Air quality index
|This article relies on references to primary sources. (August 2012)|
An air quality index (AQI) is a number used by government agencies to communicate to the public how polluted the air is currently or how polluted it is forecast to become. 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 which are not all consistent. Different countries also use different names for their indices such as Air Quality Health Index, Air Pollution Index and Pollutant Standards Index.
- 1 Definition and usage
- 2 Indices by location
- 3 See also
- 4 References
- 5 External links
Definition and usage
Air quality is defined as a measure of the condition of air relative to the requirements of one or more biotic species or to any human need or purpose. 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.
The AQI can go up (meaning worse air quality) due to a lack of dilution of air pollutants. 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 and hazy conditions. An agency might 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.
Indices by location
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 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.
|Risk:||Low (1–3)||Moderate (4–6)||High (7–10)||Very high (above 10)|
|Health Risk||Air Quality Health Index||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.|
On the 30th December 2013 Hong Kong replaced the Air Pollution Index with a new index called the Air Quality Health Index. This index is 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:
|Health risk category||AQHI|
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.
China's Ministry of Environmental Protection (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 API level is based on the level of 6 atmospheric pollutants, namely sulfur dioxide (SO2), nitrogen dioxide (NO2), suspended particulates smaller than 10 μm in aerodynamic diameter (PM10), suspended particulates smaller than 2.5 μm in aerodynamic diameter (PM2.5)， carbon monoxide (CO), and ozone (O3) measured at the monitoring stations throughout each city.
An individual score(IAQI) is assigned to the level of each pollutant and the final AQI is the highest of those 6 scores. The pollutants can be measured quite differently. PM2.5、PM10 concentration are measured as average per 24h. SO2, NO2, O3, CO are measured as average per hour. The final API value is calculated per hour according to a formula published by the MEP.
The scale for each pollutant is non-linear, as is the final AQI score. Thus an AQI of 100 does not mean twice the pollution of AQI at 50, nor does it mean 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. 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.
AQI and Health Implications (HJ 663-2012)
|0–50||Excellent||No health implications.|
|51–100||Good||Few hypersensitive individuals should reduce outdoor exercise.|
|101–150||Lightly Polluted||Slight irritations may occur, individuals with breathing or heart problems should reduce outdoor exercise.|
|151–200||Moderately Polluted||Slight irritations may occur, individuals with breathing or heart problems should reduce outdoor exercise.|
|201–300||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 Mexico City 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 smaller than 10 micrometers (PM10).
Singapore uses the Pollutant Standards Index to report on its air quality, with details of the calculation similar but not identical to that used in Malaysia and Hong Kong The PSI chart below is grouped by index values and descriptors, according to the National Environment Agency.
|PSI||Descriptor||General Health Effects|
|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.|
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.
|0–50||Good||A level that will not impact patients suffering from diseases related to air pollution.|
|51–100||Moderate||A level that 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 that may have a serious impact on patients and members of sensitive groups in case of acute exposure.|
|351–500||Hazardous||A level that may need to take emergency measures for patients and members of sensitive groups and have harmful impacts on the general public.|
The N Seoul Tower on 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 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.
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). 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.
|Air pollution banding||Value||Health messages for At-risk individuals||Health messages for General population|
|Low||1–3||Enjoy your usual outdoor activities.||Enjoy your usual outdoor activities.|
|Moderate||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.|
|High||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.|
|Very High||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 μm) 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.
|Index||Ozone, Running 8 hourly mean (μg/m3)||Nitrogen Dioxide, Hourly mean (μg/m3)||Sulphur Dioxide, 15 minute mean (μg/m3)||PM2.5 Particles, 24 hour mean (μg/m3)||PM10 Particles, 24 hour mean (μg/m3)|
|10||≥ 241||≥ 601||≥ 1065||≥ 71||≥ 101|
To present the air quality situation in European cities in a comparable and easily understandable way, all detailed measurements are transformed into a single relative figure: the Common Air Quality Index (or CAQI) Three different indices have been developed by Citeair to enable the comparison of three different time scale:.
- An hourly index, which describes the air quality today, based on hourly values and updated every hours,
- A daily index, which stands for the general air quality situation of yesterday, based on daily values and updated once a day,
- An annual index, which represents the city's general air quality conditions throughout the year and compare to European air quality norms. This index is based on the pollutants year average compare to annual limit values, and updated once a year.
However, the proposed indices and the supporting common web site www.airqualitynow.eu are designed to give a dynamic picture of the air quality situation in each city but not for compliance checking.
The hourly and daily common indices
These indices have 5 levels using a scale from 0 (very low) to > 100 (very high), it is a relative measure of the amount of air pollution. They are based on 3 pollutants of major concern in Europe: PM10, NO2, O3 and will be able to take into account to 3 additional pollutants (CO, PM2.5 and SO2) where data are also available.
The calculation of the index is based on a review of a number of existing air quality indices, and it reflects EU alert threshold levels or daily limit values as much as possible. In order to make cities more comparable, independent of the nature of their monitoring network two situations are defined:
- Background, representing the general situation of the given agglomeration (based on urban background monitoring sites),
- Roadside, being representative of city streets with a lot of traffic, (based on roadside monitoring stations)
The indices values are updated hourly (for those cities that supply hourly data) and yesterdays daily indices are presented.
Common air quality index legend:
The common annual air quality index
The common annual air quality index provides a general overview of the air quality situation in a given city all the year through and regarding to the European norms.
It is also calculated both for background and traffic conditions but its principle of calculation is different from the hourly and daily indices. It is presented as a distance to a target index, this target being derived from the EU directives (annual air quality standards and objectives):
- If the index is higher than 1: for one or more pollutants the limit values are not met.
- If the index is below 1: on average the limit values are met.
The annual index is aimed at better taking into account long term exposure to air pollution based on distance to the target set by the EU annual norms, those norms being linked most of the time to recommendations and health protection set up by World Health Organisation.
The United States Environmental Protection Agency (EPA) has developed an index called the Air Quality Index which they use to report daily air quality. This AQI is divided into six categories indicating increasing levels of health concern. An AQI value over 300 represents hazardous air quality whereas if it is below 50 the air quality is good.
The AQI is based on the five pollutants regulated by the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The EPA has established National Ambient Air Quality Standards for all of these pollutants to protect public health. An AQI value of 100 generally corresponds to the standard for the pollutant.
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:
- = the (Air Quality) index,
- = the pollutant concentration,
- = the concentration breakpoint that is ≤ ,
- = the concentration breakpoint that is ≥ ,
- = the index breakpoint corresponding to ,
- = the index breakpoint corresponding to .
|35.5||55.4||101||150||Unhealthy for Sensitive Groups|
For example, suppose a monitor records a 24-hour average fine particle (PM2.5) concentration of 12.0 micrograms per cubic meter. The equation above results in an AQI of:
corresponding to air quality in the "Good" range. For reporting the PM2.5 AQI for the current day, when less than 24 hours of hourly PM2.5 data are available to construct the 24-hour average, EPA has developed the NowCast, which uses a weighted average of the most recent 12-hour period as surrogate for the 24-hour average.
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. Monitoring data and forecasts of air quality that are color-coded in terms of the air quality index are available from the AIRNow web site. 
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.
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.
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.
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- "Environment Canada - Air - AQHI categories and explanations". Ec.gc.ca. 2008-04-16. Retrieved 2011-11-11.
- Hsu, Angel. "China’s new Air Quality Index: How does it measure up?". Retrieved 8 February 2014.
- "Air Quality Health Index". Government of the Hong Kong Special Administrative Region. Retrieved 9 February 2014.
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- "What's CAI". Air Korea. Retrieved 8 August 2012.
- "Improved Air Quality Reflected in N Seoul Tower". Chosun Ilbo. 18 May 2012. Retrieved 29 July 2012.
- COMEAP. "Review of the UK Air Quality Index". COMEAP website.
- "Daily Air Quality Index". Air UK Website. Defra.
- Garcia, Javier; Colosio, Joëlle (2002). Air-quality indices : elaboration, uses and international comparisons. Presses des MINES. ISBN 2-911762-36-3.
- "Indices definition". www.airqualitynow.eu. Retrieved 9 August 2012.
- David Mintz (February 2009). Technical Assistance Document for the Reporting of Daily Air Quality – the Air Quality Index (AQI). North Carolina: US EPA Office of Air Quality Planning and Standards. EPA-454/B-09-001. Retrieved 9 August 2012.
- Revised Air Quality Standards For Particle Pollution And Updates To The Air Quality Index (AQI). North Carolina: US EPA Office of Air Quality Planning and Standards. 2013.
- "AQI Calculator: Concentration to AQI". Retrieved 9 August 2012.
- "AirNow". Retrieved 9 August 2012..
- 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.
- CAQI in Europe- AirqualityNow website
- CAI at Airkorea.or.kr - website of South Korea Environmental Management Corp.
- 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
- 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