Pollutant Standards Index
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The Pollutant Standards Index (PSI) is a type of
air quality index An air quality index (AQI) is used by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become. AQI information is obtained by averaging readings from an air quality sensor, whi ...
, 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 Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The ter ...
(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 The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...
(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 An air quality index (AQI) is used by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become. AQI information is obtained by averaging readings from an air quality sensor, whi ...
(AQI) to incorporate new PM2.5 and
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
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 Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates suspended in air obscure visibility and the clarity of the sky. The World Meteorological Organization manual of codes includes a classification ...
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 PSI table is grouped by index values and descriptors, explaining the effects of the levels, according to Singapore's
National Environment Agency National Environment Agency (NEA) is a statutory board under the Ministry of Sustainability and the Environment of the Government of Singapore. NEA is responsible for improving and sustaining clean and green environment in Singapore. Its r ...
(NEA). 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 Sumatra is one of the Sunda Islands of western Indonesia. It is the largest island that is fully within Indonesian territory, as well as the sixth-largest island in the world at 473,481 km2 (182,812 mi.2), not including adjacent i ...
,
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...
, brought over by wind. These forest fires have been attributed to the
slash-and-burn Slash-and-burn agriculture is a farming method that involves the cutting and burning of plants in a forest or woodland to create a field called a swidden. The method begins by cutting down the trees and woody plants in an area. The downed vegeta ...
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. In June 2013, severe
haze Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates suspended in air obscure visibility and the clarity of the sky. The World Meteorological Organization manual of codes includes a classification ...
hit
Singapore Singapore (), officially the Republic of Singapore, is a sovereign island country and city-state in maritime Southeast Asia. It lies about one degree of latitude () north of the equator, off the southern tip of the Malay Peninsula, borde ...
, 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 471 on 20 October 2015 at 11 pm (GMT+8).


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. 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. 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. In terms of healthcare utilization, both country-wide Emergency Department visits and hospital admissions were increased per unit increase in PSI.


References

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External links


Past and present PSI readings in Singapore published by the NEA
Air pollution Environment of Singapore Environment of Malaysia United States Environmental Protection Agency Air quality index