Clearing the haze on PSI

What is PSI? Pollutant Standards Index (PSI) does not differ much from the Air Quality Index (AQI) used by U.S. Environmental Protection Agency (EPA) and in many cities around the world.

Both indices are calculated^1,2 based on 24-hour average concentrations of pollutants. The highest index for each pollutant (based on a calculation as depicted below) will be taken as the AQI/PSI.

Hence the PSI is a reported value of the highest pollutant within the past 24 hour.

The NEA further reports 3-hour PSI readings. During periods of smoke haze, as PM_2.5 (particulate matter <2.5mm) is the dominant pollutant of concern, the 3-hour PSI is the index calculated based on the PM_2.5 concentration levels averaged across 3 hours.

Since most studies on the health effects of PM have used the 24-hour measurements, the health advisory in Singapore takes reference from the 24-hour PSI.

For individuals who wish to make adjustments to their daily activities, one can refer to 3-hour PSI readings or 1-hour PM_2.5 concentration levels. The 1-hour PM_2.5 concentration levels gives the pollutant levels of each geographical region – North, South, East, West, Central – of Singapore. The following graph shows the corresponding AQI/PSI value of PM_2.5 concentration levels. Do note that for PM_2.5 concentration levels between 12 and 150 mcg/m3, the PSI value will be lower than the AQI value.

The online calculator to convert PM_2.5 concentration levels to AQI is available here.

How important are these numbers? Can we just rely on visibility and smell? In a Straits Times article dated 13 Mar 2014³, Dr Erik Velasco, a research scientist who studies air pollution at the Singapore-MIT Alliance for Research and Technology said, “But while the haze might smell bad, its odour cannot be used to judge how harmful it might be”. He added: "The intensity of smell cannot be used to quantify the concentration of pollutants in the air. Both visibility and smell are just qualitative indicators."

Regions for Air Quality Reporting⁴

Region	Town Centres /Areas
North	Admirality, Kranji, Woodlands, Sembawang, Yishun, Yio Chu Kang, Seletar, Sengkang
South	Holland, Queenstown, Bukit Merah, Telok Blangah, Pasir Panjang, Sentosa, Bukit Timah, Newton, Orchard, City, Marina South
East	Serangoon, Punggol, Hougang, Tampines, Pasir Ris, Loyang, Simei, Kallang, Katong, East Coast, Macpherson, Bedok, Pulau Ubin, Pulau Tekong
West	Lim Chu Kang, Choa Chu Kang, Bukit Panjang, Tuas, Jurong East, Jurong West, Jurong Industrial Estate, Bukit Batok, Hillview, West Coast, Clementi
Central	Thomson, Marymount, Sin Ming, Ang Mo Kio, Bishan, Serangoon Gardens, MacRitchie, Toa Payoh

REFERENCE

1. http://www.haze.gov.sg/docs/default-source/faq/computation-of-the-pollutant-standards-index-(psi).pdf
2. http://www.epa.gov/ttn/caaa/t1/memoranda/rg701.pdf
3. http://www.straitstimes.com/singapore/pm25-pollutants-hit-unhealthy-level-0
4. http://www.haze.gov.sg/faq