Air quality index: Difference between revisions

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Air quality is defined as a measure of the condition of air relative to the requirements of one or more biotic species and or to any human need or purpose.^[1]

The Air Quality Index (AQI) (also known as the Air Pollution Index (API) or Pollutant Standard Index (PSI) is a number used by government agencies to characterize the quality of the air at a given location. As the AQI increases, an increasingly large percentage of the population is likely to experience increasingly severe adverse health effects. To compute the AQI requires an air pollutant concentration from a monitor or model. The function used to convert from air pollutant concentration to AQI varies by pollutant, and is different in different countries. Air quality index values are divided into ranges, and each range is assigned a descriptor and a color code. Standardized public health advisories are associated with each AQI range. An agency might also encourage members of the public to take public transportation or work from home when AQI levels are high.

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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.

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.

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

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.

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.^[2]

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.^[2]

API and Health Implications (Daily Targets)^[2]

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

Singapore uses the Pollutant Standards Index to report on its air quality.^[3] The PSI chart below is grouped by index values and descriptors, according to the National Environment Agency.^[4]

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://eng.airkorea.or.kr/cai/main.jsp

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.

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

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:

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

is used, where:

${\displaystyle I}$ = the (Air Quality) index,

${\displaystyle C}$ = the pollutant concentration,

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

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

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

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

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:

${\displaystyle C_{low}}$	${\displaystyle C_{high}}$	${\displaystyle I_{low}}$	${\displaystyle I_{high}}$	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:

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

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.

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

• 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.