Pollutant Standards Index

The Pollutant Standards Index, or PSI, is a type of air quality index, which is a number used to indicate the level of pollutants in air. Initially PSI was based on five air pollutants, but since 1 April 2014 it has also included fine particulate matter (PM2.5). In addition to the PSI derived by averaging data collected for the past 24 hours, Singapore also publishes a 3h-PSI based on PM2.5 concentrations for the past 3 hours. 1-hr PM2.5 concentrations are also published every hour. Besides Singapore, some other countries also use air quality indices. However, the calculations used to derive their air quality indices may differ. Different countries also use different names for their indices such as Air Quality Health Index, Air Pollution Index and Pollutant Standards Index. The PSI is based on a scale devised by the United States Environmental Protection Agency (USEPA) to provide a way for broadcasts and newspapers to report air quality on a daily basis. The PSI has been used in a number of countries including the United States and Singapore. Since 1999, the United States EPA has replaced the Pollution Standards Index (PSI) with the Air Quality Index (AQI) to incorporate new PM2.5 and ozone standards. Prior to 1 April 2014, Singapore published the PSI and the 1-hour PM2.5 reading separately. This 3-hour PSI is unique to Singapore and was introduced in 1997 to provide additional air quality information which would better reflect a more current air quality situation. In 2016, the 3-hour PSI was phased out on the grounds that the 1-hour PM2.5 reading was a better indicator of the current air quality. The PSI considers six air pollutants - sulphur dioxide (SO2), particulate matter (PM10), fine particulate matter (PM2.5), nitrogen dioxide (NO2), carbon monoxide (CO) and ozone (O3).