Pollutant Standards Index

The Pollutant Standards Index, or PSI, provides a uniform system of measuring pollution levels for the major air pollutants. It 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. However, 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.

Singapore has yet to replace the PSI with the Air Quality Index. Instead, it publishes the PSI and the PM2.5 Concentration separately. Although PSI is derived by averaging data collected for the past 24 hours, Singapore also publishes a PSI based on data from the past 3 hours. 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.

Definition of the PSI used in Singapore
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 PSI table is grouped by index values and descriptors, explaining the effects of the levels, according to Singapore's National Environment Agency (NEA).

Note: This chart reflects the guidelines used in Singapore and may differ from other countries. Health advisories are based on the USEPA’s guidelines. Only the 24-hour PSI value and not the 3-hour PSI value is correlated to the health effects outlined in NEA’s advisories. NEA’s health advisories are determined by the worse of the 24-hour PSI and 24-hour PM2.5 value.

The NEA publishes the current readings on the last 24-hour cycle of both the PSI and the PM2.5 readings.

Record values of the PSI
Singapore has been occasionally hit by smoke haze from forest fires in nearby Sumatra, Indonesia, brought over by wind. These forest fires have been attributed to the slash-and-burn method favoured by several farmers to clear their land, as opposed to a more expensive and inconvenient mechanical approach using excavators and bulldozers. In June 2013, severe haze hit Singapore, pushing the nation's PSI into Hazardous levels for the first time in its history. Presently, the highest 3-hour PSI reading on record in Singapore is 401 on 21 June 2013 at 12 noon (GMT+8).