Pollutant Standards Index

This article is about the air pollution index used in Singapore. For air pollution indices in general, see Air quality 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.[1]

Besides Singapore, some other countries also use air quality indices. However, the calculations used to derive their air quality indices may differ.[2] 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 PM2.5 Concentration 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.[3]

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.[4]

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.[5]

During haze episodes, PM2.5 is the most significant pollutant.[6]

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).[7]

PSI Descriptor General Health Effects
0–50 Good None
51–100 Moderate Few or none for the general population
101–200 Unhealthy Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects. To stay indoors.
201-300 Very unhealthy Health warnings of emergency conditions. The entire population is more likely to be affected.
301+ Hazardous Health alert: everyone may experience more serious health effects

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.

Record values of the PSI

Main article: Southeast Asian haze

Singapore has been regularly 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 large plantation owners to clear their land, as opposed to a more expensive and inconvenient mechanical approach using excavators and bulldozers.[8] In June 2013, severe haze hit Singapore, pushing the nation's PSI into Hazardous levels for the first time in its history.[9] Presently, the highest 3-hour PSI reading on record in Singapore is 401 on 21 June 2013 at 12 noon (GMT+8).[10]


  1. "PSI Reading". National Environment Agency of Singapore. Retrieved 15 April 2014.
  2. "Frequently Asked Questions (FAQs) On Haze". National Environment Agency of Singapore. Retrieved 15 April 2014.
  3. "Govt says it will move towards publishing 24-hour PSI, PM2.5 data on hourly basis". TODAY. 20 June 2013. Retrieved 20 June 2013.
  4. "Written Reply by Dr Vivian Balakrishnan, Minister for the Environment and Water Resources to Parliamentary Question on Air Quality Reporting". Ministry of the Environment & Water Resources (Singapore). 10 September 2012. Retrieved 15 April 2014.
  5. "Computation of the Pollutant Standards Index (PSI)" (PDF). National Environment Agency of Singapore. Retrieved 15 April 2014.
  6. "Frequently Asked Questions (FAQs) On Haze". National Environment Agency of Singapore. Retrieved 13 March 2016.
  7. "PSI Readings". National Environment Agency. Retrieved 21 January 2014.
  8. "Singapore hit by highest haze levels in 16 years". BBC News. 18 June 2013. Retrieved 20 June 2013.
  9. "Haze in Singapore hits new high, PSI at 321 at 10pm". The Straits Times. 19 June 2013. Retrieved 20 June 2013.
  10. "PSI hits new all-time high of 401 on Friday". Channel NewsAsia. 21 June 2013. Retrieved 21 June 2013.

External links

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