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. Vietnam On November 12, 2019, Vietnam Environment Administration issued Decision No. 1459/QD-TCMT on promulgating Technical Guidelines for calculation and publication of Vietnam Air Quality Index (VN_AQI). See also Air pollution Air pollution measurement Indoor air quality Air pollution forecasting References External links Some of the following websites display actively updated air quality index maps; others are archived versions of inactive websites: Global: Worldwide Air Pollution: Real-time Air Quality Index Visual Map Europe: CAQI- AirqualityNow EAQI - European Environment Agency The UK Air Quality Archive North America: 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 San Francisco Bay Area Spare-the-Air - AQI explanation Asia: CAI at Airkorea.or.kr - website of South Korea Environmental Management Corp. API at JAS (Malaysian Department of Environment) Malaysia Air Pollution Index (inactive ) API at Hong Kong - Environmental Protection Department of the Government of the Hong Kong Special Administrative Region (inactive ) AQI in Thailand AQI in Vietnam AQI in Hanoi Unofficial PM25 AQI in Hanoi, Vietnam Atmosphere of Earth Air pollution Environmental indices Pollution