Air Pollution Index
The Air Pollution Index (API) is a simple and generalized way to describe the air quality in China, Hong Kong and Malaysia. It is calculated from several sets of air pollution data. In mainland China the API was replaced by an updated Air Quality Index in early 2012.
China's State Environment Protection Agency (SEPA) is responsible for measuring the level of air pollution in China. As of 28 August 2008, SEPA 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.
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)
|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.|
Hong Kong 
The API has been in use in Hong Kong since June 1995. It is measured and updated hourly by the Environmental Protection Department (EPD). Moreover, the EPD makes forecast on the API for the following day everyday.
The API is based on the level of 6 atmospheric pollutants, namely sulfur dioxide (SO2), nitrogen dioxide (NO2), respirable suspended particulates, carbon monoxide (CO), ozone (O3), lead (Pb), measured at all the monitoring stations throughout the territory.
There are 11 General Stations and 3 Roadside Stations. The former includes Central / Western, Eastern, Kwai Chung, Kwun Tong, Sha Tin, Sham Shui Po, Tai Po, Tap Mun, Tsuen Wan, Tung Chung, and Yuen Long; the latter Causeway Bay, Central, and Mong Kok.
In Hong Kong, there are two types of API: General API and Roadside API. The EPD reports the latest APIs hourly.
The index and the air quality objectives were set in 1987; and pollutant levels are measured over varying periods, in μg/m3. There are hourly, 24 hour and annual targets for sulfur dioxide and nitrogen dioxide, and 24 hour and annual targets for particulates.
The table below shows the official Health Implications of the respective API levels in Hong Kong.
|0 - 25||Low||Not expected.|
|26 - 50||Medium||Not 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).|
In 1998, the Education Bureau's recommended schools to curtail outdoor activities when the index reached 200, whereas leading healthcare advocates are urging that the level be revised to 100. The World Health Organisation revised its air quality guideline levels of sulfur dioxide, nitrogen dioxide and ozone in 2006 in light of new scientific evidence. The WHO also introduced new measurement guidelines for very small particulates which are more dangerous to pulmonary function. At the '200' level, Hong Kong levels of SO2 (800μg/m3) and NO2 (1,130μg/m3) are 40 times and 5½ times WHO guidelines respectively; the equivalent for particulates (350μg/m3) is 7 times WHO guidelines.
Similar to Hong Kong, the air quality in Malaysia is reported as the API or Air Pollutant Index. Four of the index's pollutant components (i.e., carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide) are reported in ppmv but PM10 particulate matter is reported in μg/m3.
This scale below shows the Health classifications used by the Malaysian government.
|0 - 50||Good|
|51 - 100||Moderate|
|101 - 200||Unhealthy|
|201 - 300||Very unhealthy|
If the API exceeds 500, a state of emergency is declared in the reporting area. Usually, this means that non-essential government services are suspended, and all ports in the affected area are closed. There may also be a prohibition on private sector commercial and industrial activities in the reporting area excluding the food sector.
See also 
- Air pollution
- Air Quality Index
- Air Quality Health Index
- Atmospheric dispersion modeling
- Emission standard
- European emission standards
- Pollutant Standards Index
- Pollution index based on index set 20 years ago, p5, South China Morning Post, 30 September 2007
- "Air Pollution Index". Environmental Protection Department. 2004. Archived from the original on 10 April 2007. Retrieved 18 April 2007.
- Foul-air gauge for pupils too high, say critics, p5, South China Morning Post, 30 September 2007
- Ahmad, Asmala and Hashim, Mazlan and Hashim, Md Noorazuan and Ayof, Mohd Nizam and Budi, Agus Setyo (2006), The Use of Remote Sensing and GIS to Estimate Air Quality Index (AQI) Over Peninsular Malaysia. GIS development . 5pp.