Jun 20, 2013

    Usefulness of PSI called into question

    IS THE Pollutant Standards Index (PSI) an effective measure of air quality here? That was a question some netizens posed as the haze worsened yesterday.

    Several, like Mr Phil Askey, have noted that the PSI does not factor in fine particles called PM2.5. They believe this means the PSI is not a good reflection of air quality here.

    The PSI is based on concentrations of larger particulate matter called PM10.

    PM2.5 particles, about one-thirtieth the width of a human hair, are more dangerous as they can enter the lungs or bloodstream more easily than larger dust particles.

    Mr Askey posted a suggestion on the Facebook page of Environment Minister Vivian Balakrishnan, saying that the National Environment Agency (NEA) should use another index instead of the PSI.

    He said the Air Quality Index (AQI), for instance, takes into account PM2.5 concentrations.

    There is no international consensus on air-quality indexes. The PSI was developed by the United States Environmental Protection Agency, but the US switched to the AQI in 1999.

    Malaysia uses another measure, the Air Pollution Index, which also measures PM2.5.

    In a report by The Straits Times in 2010, the NEA said it was studying other indexes that measure air quality.

    NEA had said that the PSI has served Singapore well when it comes to issuing health advisories to the public.

    The PSI is measured on a scale of zero to 400, with a health advisory issued when a reading goes beyond 100.

    NEA also noted then that the public has become used to the PSI over the years.

    The NEA measures concentrations of PM2.5 separately. Readings, averaged over 24 hours, are published on its website.

    Health advisories for different PM2.5 concentration levels are also displayed.

    For instance, when the reading is between 65 and 150 micrograms of PM2.5 per cubic m, people with heart or lung disease, children and older adults should avoid all outdoor physical activities.

    Everyone else should avoid prolonged or heavy exertion.

    The PM2.5 figures at 4pm yesterday for Singapore on the whole were 112-143.

    The highest PM2.5 concentrations then were in the northern part of Singapore, at 143. The lowest was in the central part of the island, at 112.