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PSI stands for 'Pollutant Standards Index'. It is an index of daily air quality levels and computed on the concentration of fine particulate matter (PM2.5), the dominant pollutant during haze episodes, along with other pollutants.
To assess the situation and plan ahead, use the air quality indicators that are based on an average of a 24-hour period and are related to scientific studies on the effects of particulate matter, duration of exposure and health.
To monitor throughout the day, use the readings that reflect the PM2.5 concentration levels averaged across three hours. This is only an indicative measure not tied to health advisories. There are no accompanying air quality descriptors or health advisories associated with the 3-hour PSI as there are few studies currently on the health effects of short-term exposure to PM concentration levels.
The health impact of haze is dependent on each person’s quality of health, the PSI level, and the length as well as intensity of outdoor activity. Reducing outdoor activities and physical exertion can help limit the ill effects from haze exposure.
When the air quality forecast is in the range of unhealthy and upwards, healthy persons should reduce or avoid prolonged and strenuous outdoor physical exertion. The elderly, pregnant women and children should minimise outdoor activity, while those with chronic lung or heart disease should avoid outdoor activity. Persons who are not feeling well, especially the elderly and children, and those with chronic heart or lung conditions, should seek medical attention.
The PSI readings reflect a total of six pollutants - sulphur dioxide (SO2), particulate matter (PM10), fine particulate matter (PM2.5), nitrogen dioxide (NO2), carbon monoxide (CO) and ozone (O3).
The air quality in Singapore is monitored through a network of air monitoring stations located in different parts of the island and is based on air quality reporting systems used by various countries that incorporate in their index, PM2.5 concentrations, which is considered an important component of air quality that impacts health.
Since August 2012, the PM2.5 concentrations have been included in PSI readings. And since 2014, the 3-hour PSI is also calculated based on PM2.5 concentration levels (as opposed to readings of particulate matter or PM10 over a 3-hour period). Due to the more stringent bandings for PM2.5 there will be higher 3-hour PSI readings.
As the 3-hour PSI is based on PM2.5 concentration levels averaged across 3 hours, the readings may also not correspond with the volatile hour to hour concentrations and what may be visibly observed.