Pollutant Standards Index

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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 is reported as a number on a scale of 0 to 500 and is the air quality indicator. These index figures enable the public to determine whether the air pollution levels in a particular location are good, unhealthy, hazardous or worse. The PSI is 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.

PSI in Singapore[edit]

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.[1]

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.[2] In June 2013, severe haze hit Singapore, pushing the nation's PSI into Hazardous levels for the first time in its history.[3] Presently, the highest 3-hour PSI reading on record in Singapore is 401 on 21 June 2013 at 12 noon (GMT+8).[4]

The following PSI table is grouped by index values and descriptors, explaining the effects of the various bracketed levels as coloured above, according to the National Environment Agency (NEA).

PSIDescriptorGeneral Health Effects
51–100ModerateFew or none for the general population
101–150Unhealthy for sensitive groupsMembers of sensitive groups may experience health effects. The general public is not likely to be affected.
151–200UnhealthyEveryone may begin to experience health effects; members of sensitive groups may experience more serious health effects.
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 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.[5]

API (IPU) in Malaysia[edit]

The air quality in Malaysia is reported as the API Air Pollution Index or in Malay as IPU (Indeks Pencemaran Udara), which is based closely on the PSI.[6] Four of the index's pollutant components (i.e., carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide) are reported in ppmv but PM10 particulate matter is reported in μg/m³.

Unlike the American AQI, the index number can exceed 500. Above 500, a state of emergency is declared in the reporting area. Usually, this means that non-essential government services are suspended, and all ports in the affected area closed. There may also be a prohibition on private sector commercial and industrial activities in the reporting area excluding the food sector.

AQHI in Canada[edit]

In Canada air pollution and the health risks associated are measured with the Air Quality Health Index or (AQHI). It is a health protection tool used to make decisions to reduce short-term exposure to air pollution by adjusting activity levels during increased levels of air pollution.

The Air Quality Health Index or "AQHI" is a federal program jointly coordinated by Health Canada and Environment Canada. However, the AQHI program would not be possible without the commitment and support of the provinces, municipalities and NGOs. From air quality monitoring to health risk communication and community engagement, local partners are responsible for the vast majority of work related to AQHI implementation. The AQHI provides a number from 1 to 10+ to indicate the level of health risk associated with local air quality. Occasionally, when the amount of air pollution is abnormally high, the number may exceed 10. The AQHI provides a local air quality current value as well as a local air quality maximums forecast for today, tonight and tomorrow and provides associated health advice.

Risk:Low (1-3)Moderate (4-6)High (7-10)Very high (above 10)

As it is now known that even low levels of air pollution can trigger discomfort for the sensitive population, the index has been developed as a continuum: The higher the number, the greater the health risk and need to take precautions. The index describes the level of health risk associated with this number as ‘low’, ‘moderate’, ‘high’ or ‘very high’, and suggests steps that can be taken to reduce exposure.


Health RiskAir Quality Health IndexHealth Messages
At Risk population*General Population
Low1-3Enjoy your usual outdoor activities.Ideal air quality for outdoor activities
Moderate4-6Consider reducing or rescheduling strenuous activities outdoors if you are experiencing symptoms.No need to modify your usual outdoor activities unless you experience symptoms such as coughing and throat irritation.
High7-10Reduce or reschedule strenuous activities outdoors. Children and the elderly should also take it easy.Consider reducing or rescheduling strenuous activities outdoors if you experience symptoms such as coughing and throat irritation.
Very highAbove 10Avoid strenuous activities outdoors. Children and the elderly should also avoid outdoor physical exertion.Reduce or reschedule strenuous activities outdoors, especially if you experience symptoms such as coughing and throat irritation.


It is measured based on the observed relationship of Nitrogen Dioxide (NO2), ground-level Ozone (O3) and particulate matter (PM2.5) with mortality from an analysis of several Canadian cities. Significantly, all three of these pollutants can pose health risks, even at low levels of exposure, especially among those with pre-existing health problems.

When developing the AQHI, Health Canada’s original analysis of health effects included five major air pollutants: particulate matter, ozone, and nitrogen dioxide (NO2), as well as sulphur dioxide (SO2), and carbon monoxide (CO). The latter two pollutants provided little information in predicting health effects and were removed from the AQHI formulation.

The AQHI does not measure the effects of odour, pollen, dust, heat or humidity.

See also[edit]


  1. ^ "Govt says it will move towards publishing 24-hour PSI, PM2.5 data on hourly basis". TODAY. 20 June 2013. Retrieved 20 June 2013. 
  2. ^ "Singapore hit by highest haze levels in 16 years". BBC News. 18 June 2013. Retrieved 20 June 2013. 
  3. ^ "Haze in Singapore hits new high, PSI at 321 at 10pm". The Straits Times. 19 June 2013. Retrieved 20 June 2013. 
  4. ^ "PSI hits new all-time high of 401 on Friday". Channel NewsAsia. 21 June 2013. Retrieved 21 June 2013. 
  5. ^ PSI and PM2.5 Readings and Haze Satellite Images & PSI Summary, National Environmental Agency, Singapore (downloaded June 20, 2013)
  6. ^ "Air Quality", Malaysia Department of Environment
  7. ^ "Environment Canada - Air - About the Air Quality Health Index". Ec.gc.ca. 2013-04-10. Retrieved 2013-06-23. 
  8. ^ "Environment Canada - Air - AQHI categories and explanations". Ec.gc.ca. 2012-10-17. Retrieved 2013-06-23. 

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