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An air quality measurement station in Edinburgh, Scotland

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. Air quality index values are divided into ranges, and each range is assigned 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.


Limitations of the AQI

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.

Causes of Poor Air Quality

The AQI can worsen (go up) due to lack of dilution of air emissions by 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.

Indices by location


South Korea

The Ministry of Environment of South Korea uses the Comprehensice Air-quality Index (CAI) to describe the ambient air quality based on health risk of air pollution. The index aims to help the public easily understand air quality level and protect the health of people from air pollution. - The CAI has values of 0 through 500, which are divided into six categories. The higher the CAI value, the greater the level of air pollution. - Of values of the five air pollutants, the highest is the CAI value.

For more information on the CAI please go here

CAI Description Health Implications
0-50 Good A level that will not impact patients suffering from diseases related to air pollution.
51-100 Moderate A level which may have a meager impact on patients in case of chronic exposure.
101-150 Unhealthy for sensitive groups A level that may have harmful impacts on patients and members of sensitive groups.
151-250 Unhealthy A level that may have harmful impacts on patients and members of sensitive groups (children, aged or weak people), and also cause the general public unpleasant feelings.
251-350 Very unhealthy A level which may have a serious impact on patients and members of sensitive groups in case of acute exposure.
351-500 Hazardous A level which may need to take emergency measures for patients and members of sensitive groups and have harmful impacts on the general public.


The current health classifications used by the Meteorological Service of Canada (MSC) are as follows:

API Air Quality
Health Implications
0 - 25 Good 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.
26 - 50 Fair
51 - 100 Poor
100 + Very Poor

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.

Air Pollution
Health Implications
0 - 25 Low None expected.
26 - 50 Medium None expected for the general population.
51 - 100 High Acute health effects are not expected but chronic effects may be observed if one is persistently exposed to such levels.
100 - 200 Very High People with existing heart or respiratory illnesses may notice mild aggravation of their health conditions. Generally healthy individuals may also notice some discomfort.
201 - 500 Severe People with existing heart or respiratory illnesses may experience significant aggravation of their symptoms. There may also be widespread symptoms in the healthy population (e.g. eye irritation, wheezing, coughing, phlegm and sore throats).

Mainland China

China's Minitsry of Environmental Protection ( MEP) is responsible for measuring the level of air pollution in China. As of 28 August 2008, MEP monitors daily pollution level in 86 of its major cities. The 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.[1]

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.[1]

API and Health Implications (Daily Targets)[1]

API Air Pollution
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.


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


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 uses the Pollutant Standards Index to report on its air quality.[2] The PSI chart below is grouped by index values and descriptors, according to the National Environment Agency.[3]

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
Sulphur dioxide
These gases irritate the airways of the lungs, increasing the symptoms
of those suffering from lung diseases.
Fine particles can be carried deep into the lungs where they can cause
inflammation and a worsening of heart and lung diseases

United States

The United States Environmental Protection Agency (EPA) uses the following AQI:

Air Quality Index (AQI) Values Levels of Health Concern Colors
0 to 50 Good Green
51 to 100 Moderate Yellow
101 to 150 Unhealthy for Sensitive Groups Orange
151 to 200 Unhealthy Red
201 to 300 Very Unhealthy Purple
301 to 500 Hazardous Maroon

PM2.5 24-Hour AQI Loop, Courtesy US EPA

The air quality index is a piecewise linear function of the pollutant concentration. At the boundary between AQI categories, there is a discontinuous jump of one AQI unit. To convert from concentration to AQI the equation:

 I = \frac {I_{high} -I_{low}}{ C_{high} -C_{low} }(C-C_{low}) +I_{low}

is used, where:

I = the (Air Quality) index,
C = the pollutant concentration,
Clow= the concentration breakpoint that is ≤ C,
Chigh= the concentration breakpoint that is ≥ C,
Ilow= the index breakpoint corresponding to Clow,
Ihigh= the index breakpoint corresponding to Chigh.

For breakpoints for different pollutants, see:

For example, suppose a monitor records a 24-hour average fine particle (PM2.5) concentration of 12.0 micrograms per cubic meter. EPA's table of breakpoints for PM2.5 is:

Clow Chigh Ilow Ihigh Category
0 15.4 0 50 Good
15.5 40.4 51 100 Moderate
40.5 65.4 101 150 Unhealthy for Sensitive Groups
65.5 150.4 151 200 Unhealthy
150.5 250.4 201 300 Very Unhealthy
250.5 350.4 301 400 Hazardous
350.5 500.4 401 500 Hazardous

The equation above results in an AQI of:

 \frac{50 -0}{15.4 -0}(12.0-0) +0=39,

corresponding to air quality in the "Good" range.

If multiple pollutants are measured at a monitoring site, then the largest or "dominant" AQI value is reported for the location.

To convert an air pollutant concentration to an AQI, EPA has developed a calculator:
To convert an air quality index to an air pollutant concentration, see:
Current ambient monitoring data and forecasts of air quality that are color-coded in terms of the air quality index are available at:

The Clean Air Act (USA) (1990) requires 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.


In the context of this article about air quality:

  • ppbv=parts per billion by volume = volume of pollutant gas per billion volumes of ambient air
  • ppmv = parts per million by volume = volume of pollutant gas per million volumes of ambient air
  • PM2.5 = particulate matter smaller than 2.5 μm in aerodynamic diameter
  • PM10 = particulate matter smaller than 10 μm in aerodynamic diameter
  • μg/m³ = micrograms per cubic metre of ambient air
  • μm = micrometre

Air quality by country or region

See also


External links


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