Pollutant Standards Index

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.[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 (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.[3] 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.[4]

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.[5][6]

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.[7]

During haze episodes, PM2.5 is the most significant pollutant.[8]

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).[9]

PSIDescriptorGeneral Health Effects
51–100ModerateFew or none for the general population
101–200UnhealthyEveryone may begin to experience health effects; members of sensitive groups may experience more serious health effects. To stay indoors.
201–300Very unhealthyHealth warnings of emergency conditions. The entire population is more likely to be affected.
301+HazardousHealth 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 US EPA'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

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.[10] In June 2013, severe haze hit Singapore, pushing the nation's PSI into Hazardous levels for the first time in its history.[11] Presently, the highest 3-hour PSI reading on record in Singapore is 471 on 20 October 2015 at 11 pm (GMT+8).[12]

Association with health outcomes

Singapore's computation of PSI and NEA's definitions of PSI ranges have been shown to correlate with a number of health outcomes including all-cause mortality.[13] For sudden cardiac deaths, every increment of 30 units in PSI correlated with 8.15% increased risk of out-of-hospital cardiac arrest on the same day of exposure.[14] This risk was found to remain elevated for 1–5 days after exposure. Similar short-term associations were subsequently found for acute myocardial infarction and acute ischemic stroke in analyses of national registries.[15][16] In terms of healthcare utilization, both country-wide Emergency Department visits and hospital admissions were increased per unit increase in PSI.[17]


  1. "PSI Reading". National Environment Agency of Singapore. Retrieved 15 April 2014.
  2. "Frequently Asked Questions (FAQs) On Haze". National Environment Agency of Singapore. Archived from the original on 15 March 2014. 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. Archived from the original on 24 June 2013. Retrieved 20 June 2013.
  4. "National Environment Agency - Air Pollution FAQ".
  5. "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. Archived from the original on 16 April 2014. Retrieved 15 April 2014.
  6. "What is Air Quality Index AQI". Business Standard India.
  7. "Computation of the Pollutant Standards Index (PSI)" (PDF). National Environment Agency of Singapore. Retrieved 15 April 2014.
  8. "Frequently Asked Questions (FAQs) On Haze". National Environment Agency of Singapore. Retrieved 13 March 2016.
  9. "PSI Readings". National Environment Agency. Archived from the original on 1 February 2014. Retrieved 21 January 2014.
  10. "Singapore hit by highest haze levels in 16 years". BBC News. 18 June 2013. Retrieved 20 June 2013.
  11. "Haze in Singapore hits new high, PSI at 321 at 10pm". The Straits Times. 19 June 2013. Retrieved 20 June 2013.
  12. "PM2.5 levels hit 471 as haze situation worsens". Today. 20 October 2015.
  13. Cheong, Kang Hao; Ngiam, Nicholas Jinghao; Morgan, Geoffrey G.; Pek, Pin Pin; Tan, Benjamin Yong-Qiang; Lai, Joel Weijia; Koh, Jin Ming; Ong, Marcus Eng Hock; Ho, Andrew Fu Wah (6 September 2019). "Acute Health Impacts of the Southeast Asian Transboundary Haze Problem-A Review". International Journal of Environmental Research and Public Health. 16 (18): 3286. doi:10.3390/ijerph16183286. ISSN 1660-4601. PMC 6765769. PMID 31500215.
  14. Ho, Andrew Fu Wah; Wah, Win; Earnest, Arul; Ng, Yih Yng; Xie, Zhenjia; Shahidah, Nur; Yap, Susan; Pek, Pin Pin; Liu, Nan (November 2018). "Health impacts of the Southeast Asian haze problem – A time-stratified case crossover study of the relationship between ambient air pollution and sudden cardiac deaths in Singapore". International Journal of Cardiology. 271: 352–358. doi:10.1016/j.ijcard.2018.04.070. PMID 30223374. S2CID 52282745.
  15. Ho, Andrew F. W.; Zheng, Huili; De Silva, Deidre A.; Wah, Win; Earnest, Arul; Pang, Yee H.; Xie, Zhenjia; Pek, Pin P.; Liu, Nan (November 2018). "The Relationship Between Ambient Air Pollution and Acute Ischemic Stroke: A Time-Stratified Case-Crossover Study in a City-State With Seasonal Exposure to the Southeast Asian Haze Problem". Annals of Emergency Medicine. 72 (5): 591–601. doi:10.1016/j.annemergmed.2018.06.037. ISSN 1097-6760. PMID 30172448. S2CID 52144033.
  16. Ho, Andrew Fu Wah; Zheng, Huili; Earnest, Arul; Cheong, Kang Hao; Pek, Pin Pin; Seok, Jeon Young; Liu, Nan; Kwan, Yu Heng; Tan, Jack Wei Chieh (19 March 2019). "Time‐Stratified Case Crossover Study of the Association of Outdoor Ambient Air Pollution With the Risk of Acute Myocardial Infarction in the Context of Seasonal Exposure to the Southeast Asian Haze Problem". Journal of the American Heart Association. 8 (6): e011272. doi:10.1161/JAHA.118.011272. ISSN 2047-9980. PMC 6475051. PMID 31112443.
  17. Chan, Sze Ling; Ho, Andrew Fw; Ding, Huicong; Liu, Nan; Earnest, Arul; Koh, Mariko S.; Chuah, Jolyn St; Lau, Zheng Yi; Tan, Kelvin Bryan; Zheng, Huili; Morgan, Geoffrey G. (February 2020). "Impact of Air Pollution and Trans-Boundary Haze on Nation-Wide Emergency Department Visits and Hospital Admissions in Singapore". Annals of the Academy of Medicine, Singapore. 49 (2): 78–87. doi:10.47102/annals-acadmedsg.2019209. ISSN 0304-4602. PMID 32246709. S2CID 214786174.
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