Air quality

Not to be confused with Qualities of air.

"Bad air quality" redirects here. For the obsolete medical theory, see Bad air.

An air quality measurement station in Edinburgh, Scotland

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. Air quality indices (AQI) are numbers 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

Signboard in Gulfton, Houston indicating an ozone watch

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, leading to haze.

Indices by location

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 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 behavioral 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.

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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.

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.

API	Air Pollution Level	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.
101 - 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 Ministry 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.

API Mechanics
An individual score is assigned to the level of each pollutant and the final API is the highest of those 5 scores. The pollutants can be measured quite differently. SO₂, NO₂ and PM₁₀ concentration are measured as average per day. CO and O₃ are more harmful and are measured as average per hour. The final API value is calculated per day.

The scale for each pollutant is non-linear, as is the final API score. Thus an API of 100 does not mean twice the pollution of API at 50, nor does it mean twice as harmful. While an API of 50 from day 1 to 182 and API of 100 from day 183 to 365 does provide an annual average of 75, it does not mean the pollution is acceptable even if the benchmark of 100 is deemed safe. This is because the benchmark is a 24 hour target. The annual average must match against the annual target. It is entirely possible to have safe air every day of the year but still fail the annual pollution benchmark.

API and Health Implications (Daily Targets)

API	Air Pollution Level	Health Implications
0 - 50	Excellent	No health implications
51 -100	Good	No health implications
101-150	Slightly Polluted	Slight irritations may occur, individuals with breathing or heart problems should reduce outdoor exercise.
151-200	Lightly Polluted	Slight irritations 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.

Mexico

Main article: Índice Metropolitano de la Calidad del Aire

The air quality in Mexico is reported in IMECAs. The IMECA is calculated using the measurements of average times of the chemicals ozone (O₃), sulphur dioxide (SO₂), nitrogen dioxide (NO₂), carbon monoxide (CO) and particles lower than 10 micrometers (PM₁₀).

Singapore

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

PSI	Descriptor	General Health Effects
0 - 50		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 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 http://www.airkorea.or.kr/airkorea/eng/

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 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.

United Kingdom

AEA Technology issue air quality forecasts for the UK on behalf of DEFRA wherein the level of pollution is described either as an index (ranging from 1 to 10) or as a banding (low, moderate, high or very high). These levels are based on the health effects of each pollutant.

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

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

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

Europe

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:

Pollution	Index Value
Very low	0/25
Low	25/50
Medium	50/75
High	75/100
Very high	>100

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.

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

PM_2.5 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,

$C_{low}$= the concentration breakpoint that is ≤ $C$,

$C_{high}$= the concentration breakpoint that is ≥ $C$,

$I_{low}$= the index breakpoint corresponding to $C_{low}$,

$I_{high}$= the index breakpoint corresponding to $C_{high}$.

For breakpoints for different pollutants, see: http://www.epa.gov/airnow/aqi_tech_assistance.pdf.

For example, suppose a monitor records a 24-hour average fine particle (PM_2.5) concentration of 12.0 micrograms per cubic meter. EPA's table of breakpoints for PM_2.5 is:

$C_{low}$	$C_{high}$	$I_{low}$	$I_{high}$	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: http://airnow.gov/index.cfm?action=aqi.conc_aqi_calc.
To convert an air quality index to an air pollutant concentration, see: http://airnow.gov/index.cfm?action=aqi.aqi_conc_calc.
Current ambient monitoring data and forecasts of air quality that are color-coded in terms of the air quality index are available at: http://www.airnow.gov/.

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.

Abbreviations in this article

In the context of this article about air quality:

• ppbv=parts per billion by volume = a portion anomaly 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
• PM_2.5 = 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

• * British Columbia

• * Hong Kong

See also

• Air pollution
• Air Pollution Index
• Atmospheric dispersion modeling
• Barbecue
• Grilling
• Emission standard
• Radon mitigation

• European emission standards
• Haze
• Indoor air quality
• National Ambient Air Quality Standards - U.S. standards for EPA intervention
• Pollutant Standards Index
• Smog

External links

• 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
• The pollution index of the UK Met Office
• API at JAS (Malaysian Department of Environment)
• API at Hong Kong - Environmental Protection Department of the Government of the Hong Kong Special Administrative Region
• San Francisco Bay Area Spare-the-Air - AQI explanation
• Malaysia Air Pollution Index
• AQI in Thailand provinces and in Bangkok
• The American Lung Association declares EPA standards fall short.