₂), suspended particulates (PM₁₀), carbon monoxide (CO), and ozone (O₃) measured at the monitoring stations throughout each city.

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 pollutants can be measured quite differently. SO₂, NO₂ and PM₁₀ concentration are measured as average per day. CO and O₃ are more harmful and are measured as average per hour. The final API value is calculated per 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. While an API of 50 from day 1 to 182 and API of 100 from day 183 to 365 does provide an annual average of 75, it does not mean the pollution is acceptable even if the benchmark of 100 is deemed safe. This is because the benchmark is a 24 hour target. The annual average must match against the annual target. It is entirely possible to have safe air every day of the year but still fail the annual pollution benchmark.

API and Health Implications (Daily Targets)

API	Air Pollution Level	Health Implications
0 - 50	Excellent	No health implications
51 -100	Good	No health implications
101-150	Slightly Polluted	Slight irrations may occur, individuals with breathing or heart problems should reduce outdoor exercise.
151-200	Lightly Polluted	Slight irrations may occur, individuals with breathing or heart problems should reduce outdoor exercise.
201-250	Moderately Polluted	Healthy people will be noticeably affected. People with breathing or heart problems will experience reduced endurance in activities. These individuals and elders should remain indoors and restrict activities.
251-300	Heavily Polluted	Healthy people will be noticeably affected. People with breathing or heart problems will experience reduced endurance in activities. These individuals and elders should remain indoors and restrict activities.
300+	Severely Polluted	Healthy people will experience reduced endurance in activities. There may be strong irritations and symptoms and may trigger other illnesses. Elders and the sick should remain indoors and avoid exercise. Healthy individuals should avoid out door activities.

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 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 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 (O₃), sulphur dioxide (SO₂), nitrogen dioxide (NO₂), carbon monoxide (CO) and particles lower than 10 micrometers (PM₁₀).

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

Air Quality Index (AQI) Health Effect 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 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.