The Pollutant Standards Index (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 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. History The PSI is based on a scale devised by the United States Environmental Protection Agency (EPA) 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 US 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. Definition of the PSI used in Singapore 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). The concentrations of these pollutants in the ambient air are measured via a network of air monitoring stations located around Singapore. A sub-index value is computed for each pollutant based on the pollutant's ambient air concentration. The highest sub-index value is then taken as the PSI value. In other words, the PSI is determined by the pollutant with the most significant concentration. During haze episodes, PM2.5 is the most significant pollutant. The PSI is reported as a number on a scale of 0 to 500. The index figures enable the public to determine whether the air pollution levels in a particular location are good, unhealthy, hazardous or worse. The following