Higher reading for same air quality as PSI tweak kicks in

SINGAPORE - Singapore's main air quality index has been tweaked to reflect more precisely the smallest, most harmful pollution particles.

The change, which took effect yesterday, means that the Pollutant Standards Index (PSI) reading will tend to be higher even if the air quality is the same as before.

More days will be classified as "moderate" under the new index, when they would have been "good" previously.

The tiniest particles are known as PM2.5 - less than 2.5 microns in diameter or a 30th of the diameter of a human hair.

The finer the particles, the more harmful their effects, as fine particles can enter the lungs; very fine ones can slip into the blood and even enter the brain.

These tiny particles have always been included in the PSI, but until now, they were part of a component called PM10 - less than 10 microns in diameter.

But the proportion of PM2.5 within the wider PM10 category may vary, so having PM2.5 as a separate component will better reflect its impact.

The PSI is calculated based on the highest reading of its components - PM10, sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide. PM2.5 has now been added as a sixth component.

The level of each component corresponds to a PSI level, based on a formula.

For example, at 11am on Tuesday, the 24-hour PSI reading for the central region was 59, based on the highest component, PM2.5. There were 19 micrograms per cubic metre of PM2.5 measured, which corresponds to a PSI reading of 59.

Under the previous system, the 24-hour PSI reading for the central region would have been 28, based on the 28 micrograms per cubic metre of PM10.

The change was made to simplify the readings, said Minister for the Environment and Water Resources Vivian Balakrishnan when announcing it in his ministry's budget debate last month.

Previously, PM2.5 levels were reported separately from the PSI reading.

Public reactions to the change have been mixed.

Some were confused by the tweak, while others did not agree with the new PSI figures.

Others, however, welcomed the change. Commenting on The Straits Times' website, reader James Lee wrote: "It is good news that PM2.5 is FINALLY taken into consideration."

PM2.5 particles have in fact always been included in the PSI, but not as a standalone component of the index.

He also wondered how much higher last year's record three-hour PSI of 401, measured on June 21 during the country's most severe bout of haze, would be when translated into the current PSI system.

At noon on June 21 last year, the 24-hour average PM2.5 reading ranged from 232 micrograms to 292 micrograms per cubic metre in various parts of the island. Under the new system, that would have translated into a 24-hour PSI that is in the very unhealthy (201-300) or hazardous (301-400) range.

Under the older system, the 24-hour PSI based on PM10 readings then was lower, at 181 to 232, in the unhealthy to very unhealthy range.

The 24-hour PSI reading is available on the National Environment Agency's website www.nea.gov.sg and is updated every hour.

Measured pollutant concentrations, and the PSI readings that they correspond to, are also available at www.haze.gov.sg

caiwj@sph.com.sg

This article was published on April 2 in The Straits Times.

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