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An '''air quality index''' (AQI) is a number used by [[Government agency|government agencies]] <ref>{{cite web|url=http://www.airnow.gov/index.cfm?action=airnow.international|title=International Air Quality|accessdate=20 August 2015}}</ref> to communicate to the public how [[Air pollution|polluted]] the air currently is or how polluted it is forecast to become.<ref>{{cite web|url=http://airquality.weather.gov/|title=NOAA's National Weather Service/Environmental Protection Agency - United States Air Quality Forecast Guidance|author=National Weather Service Corporate Image Web Team|accessdate=20 August 2015}}</ref><ref>https://www.gmes-atmosphere.eu/services/raq/raq_nrt/</ref> As the AQI increases, an increasingly large percentage of the population is likely to experience increasingly severe adverse health effects. Different countries have their own air quality indices, corresponding to different national air quality standards. Some of these are the [[Air Quality Health Index (Canada)]], the [[Air Pollution Index]] (Malaysia), and the [[Pollutant Standards Index]] (Singapore).
[[Image:Smog over Almaty.jpg|thumb|250px|[[Smog]] builds up under an [[Inversion (meteorology)|inversion]] in [[Almaty]], [[Kazakhstan]] resulting in a high AQI]]
[[File:25August 2007 Greek fires.jpg|thumb|200px|Wildfires give rise to an elevated AQI in parts of [[Greece]]]]


== Definition and usage ==
[[Image:Edinburgh air quality measurement station dsc06786.jpg|thumb|An air quality measurement station in [[Edinburgh]], [[Scotland]]]]
Computation of the AQI requires an air pollutant concentration over a specified averaging period, obtained from an [[Environmental monitoring#air quality monitoring|air monitor]] or [[atmospheric dispersion modeling|model]]. Taken together, concentration and time represent the [[dose response|dose]] of the air pollutant. Health effects corresponding to a given dose are established by epidemiological research.<ref>{{cite web|url=http://www.epa.gov/risk_assessment/dose-response.htm|title=Step 2 - Dose-Response Assessment|accessdate=20 August 2015}}</ref> Air pollutants vary in potency, and the function used to convert from air pollutant concentration to AQI varies by pollutant. Its air quality index values are typically grouped into ranges. Each range is assigned a descriptor, a color code, and a standardized public health advisory.


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The AQI can increase due to an increase of air emissions (for example, during rush hour traffic or when there is an upwind forest fire) or from a lack of dilution of air pollutants. [[air stagnation|Stagnant air]], often caused by an [[anticyclone]], [[temperature inversion]], or low [[wind]] speeds lets air pollution remain in a local area, leading to high concentrations of pollutants, chemical reactions between air contaminants and [[haze|hazy]] conditions.<ref>{{cite web|author=Myanmar government|date=2007|url=http://www.kjc.gov.my/english/education/weather/haze01.html|accessdate =2007-02-11|title=Haze|archivedate=27 January 2007|archiveurl=https://web.archive.org/web/20070127214023/http://www.kjc.gov.my/english/education/weather/haze01.html}}</ref>
[[File:SignboardAirQualityHouston.JPG|thumb|Signboard in [[Gulfton, Houston|Gulfton]], [[Houston]] indicating an [[ozone]] watch]]
On a day when the AQI is predicted to be elevated due to fine particle pollution, an agency or public health organization might:
* advise sensitive groups, such as the elderly, children, and those with respiratory or cardiovascular problems to avoid outdoor exertion.<ref>{{cite web|url=http://www.lung.org/healthy-air/outdoor/air-quality-index.html |title=Air Quality Index - American Lung Association |work=American Lung Association |accessdate=20 August 2015 |deadurl=yes |archiveurl=https://web.archive.org/web/20150828143922/http://www.lung.org/healthy-air/outdoor/air-quality-index.html |archivedate=28 August 2015 }}</ref>
* declare an [[Ozone Action Day|"action day"]] to encourage voluntary measures to reduce air emissions, such as using public transportation.<ref>{{cite web|url=http://www.sparetheair.org/Make-a-Difference/Spare-the-Air-Every-Day/Summer.aspx|title=Spare the Air - Summer Spare the Air|accessdate=20 August 2015}}</ref>
*recommend the use of masks to keep fine particles from entering the lungs<ref>{{cite web|url=https://www.moh.gov.sg/content/moh_web/home/pressRoom/Current_Issues/2014/haze/faq--use-of-masks-and-availability-of-masks.html|title=FAQ: Use of masks and availability of masks|accessdate=20 August 2015|archive-url=https://web.archive.org/web/20180622004913/https://www.moh.gov.sg/content/moh_web/home/pressRoom/Current_Issues/2014/haze/faq--use-of-masks-and-availability-of-masks.html|archive-date=2018-06-22|dead-url=yes|df=}}</ref>

During a period of very poor air quality, such as an [[air pollution episode]], when the AQI indicates that acute exposure may cause significant harm to the public health, agencies may invoke emergency plans that allow them to order major emitters (such as coal burning industries) to curtail emissions until the hazardous conditions abate.<ref>{{cite web |url=http://cleanairinitiative.org/portal/sites/default/files/presentations/1_en_air_pollution_emergency_response_system_-_us_experience_1.pdf |title=Archived copy |accessdate=2014-09-25 |deadurl=yes |archiveurl=https://web.archive.org/web/20140903112519/http://cleanairinitiative.org/portal/sites/default/files/presentations/1_en_air_pollution_emergency_response_system_-_us_experience_1.pdf |archivedate=2014-09-03 |df= }}</ref>

Most air contaminants do not have an associated AQI. Many countries monitor [[ground-level ozone]], [[Atmospheric particulate matter|particulates]], [[sulfur dioxide]], [[carbon monoxide]] and [[nitrogen dioxide]], and calculate air quality indices for these pollutants.<ref name="aqi_basic"/>

The definition of the AQI in a particular nation reflects the discourse surrounding the development of national air quality standards in that nation.<ref>{{cite web|url=https://www.wsj.com/articles/the-epas-latest-threat-to-economic-growth-1407970689|title=The EPA's Latest Threat to Economic Growth|author=Jay Timmons|date=13 August 2014|work=WSJ|accessdate=20 August 2015}}</ref> A website allowing government agencies anywhere in the world to submit their real-time air monitoring data for display using a common definition of the air quality index has recently become available.<ref>{{cite web|url=http://waqi.info/|title=World Air Quality Index|accessdate=20 August 2015}}</ref>

==Indices by location==

===Canada===
{{main article|Air Quality Health Index (Canada)}}

Air quality in Canada has been reported for many years with provincial Air Quality Indices (AQIs). Significantly, AQI values reflect air quality management objectives, which are based on the lowest achievable emissions rate, and not exclusively concern for human health. The [[Air Quality Health Index (Canada)|Air Quality Health Index]] or ('''AQHI''') is a scale designed to help understand the impact of air quality on health.
It is a health protection tool used to make decisions to reduce short-term exposure to air pollution by adjusting activity levels during increased levels of air pollution. The Air Quality Health Index also provides advice on how to improve air quality by proposing behavioural change to reduce the environmental footprint. This index pays particular attention to people who are sensitive to air pollution. It provides them with advice on how to protect their health during air quality levels associated with low, moderate, high and very high health risks.

The '''Air Quality Health Index''' provides a number from 1 to 10+ to indicate the level of health risk associated with local air quality. On occasion, when the amount of air pollution is abnormally high, the number may exceed 10. The AQHI provides a local air quality current value as well as a local air quality maximums forecast for today, tonight, and tomorrow, and provides associated health advice.<ref>{{cite web|url=http://www.ec.gc.ca/cas-aqhi/default.asp?lang=En&n=79A8041B-1 |title=Environment Canada - Air - AQHI categories and explanations |publisher=Ec.gc.ca |date=2008-04-16 |accessdate=2011-11-11}}</ref>

{| style="margin:auto; background:Black; color:White;"
|- style="text-align:center;"
| style="width:35px; height:35px; background:#6f0;"| '''1''' || style="text-align:center; width:35px; height:35px; background:#9f0;"| '''2''' || style="text-align:center; width:35px; height:35px; background:#cf0;"| '''3''' || style="text-align:center; width:35px; height:35px; background:#ff0;"| '''4''' || style="height:35px; height:35px; text-align:center; width:35px; text-align:center; width:35px; background:#fc0;"| '''5''' || style="text-align:center; width:35px; height:35px; background:#f90;"| '''6''' || style="text-align:center; width:35px; height:35px; background:#f60;"| '''7''' || style="text-align:center; width:35px; height:35px; background:#f30;"| '''8''' || style="width:18px; width:35px; text-align:center; text-align:center; height:35px; background:#f00;"| '''9'''|| style="width:18px; width:35px; text-align:center; text-align:center; height:35px; background:#c00;"| '''10''' || style="text-align:center; width:35px; height:35px; background:#900;"| '''+'''
|}
{| style="margin:auto; background:white; color:black;"
|-
||'''Risk:'''|| Low '''(1–3)'''|| Moderate '''(4–6)'''|| High '''(7–10)''' || Very high '''(above 10)'''
|}

{| class="wikitable"
|-
! Health Risk !! Air Quality Health Index !!colspan="2" style="text-align: center;"|Health Messages
|-
| || || '''At Risk population'''||'''*General Population'''
|-
| '''Low''' || '''1–3'''|| '''Enjoy''' your usual outdoor activities.||'''Ideal''' air quality for outdoor activities
|-
| '''Moderate''' || '''4–6''' || '''Consider reducing''' or rescheduling strenuous activities outdoors if you are experiencing symptoms.||'''No need to modify''' your usual outdoor activities unless you experience symptoms such as coughing and throat irritation.
|-
| '''High''' || '''7–10''' || '''Reduce''' or reschedule strenuous activities outdoors. Children and the elderly should also take it easy.||'''Consider reducing''' or rescheduling strenuous activities outdoors if you experience symptoms such as coughing and throat irritation.
|-
| '''Very high''' || '''Above 10''' || '''Avoid''' strenuous activities outdoors. Children and the elderly should also avoid outdoor physical exertion.||'''Reduce''' or reschedule strenuous activities outdoors, especially if you experience symptoms such as coughing and throat irritation.
|}

===Hong Kong===
On the 30 December 2013 [[Hong Kong]] replaced the [[Air Pollution Index#Hong Kong|Air Pollution Index]] with a new index called the ''Air Quality Health Index''.<ref>{{cite web|url=http://hsu.me/2012/03/chinas-new-air-quality-index-how-does-it-measure-up/ |title=China’s new Air Quality Index: How does it measure up? |accessdate=8 February 2014 |last=Hsu |first=Angel |deadurl=yes |archiveurl=https://web.archive.org/web/20130717033733/http://hsu.me/2012/03/chinas-new-air-quality-index-how-does-it-measure-up/ |archivedate=17 July 2013 }}</ref> This index, reported by the [[Environmental Protection Department]], is measured on a scale of 1 to 10+ and considers four air pollutants: ozone; nitrogen dioxide; sulphur dioxide and particulate matter (including PM10 and PM2.5). For any given hour the AQHI is calculated from the sum of the percentage excess risk of daily hospital admissions attributable to the 3-hour moving average concentrations of these four pollutants. The AQHIs are grouped into five AQHI health risk categories with health advice provided:<ref>{{cite web | url = http://www.gov.hk/en/residents/environment/air/aqhi.htm | title = Air Quality Health Index| accessdate=9 February 2014| publisher = Government of the Hong Kong Special Administrative Region}}</ref>

{| class="wikitable"
|-
! Health risk category !! AQHI
|-
|rowspan="3" bgcolor="green"|Low || 1
|-
| bgcolor="aaaaaa"| 2
|-
| 3
|-
| rowspan="3" bgcolor="orange"|Medium || bgcolor="aaaaaa"| 4
|-
| 5
|-
| bgcolor="aaaaaa"| 6
|-
| bgcolor="red"|High || 7
|-
| rowspan="3" bgcolor="#930"|Very High || bgcolor="aaaaaa"| 8
|-
| 9
|-
| bgcolor="aaaaaa"| 10
|-
| style="background: black; color: white"|Serious || 10+
|}

Each of the health risk categories has advice with it. At the ''low'' and ''moderate'' levels the public are advised that they can continue normal activities. For the ''high'' category, children, the elderly and people with heart or respiratory illnesses are advising to reduce outdoor physical exertion. Above this (''very high'' or ''serious'') the general public are also advised to reduce or avoid outdoor physical exertion.

=== Mainland China ===
China's Ministry of Environmental Protection ([[Ministry of Environmental Protection of the People's Republic of China|MEP]]) is responsible for measuring the level of air pollution in China. As of 1 January 2013, MEP monitors daily pollution level in 163 of its major cities. The AQI level is based on the level of six atmospheric pollutants, namely [[sulfur dioxide]] (SO<sub>2</sub>), [[nitrogen dioxide]] (NO<sub>2</sub>), suspended particulates smaller than 10 μみゅーm in [[aerodynamic diameter]] (PM<sub>10</sub>)<ref name=":0">{{Cite web|url=http://kjs.mep.gov.cn/hjbhbz/bzwb/dqhjbh/dqhjzlbz/201203/W020120410330232398521.pdf|title=环境空そら气质量りょう标准|last=|first=|date=2012-03-02|website=Ministry of Environmental Protection of the People's Republic of China|language=Chinese|trans-title=Ambient Air Quality Standards|access-date=2018-02-02}}</ref>, suspended particulates smaller than 2.5 μみゅーm in [[aerodynamic diameter]] (PM<sub>2.5</sub>)<ref name=":0" />，[[carbon monoxide]] (CO), and [[ozone]] (O<sub>3</sub>) measured at the monitoring stations throughout each city.<ref name="sepa.gov.cn">{{cite web|url=http://kjs.mep.gov.cn/hjbhbz/bzwb/jcffbz/201203/W020120410332725219541.pdf|title=环境空そら气质量りょう指数しすう（AQI) 技わざ术规定じょう（试行）|last=|first=|date=2012-03-02|website=Ministry of Environmental Protection of the People's Republic of China|language=Chinese|trans-title=Technical Regulation on Ambient Air Quality Index (on trial)|access-date=2018-02-02}}</ref>

'''AQI Mechanics'''<br />An individual score (Individual Air Quality Index, IAQI) is assigned to each pollutant and the final AQI is ''the highest'' of these six scores. The final AQI value can be calculated either per hour or per 24 hours. The concentrations of pollutants can be measured quite differently. If the AQI value is calculated hourly, then SO<sub>2</sub>, NO<sub>2</sub>, CO concentrations are measured as average per 24h, O<sub>3</sub> concentration is measured as average per hour and the [[moving average]] per 8h, PM<sub>2.5</sub> and PM<sub>10</sub> concentrations are measured as average per hour and per 24h. If the AQI value is calculated per 24h, then SO<sub>2</sub>, NO<sub>2</sub>, CO, PM<sub>2.5</sub> and PM<sub>10</sub> concentrations are measured as average per 24h, while O<sub>3</sub> concentration is measured as the maximum 1h average and the maximum 24h [[moving average]]. The IAQI of each pollutant is calculated according to a formula published by the [[Ministry of Ecology and Environment|MEP]].<ref name="sepa.gov.cn" />

The score for each pollutant is non-linear, as is the final AQI score. Thus an AQI of 300 does not mean twice the pollution of AQI at 150, nor does it mean the air is twice as harmful. The concentration of a pollutant when its IAQI is 100 does not equal twice its concentration when its IAQI is 50, nor does it mean the pollutant is twice as harmful. While an AQI of 50 from day 1 to 182 and AQI 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. Because the benchmark is a 24-hour target, and the annual average must match the annual target, it is entirely possible to have safe air every day of the year but still fail the annual pollution benchmark.<ref name="sepa.gov.cn"/>

'''AQI and Health Implications (HJ 633—2012)'''<ref name="sepa.gov.cn"/>
{| class="wikitable"
|-
! style="text-align:center; width:100px;"|AQI
!Air Pollution Level!! style="text-align:center; width:150px;" |Air Pollution<br />Category !! Health Implications
!Recommended Precautions
|-
| style="text-align:center;"|0–50
|Level 1|| style="text-align:center;background-color:#0f0;color:#000" |Excellent||No health implications.
|Everyone can continue their outdoor activities normally.
|-
| style="text-align:center;"|51–100
|Level 2|| style="text-align:center;background-color:#ff0;color:#000" |Good||Some pollutants may slightly affect very few hypersensitive individuals.
|Only very few hypersensitive people should reduce outdoor activities.
|-
| style="text-align:center;"|101–150
|Level 3|| style="text-align:center;background-color:#f90;color:#000" |Lightly Polluted||Healthy people may experience slight irritations and sensitive individuals will be slightly affected to a larger extent.
|Children, seniors and individuals with respiratory or heart diseases should reduce sustained and high-intensity outdoor exercises.
|-
| style="text-align:center;"|151–200
|Level 4|| style="text-align:center;background-color:#f00;color:#000" |Moderately Polluted||Sensitive individuals will experience more serious conditions. The hearts and respiratory systems of healthy people may be affected.
|Children, seniors and individuals with respiratory or heart diseases should avoid sustained and high-intensity outdoor exercises. General population should moderately reduce outdoor activities.
|-
| style="text-align:center;"|201–300
|Level 5|| style="text-align:center;background-color:#540099;color:#fff" |Heavily Polluted||Healthy people will commonly show symptoms. People with respiratory or heart diseases will be significantly affected and will experience reduced endurance in activities.
|Children, seniors and individuals with heart or lung diseases should stay indoors and avoid outdoor activities. General population should reduce outdoor activities.
|-
| style="text-align:center;"|>300
|Level 6|| style="text-align:center;background-color:#800000;color:#fff" |Severely Polluted||Healthy people will experience reduced endurance in activities and may also show noticeably strong symptoms. Other illnesses may be triggered in healthy people. Elders and the sick should remain indoors and avoid exercise. Healthy individuals should avoid outdoor activities.
|Children, seniors and the sick should stay indoors and avoid physical exertion. General population should avoid outdoor activities.
|}

===India===
The National Air Quality Index (AQI) was launched in [[New Delhi]] on 17 September 2014 under the [[Swachh Bharat Abhiyan]].<ref>{{cite web|url=https://www.washingtonpost.com/blogs/worldviews/wp/2014/10/17/india-launches-its-own-air-quality-index-can-its-numbers-be-trusted/|title=India launches its own Air Quality Index. Can its numbers be trusted?|author=Rama Lakshmi|date=17 October 2014|work=Washington Post|accessdate=20 August 2015}}</ref>

The [[Central Pollution Control Board]] along with State Pollution Control Boards has been operating National Air Monitoring Program (NAMP) covering 240 cities of the country having more than 342 monitoring stations.<ref>{{Cite web|url=http://www.ppsthane.com/blog/ambient-air-quality-monitoring-stations|title=Ambient Air Quality Monitoring Stations|date=2016-08-15|language=en-US|access-date=2016-08-16}}</ref> An Expert Group comprising medical professionals, air quality experts, academia, advocacy groups, and SPCBs was constituted and a technical study was awarded to [[IIT Kanpur]]. IIT Kanpur and the Expert Group recommended an AQI scheme in 2014.<ref>{{cite web |url=http://home.iitk.ac.in/~mukesh/indian%20air%20quality.html |title=Archived copy |accessdate=2014-10-21 |deadurl=yes |archiveurl=https://web.archive.org/web/20141107224911/http://home.iitk.ac.in/~mukesh/indian%20air%20quality.html |archivedate=2014-11-07 |df= }}</ref> While the earlier measuring index was limited to three indicators, the new index measures eight parameters.<ref>{{cite web|url=http://pib.nic.in/newsite/PrintRelease.aspx?relid=110654|title=National Air Quality Index (AQI) launched by the Environment Minister AQI is a huge initiative under ‘Swachh Bharat’|accessdate=20 August 2015}}</ref> The continuous monitoring systems that provide data on near real-time basis are installed in New Delhi, [[Mumbai]], [[Pune]] and [[Ahmedabad]].<ref>{{cite web|url=http://economictimes.indiatimes.com/news/politics-and-nation/india-launches-index-to-measure-air-quality/etvertical_articleshow/44855452.cms|title=India launches index to measure air quality|work=timesofindia-economictimes|accessdate=20 August 2015}}</ref>

There are six AQI categories, namely Good, Satisfactory, Moderately polluted, Poor, Very Poor, and Severe. The proposed AQI will consider eight pollutants (PM{{sub|10}}, PM{{sub|2.5}}, NO{{sub|2}}, SO{{sub|2}}, CO, O{{sub|3}}, NH{{sub|3}}, and Pb) for which short-term (up to 24-hourly averaging period) National Ambient Air Quality Standards are prescribed.<ref>{{cite web|url=http://www.cpcb.nic.in|title=::: Central Pollution Control Board :::|accessdate=20 August 2015}}</ref> Based on the measured ambient concentrations, corresponding standards and likely health impact, a sub-index is calculated for each of these pollutants. The worst sub-index reflects overall AQI. Likely health impacts for different AQI categories and pollutants have also been suggested, with primary inputs from the medical experts in the group. The AQI values and corresponding ambient concentrations (health breakpoints) as well as associated likely health impacts for the identified eight pollutants are as follows:

{| class="wikitable"
|+ AQI Category, Pollutants and Health Breakpoints
| style="background: #9cf; colour: black; font-weight:bold" | AQI Category (Range)
| style="background: #9cf; colour: black; font-weight:bold" | PM{{sub|10}} (24hr)
| style="background: #9cf; colour: black; font-weight:bold" | PM{{sub|2.5}} (24hr)
| style="background: #9cf; colour: black; font-weight:bold" | NO{{sub|2}} (24hr)
| style="background: #9cf; colour: black; font-weight:bold" | O{{sub|3}} (8hr)
| style="background: #9cf; colour: black; font-weight:bold" | CO (8hr)
| style="background: #9cf; colour: black; font-weight:bold" | SO{{sub|2}} (24hr)
| style="background: #9cf; colour: black; font-weight:bold" | NH{{sub|3}} (24hr)
| style="background: #9cf; colour: black; font-weight:bold" | Pb (24hr)
|-
| style="background:#0c0; colour: black" | Good (0–50)
| style="background:#0c0; colour: black" | 0–50
| style="background:#0c0; colour: black" | 0–30
| style="background:#0c0; colour: black" | 0–40
| style="background:#0c0; colour: black" | 0–50
| style="background:#0c0; colour: black" | 0–1.0
| style="background:#0c0; colour: black" | 0–40
| style="background:#0c0; colour: black" | 0–200
| style="background:#0c0; colour: black" | 0–0.5
|-
| style="background:#6c0; colour: black" | Satisfactory (51–100)
| style="background:#6c0; colour: black" | 51–100
| style="background:#6c0; colour: black" | 31–60
| style="background:#6c0; colour: black" | 41–80
| style="background:#6c0; colour: black" | 51–100
| style="background:#6c0; colour: black" | 1.1–2.0
| style="background:#6c0; colour: black" | 41–80
| style="background:#6c0; colour: black" | 201–400
| style="background:#6c0; colour: black" | 0.5–1.0
|-
| style="background: #ff0; colour: black" | Moderately polluted (101–200)
| style="background: #ff0; colour: black" | 101–250
| style="background: #ff0; colour: black" | 61–90
| style="background: #ff0; colour: black" | 81–180
| style="background: #ff0; colour: black" | 101–168
| style="background: #ff0; colour: black" | 2.1–10
| style="background: #ff0; colour: black" | 81–380
| style="background: #ff0; colour: black" | 401–800
| style="background: #ff0; colour: black" | 1.1–2.0
|-
| style="background: #f90; colour: black" | Poor (201–300)
| style="background: #f90; colour: black" | 251–350
| style="background: #f90; colour: black" | 91–120
| style="background: #F90; colour: black" | 181–280
| style="background: #F90; colour: black" | 169–208
| style="background: #F90; colour: black" | 10–17
| style="background: #f90; colour: black" | 381–800
| style="background: #F90; colour: black" | 801–1200
| style="background: #F90; colour: black" | 2.1–3.0
|-
| style="background:red; colour: black" | Very poor (301–400)
| style="background:red; colour: black" | 351–430
| style="background:red; colour: black" | 121–250
| style="background:red; colour: black" | 281–400
| style="background:red; colour: black" | 209–748
| style="background:red; colour: black" | 17–34
| style="background:red; colour: black" | 801–1600
| style="background:red; colour: black" | 1200–1800
| style="background:red; colour: black" | 3.1–3.5
|-
| style="background:brown; colour: black" | Severe (401–500)
| style="background:brown; colour: black" | 430+
| style="background:brown; colour: black" | 250+
| style="background:brown; colour: black" | 400+
| style="background:brown; colour: black" | 748+
| style="background:brown; colour: black" | 34+
| style="background:brown; colour: black" | 1600+
| style="background:brown; colour: black" | 1800+
| style="background:brown; colour: black" | 3.5+
|}

{| class="wikitable"
! AQI
! Associated Health Impacts
|-
| Good (0–50)
| Minimal impact
|-
| Satisfactory (51–100)
| May cause minor breathing discomfort to sensitive people.
|-
| Moderately polluted (101–200)
| May cause breathing discomfort to people with lung disease such as asthma, and discomfort to people with heart disease, children and older adults.
|-
| Poor (201–300)
| May cause breathing discomfort to people on prolonged exposure, and discomfort to people with heart disease.
|-
| Very poor (301–400)
| May cause respiratory illness to the people on prolonged exposure. Effect may be more pronounced in people with lung and heart diseases.
|-
| Severe (401–500)
| May cause respiratory impact even on healthy people, and serious health impacts on people with lung/heart disease. The health impacts may be experienced even during light physical activity.
|}

===Mexico===
{{Main article|Índice Metropolitano de la Calidad del Aire}}

The air quality in [[Mexico City]] is reported in IMECAs. The IMECA is calculated using the measurements of average times of the chemicals ozone (O<sub>3</sub>), sulphur dioxide (SO<sub>2</sub>), nitrogen dioxide (NO<sub>2</sub>), carbon monoxide (CO), particles smaller than 2.5 micrometers (PM<sub>2.5</sub>), and particles smaller than 10 micrometers (PM<sub>10</sub>).<ref>{{Cite web|url=http://www.aire.cdmx.gob.mx/default.php?opc=%2527ZaBhnmI=&dc=%2527Yg==|title=Dirección de Monitoreo Atmosférico|website=www.aire.cdmx.gob.mx|access-date=2016-06-15}}</ref>

===Singapore===
[[Singapore]] uses the [[Pollutant Standards Index]] to report on its air quality,<ref>{{cite web|url=http://app.mewr.gov.sg/web/Contents/Contents.aspx?ContId=52 |title=MEWR - Key Environment Statistics - Clean Air |publisher=App.mewr.gov.sg |date=2011-06-08 |accessdate=2011-11-11 |deadurl=yes |archiveurl=https://web.archive.org/web/20111009052449/http://app.mewr.gov.sg/web/Contents/Contents.aspx?ContId=52 |archivedate=2011-10-09 }}</ref> with details of the calculation similar but not identical to that used in Malaysia and Hong Kong<ref>.{{cite web|url=http://app2.nea.gov.sg/data/cmsresource/20090401682645441976.pdf |title=National Environment Agency - Calculation of PSI |accessdate=2012-06-15 |deadurl=yes |archiveurl=https://web.archive.org/web/20130515095923/http://app2.nea.gov.sg/data/cmsresource/20090401682645441976.pdf |archivedate=2013-05-15 }}</ref>
The PSI chart below is grouped by index values and descriptors, according to the [[National Environment Agency]].<ref>{{cite web|url=http://app2.nea.gov.sg/psi_hazeplan.aspx |title=National Environment Agency |publisher=App2.nea.gov.sg |accessdate=2011-11-11 |deadurl=yes |archiveurl=https://web.archive.org/web/20111125033900/http://app2.nea.gov.sg/psi_hazeplan.aspx |archivedate=2011-11-25 }}</ref>

{|class="wikitable"
|-
! style="width:100px;"|PSI !! style="width:150px;"|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.
|}

===South Korea===
The Ministry of Environment of South Korea uses the Comprehensive Air-quality Index (CAI) to describe the ambient air quality based on the health risks of air pollution. The index aims to help the public easily understand the air quality and protect people's health. The CAI is on a scale from 0 to 500, which is 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. The index also has associated health effects and a colour representation of the categories as shown below.<ref>{{cite web|url=http://www.airkorea.or.kr/eng/cai/cai1| title=What's CAI| publisher=Air Korea| accessdate = 25 October 2015}}</ref>

{| class="wikitable mw-collapsible"
|-
! style="width:150px;"| CAI !! style="width:200px;"| Description!! Health Implications
|-
| style="text-align:center;"|0–50|| style="text-align:center; background:#00f;color:white"|Good||A level that will not impact patients suffering from diseases related to air pollution.
|-
| style="text-align:center;"|51–100|| style="background:#0f0; text-align:center; color:#000;"|Moderate||A level that may have a meager impact on patients in case of chronic exposure.
|-
| style="text-align:center;"|101–150|| style="text-align:center; background:#ff0;"|Unhealthy for sensitive groups||A level that may have harmful impacts on patients and members of sensitive groups.
|-
| style="text-align:center;"|151–250|| style="background:#f90; text-align:center; color:#fff;"|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.
|-
| style="text-align:center;"|251–350|| style="text-align:center; background:#f00;" |Very unhealthy||A level that may have a serious impact on patients and members of sensitive groups in case of acute exposure.
|-
| style="text-align:center;"|351-500|| style="text-align:center; background:#900;" |Hazardous||A level that may have a serious impact on patients and members of sensitive groups in case of acute exposure.
|-
|}

The [[N Seoul Tower]] on [[Namsan (Seoul)|Namsan Mountain]] in central [[Seoul, South Korea]], is illuminated in blue, from sunset to 23:00 and 22:00 in winter, on days where the [[Air quality#South Korea|air quality in Seoul]] is 45 or less. During the spring of 2012, the Tower was lit up for 52 days, which is four days more than in 2011.<ref>{{cite news|title=Improved Air Quality Reflected in N Seoul Tower|url=http://english.chosun.com/site/data/html_dir/2012/05/18/2012051801102.html|accessdate=29 July 2012|newspaper=Chosun Ilbo|date=18 May 2012|deadurl=yes|archiveurl=https://web.archive.org/web/20140304200802/http://english.chosun.com/site/data/html_dir/2012/05/18/2012051801102.html|archivedate=4 March 2014|df=}}</ref>

===United Kingdom===
The most commonly used air quality index in the UK is the ''Daily Air Quality Index'' recommended by the [[Committee on Medical Effects of Air Pollutants]] (COMEAP).<ref>{{cite web|url=http://comeap.org.uk/documents/130-review-of-the-uk-air-quality-index.html |archive-url=https://web.archive.org/web/20111115230335/http://www.comeap.org.uk/documents/130-review-of-the-uk-air-quality-index.html |dead-url=yes |archive-date=2011-11-15 |title=Review of the UK Air Quality Index |author=COMEAP |work=COMEAP website }}</ref> This index has ten points, which are further grouped into 4 bands: low, moderate, high and very high. Each of the bands comes with advice for at-risk groups and the general population.<ref name="defra_AQI">{{cite web|url=http://uk-air.defra.gov.uk/air-pollution/daqi|title=Daily Air Quality Index | publisher =[[Defra]]|work=Air UK Website}}</ref>

{| class="wikitable"
|-
! Air pollution banding !! Value!! Health messages for At-risk individuals!! Health messages for General population
|-
|style="color:#009900;font-weight:bold"| Low ||style="color:#009900;font-weight:bold"| 1–3|| Enjoy your usual outdoor activities.|| Enjoy your usual outdoor activities.
|-
|style="color:#FF9900;font-weight:bold"| Moderate||style="color:#FF9900;font-weight:bold"| 4–6|| Adults and children with lung problems, and adults with heart problems, who experience symptoms, should consider reducing strenuous physical activity, particularly outdoors. || Enjoy your usual outdoor activities.
|-
|style="color:#FF0000;font-weight:bold"| High||style="color:#FF0000;font-weight:bold"| 7–9|| Adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms. People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion. || Anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors.
|-
|style="color:#990099;font-weight:bold"| Very High||style="color:#990099;font-weight:bold" | 10|| Adults and children with lung problems, adults with heart problems, and older people, should avoid strenuous physical activity. People with asthma may find they need to use their reliever inhaler more often. || Reduce physical exertion, particularly outdoors, especially if you experience symptoms such as cough or sore throat.
|}

The index is based on the concentrations of 5 pollutants. The index is calculated from the concentrations of the following pollutants: Ozone, Nitrogen Dioxide, Sulphur Dioxide, PM2.5 (particles with an aerodynamic diameter less than 2.5 μみゅーm) and PM10. The breakpoints between index values are defined for each pollutant separately and the overall index is defined as the maximum value of the index. Different averaging periods are used for different pollutants.<ref name="defra_AQI" />

{| class="wikitable"
|-
! Index !! Ozone, Running 8 hourly mean (μみゅーg/m<sup>3</sup>) !! Nitrogen Dioxide, Hourly mean (μみゅーg/m<sup>3</sup>) !! Sulphur Dioxide, 15 minute mean (μみゅーg/m<sup>3</sup>) !! PM<sub>2.5</sub> Particles, 24 hour mean (μみゅーg/m<sup>3</sup>) !! PM<sub>10</sub> Particles, 24 hour mean (μみゅーg/m<sup>3</sup>)
|-style="background-color:#cfc; color:#444;font-weight:bold"
| 1|| 0–33 || 0–67 || 0–88 || 0–11 || 0–16
|-style="background-color:#6f6;color:#333;font-weight:bold"
| 2|| 34–66 || 68–134|| 89–177 || 12–23 || 17–33
|-style="background-color:#0f0;color:#222;font-weight:bold"
| 3|| 67–100 || 135–200 || 178–266 || 24–35 || 34–50
|-style="background-color:#9f0;color:#666;font-weight:bold"
| 4|| 101–120 || 201–267 || 267–354 || 36–41 || 51–58
|-style="background-color:#ff0;color:#666;font-weight:bold"
| 5|| 121–140 || 268–334|| 355–443 || 42–47 || 59–66
|-style="background-color:#fc0;color:#222;font-weight:bold"
| 6|| 141–160 || 335–400|| 444–532 || 48–53 || 67–75
|-style="background-color:#f60;color:#222;font-weight:bold"
| 7|| 161–187 || 401–467|| 533–710 || 54–58 || 76–83
|-style="background-color:#f30;color:#080805;font-weight:bold"
| 8|| 188-213 || 468–534|| 711–887 || 59–64 || 84–91
|-style="background-color:#f00;color:#fff;font-weight:bold"
| 9|| 214–240 || 535–600|| 888–1064 || 65–70 || 92–100
|-style="background-color:#f06;color:#FFF;font-weight:bold"
| 10|| ≥ 241 || ≥ 601 || ≥ 1065 || ≥ 71 || ≥ 101
|}

===Europe===
The ''Common Air Quality Index'' (CAQI)<ref name="aqnow_indices_web" /> is an air quality index used in Europe since 2006.<ref name="CAQI_definition" /> In November 2017, the [[European Environment Agency]] announced the ''European Air Quality Index'' (EAQI) and started encouraging its use on websites and for other ways of informing the public about air quality.<ref name="EAQI_announce" />

====CAQI====
{{As of|2012}}, the [[European Union|EU]]-supported project ''CiteairII'' argued that the CAQI had been evaluated on a "large set" of data, and described the CAQI's motivation and definition. ''CiteairII'' stated that having an air quality index that would be easy to present to the general public was a major motivation, leaving aside the more complex question of a health-based index, which would require, for example, effects of combined levels of different pollutants. The main aim of the CAQI was to have an index that would encourage wide comparison across the EU, without replacing local indices. ''CiteairII'' stated that the "main goal of the CAQI is not to warn people for possible adverse health effects of poor air quality but to attract their attention to urban air pollution and its main source (traffic) and help them decrease their exposure."<ref name="CAQI_definition" />

The CAQI is a number on a scale from 1 to 100, where a low value means good air quality and a high value means bad air quality. The index is defined in both hourly and daily versions, and separately near roads (a "roadside" or "traffic" index) or away from roads (a "background" index). {{As of|2012}}, the CAQI had two mandatory components for the roadside index, NO<sub>2</sub> and PM<sub>10</sub>, and three mandatory components for the background index, NO<sub>2</sub>, PM<sub>10</sub> and O<sub>3</sub>. It also included optional pollutants PM<sub>2.5</sub>, CO and SO<sub>2</sub>. A "sub-index" is calculated for each of the mandatory (and optional if available) components. The CAQI is defined as the sub-index that represents the worst quality among those components.<ref name="CAQI_definition" />

Some of the key pollutant densities in &mu;g/m<sup>3</sup> for the hourly background index, the corresponding sub-indices, and five CAQI ranges and verbal descriptions are as follows.<ref name="CAQI_definition" />
{| class="wikitable" style="text-align:center;"
|-
! Qualitative name !! Index or sub-index !! colspan="4"|Pollutant (hourly) density in &mu;g/m{{sup|3}}
|-
! colspan="2"| !! NO<sub>2</sub> !! PM<sub>10</sub> !! O<sub>3</sub> !! PM<sub>2.5</sub>(optional)
|-
| Very low || style="background:#79bc6a;" |0–25 ||0–50 ||0–25 ||0–60 ||0–15
|-
| Low || style="background:#bbcf4c;" |25–50 ||50–100 ||25–50 ||60–120 ||15–30
|-
| Medium || style="background:#eec20b;" |50–75 ||100–200||50–90 ||120–180||30–55
|-
| High || style="background:#f29305;" |75–100 ||200–400||90–180||180–240||55–110
|-
| Very high || style="background:#e8416f;"|>100 ||>400 ||>180 ||>240 ||>110
|-
|}

Frequently updated CAQI values and maps are shown on the ''www.airqualitynow.eu'' and other websites.<ref name="aqnow_indices_web" /> A separate ''Year Average Common Air Quality Index'' (YACAQI) is also defined, in which different pollutant sub-indices are separately normalised to a value typically near unity. For example, the yearly averages of NO<sub>2</sub>, PM<sub>10</sub> and PM<sub>2.5</sub> are divided by 40 &mu;g/m^3, 40 &mu;g/m^3 and 20 &mu;g/m^3, respectively. The overall background or traffic YACAQI for a city is the [[arithmetic mean]] of a defined subset of these sub-indices.<ref name="CAQI_definition" />

===United States===
[[Image:Pm25-24a-super.gif|thumb|PM<sub>2.5</sub> 24-Hour AQI Loop, Courtesy US EPA]]
The [[United States Environmental Protection Agency]] (EPA) has developed an Air Quality Index that is used to report air quality. This AQI is divided into six categories indicating increasing levels of health concern. An AQI value over 300 represents hazardous air quality and below 50 the air quality is good.<ref name="aqi_basic">{{cite web|url=http://www.airnow.gov/index.cfm?action=aqibasics.aqi | title = Air Quality Index (AQI) - A Guide to Air Quality and Your Health|date= 9 December 2011 | accessdate = 8 August 2012| publisher = US EPA}}</ref>
{|
|-
|
{| border="2" cellspacing="0" style="width:480px; float:left;"
|- style="vertical-align:top; background:#036;"
| style="width:31%;"| '''<span style="color:#fff; font-family:arial; font-size:1em;">Air Quality Index (AQI) Values</span>'''|| style="background:#036; width:32%;"|<span style="color:#fff; font-family:arial; font-size:1em;">'''Levels of Health Concern'''</span>|| style="background:#036; width:37%;"|<span style="color:#fff; font-family:arial; font-size:1em;">'''Colors'''</span>
|- style="vertical-align:top; background:#00e400;"
| style="width:31%;"| 0 to 50|| style="width:32%;"|Good|| style="width:37%;"|Green
|- style="vertical-align:top; background:#ff0;"
| style="width:31%;"| 51 to 100|| style="width:32%;"|Moderate|| style="width:37%;"|Yellow
|- style="vertical-align:top; background:#ff7e00;"
| style="width:31%;"| 101 to 150|| style="width:32%;"|Unhealthy for Sensitive Groups|| style="width:37%;"|Orange
|- style="vertical-align:top; background:#f00;"
| style="width:31%;"| <span style="color:#fff;">151 to 200</span>|| style="width:32%;"|<span style="color:#fff;">Unhealthy</span>|| style="width:37%;"|<span style="color:#fff;">Red</span>
|- style="vertical-align:top; background:#99004c;"
| style="width:31%;"| <span style="color:#fff;">201 to 300</span>|| style="width:32%;"|<span style="color:#fff;">Very Unhealthy</span>|| style="width:37%;"|<span style="color:#fff;">Purple</span>
|- style="vertical-align:top; background:#7e0023;"
| style="width:31%;"| <span style="color:#fff;">301 to 500</span>|| style="width:32%;"|<span style="color:#fff;">Hazardous</span>|| style="width:37%;"|<span style="color:#fff;">Maroon</span>
|}
|}

The AQI is based on the five "criteria" pollutants regulated under the [[Clean Air Act (USA) (1990)|Clean Air Act]]: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The EPA has established [[National Ambient Air Quality Standards]] (NAAQS) for each of these pollutants in order to protect public health. An AQI value of 100 generally corresponds to the level of the NAAQS for the pollutant.<ref name="aqi_basic" /> 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.

====Computing the AQI====

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 this equation is used:<ref>http://www.epa.gov/airnow/aqi-technical-assistance-document-dec2013.pdf</ref>

<math> I = \frac {I_{high} -I_{low}}{ C_{high} -C_{low} }(C-C_{low}) +I_{low} </math>

where:

:<math>I</math> = the (Air Quality) index,
:<math>C</math> = the pollutant concentration,
:<math>C_{low}</math>= the concentration breakpoint that is ≤ <math>C</math>,
:<math>C_{high}</math>= the concentration breakpoint that is ≥ <math>C</math>,
:<math>I_{low}</math>= the index breakpoint corresponding to <math>C_{low}</math>,
:<math>I_{high}</math>= the index breakpoint corresponding to <math>C_{high}</math>.

EPA's table of breakpoints is:<ref name="tech_aqi">{{cite book|title=Technical Assistance Document for the Reporting of Daily Air Quality – the Air Quality Index (AQI)|author=David Mintz|publisher=US EPA Office of Air Quality Planning and Standards| location = North Carolina|date=February 2009|id=EPA-454/B-09-001|url=http://www.epa.gov/airnow/aqi_tech_assistance.pdf|accessdate=9 August 2012}}</ref><ref name="tech_aqi2">{{cite book|title=Revised Air Quality Standards For Particle Pollution And Updates To The Air Quality Index (AQI)|publisher=US EPA Office of Air Quality Planning and Standards| location = North Carolina|date=2013|url=http://www.epa.gov/pm/2012/decfsstandards.pdf}}</ref><ref>http://www3.epa.gov/airquality/ozonepollution/pdfs/20151001fr.pdf</ref>
{| class="wikitable" style="text-align:left;"
|-
|'''[[Ozone|O<sub>3</sub>]] (ppb)'''||'''[[Ozone|O<sub>3</sub>]] (ppb)'''||'''[[Particulates|PM<sub>2.5</sub>]] (µg/m<sup>3</sup>)'''||'''[[Particulates|PM<sub>10</sub>]] (µg/m<sup>3</sup>)'''||'''[[Carbon monoxide|CO]] (ppm)'''||'''[[Sulfur dioxide|SO<sub>2</sub>]] (ppb)'''|| '''[[Nitrogen dioxide|NO<sub>2</sub>]] (ppb)'''||'''AQI'''|| '''AQI'''
|-
|''C<sub>low</sub>'' - ''C<sub>high</sub> (avg)''||''C<sub>low</sub>'' - ''C<sub>high</sub> (avg)''||''C<sub>low</sub>''- ''C<sub>high</sub> (avg)''||''C<sub>low</sub>'' - ''C<sub>high</sub> (avg)''||''C<sub>low</sub>'' - ''C<sub>high</sub> (avg)''||''C<sub>low</sub>'' - ''C<sub>high</sub> (avg)''||''C<sub>low</sub>'' - ''C<sub>high</sub> (avg)''||''I<sub>low</sub>'' - ''I<sub>high</sub>''|| '''Category'''
|-
|0-54 (8-hr) || - ||0.0-12.0 (24-hr)|| 0-54 (24-hr)|| 0.0-4.4 (8-hr)|| 0-35 (1-hr)|| 0-53 (1-hr)|| 0-50|| style="background:#00e400;" |Good
|-
|55-70 (8-hr)|| - ||12.1-35.4 (24-hr)|| 55-154 (24-hr)|| 4.5-9.4 (8-hr)|| 36-75 (1-hr)|| 54-100 (1-hr)|| 51-100|| style="background:#ff0;" |Moderate
|-
|71-85 (8-hr)|| 125-164 (1-hr)|| 35.5-55.4 (24-hr)|| 155-254 (24-hr)|| 9.5-12.4 (8-hr)|| 76-185 (1-hr)|| 101-360 (1-hr)|| 101-150 || style="background:#ff7e00;" |Unhealthy for Sensitive Groups
|-
|86-105 (8-hr)|| 165-204 (1-hr)|| 55.5-150.4 (24-hr)|| 255-354 (24-hr)|| 12.5-15.4 (8-hr)|| 186-304 (1-hr)|| 361-649 (1-hr)|| 151-200|| style="background:#f00; color:#fff;" |Unhealthy
|-
|106-200 (8-hr)|| 205-404 (1-hr)|| 150.5-250.4 (24-hr)|| 355-424 (24-hr)|| 15.5-30.4 (8-hr)|| 305-604 (24-hr)|| 650-1249 (1-hr)|| 201-300|| style="background:#99004c; color:#fff;" |Very Unhealthy
|-
| - || 405-504 (1-hr)|| 250.5-350.4 (24-hr)|| 425-504 (24-hr)|| 30.5-40.4 (8-hr)|| 605-804 (24-hr)||| 1250-1649 (1-hr)|| 301-400|| style="background:#7e0023; color:#fff;" rowspan = 2 | Hazardous
|-
| - || 505-604 (1-hr)|| 350.5-500.4 (24-hr)|| 505-604 (24-hr)|| 40.5-50.4 (8-hr)|| 805-1004 (24-hr)|| 1650-2049 (1-hr)|| 401-500
|}
Suppose a monitor records a 24-hour average fine particle (PM<sub>2.5</sub>) concentration of 12.0 micrograms per cubic meter. The equation above results in an AQI of:
:<math> \frac{50 -0}{12.0 -0}(12.0-0) +0=50</math>,
corresponding to air quality in the "Good" range. To convert an air pollutant concentration to an AQI, EPA has developed a calculator.<ref>{{cite web|url=https://www.airnow.gov/aqi/aqi-calculator|title=AQI Calculator|accessdate=7 October 2018}}</ref>

If multiple pollutants are measured at a monitoring site, then the largest or "dominant" AQI value is reported for the location. The ozone AQI between 100 and 300 is computed by selecting the larger of the AQI calculated with a 1-hour ozone value and the AQI computed with the 8-hour ozone value.

8-hour ozone averages do not define AQI values greater than 300; AQI values of 301 or greater are calculated with 1-hour ozone concentrations. 1-hour SO<sub>2</sub> values do not define higher AQI values greater than 200. AQI values of 201 or greater are calculated with 24-hour SO<sub>2</sub> concentrations.

Real time monitoring data from continuous monitors are typically available as 1-hour averages. However, computation of the AQI for some pollutants requires averaging over multiple hours of data. (For example, calculation of the ozone AQI requires computation of an 8-hour average and computation of the PM<sub>2.5</sub> or PM<sub>10</sub> AQI requires a 24-hour average.) To accurately reflect the current air quality, the multi-hour average used for the AQI computation should be centered on the current time, but as concentrations of future hours are unknown and are difficult to estimate accurately, EPA uses surrogate concentrations to estimate these multi-hour averages. For reporting the PM<sub>2.5</sub>, PM<sub>10</sub> and ozone air quality indices, this surrogate concentration is called the [[Nowcast (Air Quality Index)|NowCast]]. The Nowcast is a particular type of weighted average that provides more weight to the most recent air quality data when air pollution levels are changing.<ref>{{cite web|url=http://www.airnowapi.org/aq101|title=AirNow API Documentation|accessdate=20 August 2015}}</ref><ref>{{cite web|url=http://airnow.supportportal.com/link/portal/23002/23002/Article/16115/How-are-your-ozone-maps-calculated|title=How are your ozone maps calculated?|accessdate=20 August 2015}}</ref> There is a free email subscription service for New York inhabitants - AirNYC.<ref>{{Cite web|url=https://aerostate.io/cases/airnyc/|title=AirNYC - Air quality in New York. Online.|last=|first=|date=|website=|access-date=10 August 2017}}</ref> Subscribers get notification about AQI values changes for selected location (eg home address), based on air quality conditions.

====Public Availability of the AQI====

Real time monitoring data and forecasts of air quality that are color-coded in terms of the air quality index are available from EPA's AirNow web site.<ref>{{cite web|url=http://www.airnow.gov/|title=AirNow|accessdate=9 August 2012}}.</ref> Historical air monitoring data including AQI charts and maps are available at EPA's AirData website.<ref>{{cite web|url=http://www.epa.gov/airdata/|title=AirData - US Environmental Protection Agency|accessdate=20 August 2015}}</ref> Detailed map about current AQI level and its two day forecast is available from Aerostate web site.<ref>{{Cite web|url=https://browser.aerostate.io/|title=Aerostate AQI map|last=|first=|date=|website=aerostate.io|access-date=13 February 2018}}</ref>

====History of the AQI====

The AQI made its debut in 1968, when the National Air Pollution Control Administration undertook an initiative to develop an air quality index and to apply the methodology to [[Metropolitan Statistical Areas]]. The impetus was to draw public attention to the issue of air pollution and indirectly push responsible local public officials to take action to control sources of pollution and enhance air quality within their jurisdictions.

Jack Fensterstock, the head of the National Inventory of Air Pollution Emissions and Control Branch, was tasked to lead the development of the methodology and to compile the air quality and emissions data necessary to test and calibrate resultant indices.<ref>J.C Fensterstock et al., " The Development and Utilization of an Air Quality Index," Paper No. 69–73, presented at the 62nd Annual Meeting of the Air Pollution Control Administration, June 1969.</ref>

The initial iteration of the air quality index used standardized ambient pollutant concentrations to yield individual pollutant indices. These indices were then weighted and summed to form a single total air quality index. The overall methodology could use concentrations that are taken from ambient monitoring data or are predicted by means of a diffusion model. The concentrations were then converted into a standard statistical distribution with a preset mean and standard deviation. The resultant individual pollutant indices are assumed to be equally weighted, although values other than unity can be used. Likewise, the index can incorporate any number of pollutants although it was only used to combine SOx, CO, and TSP because of a lack of available data for other pollutants.

While the methodology was designed to be robust, the practical application for all metropolitan areas proved to be inconsistent due to the paucity of ambient air quality monitoring data, lack of agreement on weighting factors, and non-uniformity of air quality standards across geographical and political boundaries. Despite these issues, the publication of lists ranking metropolitan areas achieved the public policy objectives and led to the future development of improved indices and their routine application.

==See also==
{{Portal|Environment}}
*[[Air pollution]]
*[[Indoor air quality]]

==References==
{{Reflist|30em|refs=

<ref name="aqnow_indices_web">{{cite web|url=http://www.airqualitynow.eu/about_indices_definition.php|title=Indices definition|publisher=Air quality|accessdate=9 August 2012}}</ref>

<ref name="CAQI_definition">{{cite web| last1 =CiteairII — Common Information to European Air| first1 =| last2 =| first2 =| authorlink =| coauthors =| title =CAQI Air quality index — Comparing Urban Air Quality across Borders – 2012| work =| publisher =| date =2012-07-09| url =https://www.airqualitynow.eu/download/CITEAIR-Comparing_Urban_Air_Quality_across_Borders.pdf| format =[[PDF]]| doi =| accessdate =2018-02-18| archiveurl =https://www.webcitation.org/6xKhR4oXJ?url=https://www.airqualitynow.eu/download/CITEAIR-Comparing_Urban_Air_Quality_across_Borders.pdf| archivedate =2018-02-18| deadurl =no| df =}}</ref>

<ref name="EAQI_announce">{{cite web| last1 =| first1 =| last2 =| first2 =| authorlink =European Environment Agency| coauthors =| title =European Air Quality Index: current air quality information at your finger tips| work =| publisher =European Environment Agency| date =2017-11-16| url =https://www.eea.europa.eu/highlights/european-air-quality-index-current| format =| doi =| accessdate =2018-02-19| archiveurl =https://www.webcitation.org/6xKp6ngKA?url=https://www.eea.europa.eu/highlights/european-air-quality-index-current| archivedate =2018-02-19| deadurl =no| df =}}</ref>


}}

==External links==
Some of the following websites display actively updated air quality index maps; others are archived versions of inactive websites:
*Global:
**[http://aqicn.org/map/world/#@g/2.0574/7.9102/2z Worldwide Air Pollution: Real-time Air Quality Index Visual Map]
**[https://aerostate.io/ AQI at Aerostate.io] - Aerostate website about AQI real time data, forecasts, maps, analytics
*Europe:
**[http://www.airqualitynow.eu/index.php CAQI]- AirqualityNow
**[http://www.eea.europa.eu/themes/air/air-quality-index EAQI] - [[European Environment Agency]]
**[http://www.airquality.co.uk/archive/index.php The UK Air Quality Archive]
*North America:
**[http://airnow.gov AQI at airnow.gov] - cross-agency U.S. Government site
**[https://web.archive.org/web/20070623011137/http://air.state.nm.us/ New Mexico Air Quality and API data] - Example of how New Mexico Environment Department publishes their Air Quality and API data.
**[https://web.archive.org/web/20060706053637/http://www.msc-smc.ec.gc.ca/aq_smog/index_e.cfm AQI at Meteorological Service of Canada]
**[https://web.archive.org/web/20070613193732/http://www.sparetheair.org/data/air_quality.htm San Francisco Bay Area Spare-the-Air] - AQI explanation
*Asia:
**[http://eng.airkorea.or.kr/cai/main.jsp CAI at Airkorea.or.kr] - website of South Korea Environmental Management Corp.
**[http://www.doe.gov.my/index.php?option=com_content&task=view&id=188&Itemid=370&lang=en API at JAS (Malaysian Department of Environment)]
**[https://web.archive.org/web/20071009165620/http://haze.net.my/ Malaysia Air Pollution Index] ''(inactive {{as of|2018|02|19|lc=yes}})''
**[https://web.archive.org/web/20070410055414/http://www.epd-asg.gov.hk/ API at Hong Kong] - Environmental Protection Department of the Government of the Hong Kong Special Administrative Region ''(inactive {{as of|2018|02|19|lc=yes}})''
**[http://aqmthai.com/index.php?lang=en AQI in Thailand]
**[http://www.aqivn.org/en/ Unofficial PM25 AQI in Hanoi, Vietnam]

{{Pollution}}
{{Natural resources}}

[[Category:Atmosphere of Earth]]
[[Category:Air pollution]]
[[Category:Environmental indices]]
[[Category:Air quality index]]

Revision as of 15:26, 7 November 2018

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