Explainer: Confused over AQI, PSI, PM2.5? With the haze threat back, here’s how to interpret the various air quality readings

SINGAPORE — The threat of haze is back. Between reading the daily advisories sent out by the authorities and deciding whether to buy that air purifier, it could prove useful to have a deeper understanding of the various air quality readings available online.

As of Friday (Sept 13), the 24-hour Pollutant Standards Index (PSI) in Singapore — the main measure of air quality used by the authorities here — was in the “moderate” range, which is anywhere between 51 and 100 on the index. At 10pm, the 24-hour PSI in Singapore was between 89 and 99, just shy of the "unhealthy" range.

The 24-hour PSI, updated hourly on the National Environment Agency’s (NEA’s) website, would enter the “unhealthy” range if it crosses the 100-mark. Above 300 is considered “hazardous”.

The PSI takes into account, among other types of pollutants, tiny air particles known as PM2.5, which refers to particulate matter that is up to 2.5 micrometres, or microns, in size.

These particles are among the potentially most harmful to human health, and the main pollutant during haze.

‘SLIGHTLY HAZY CONDITIONS’

The NEA gives a separate PM2.5 reading. At 10pm, the one-hour PM2.5 concentration readings were in the “normal” and “elevated” bands.

The agency said on Friday evening that slightly hazy conditions have been persisting in Singapore as smoke haze from central and southern Sumatra continues to be blown in by prevailing winds.

It added that there is a risk of “increased haziness” if the winds continue to blow from the south or south-west.

Besides PSI readings and PM2.5, other readings are available online, such as the World Air Quality Index website (aqicn.org).

For example, Singapore’s AQI was 132 at 6pm on Friday, which is classified as “unhealthy” by the website, a non-profit enterprise with support from a range of government agencies worldwide.

With multiple air quality indexes, which one should you be referring to?

CUTTING THROUGH THE CLUTTER

There are no international guidelines on how air quality indices should be computed. Countries adopt different index systems based on their local needs and circumstances.

The PSI on NEA’s website is calculated based on six pollutants — fine particulate matter (PM2.5), particulate matter (PM10), sulphur dioxide (SO2), carbon monoxide (CO), ozone (O3) and nitrogen dioxide (NO2).

Singapore incorporated PM2.5 into the PSI readings only in 2014, a move which was lauded by respiratory and climate change experts as it would give a more accurate gauge of air quality standards.

In contrast, the NEA said that the aqicn.org website computes its AQI using the United States Environmental Protection Agency's reporting system. The aqicn.org does not follow Singapore’s system of computing PSI.

In addition, aqicn.org relies on the 24-hour average of PM2.5 concentration data that is posted every hour by the NEA.

Thus, the two indexes are not directly comparable.

WHAT DO PSI AND PM2.5 READINGS MEAN?

The 24-hour PSI is accompanied with a corresponding health advisory, which can be used as the basis for major decisions or planning ahead, such as whether schools should be closed.

The health advisory is based on the 24-hour PSI (which incorporates the 24-hour PM2.5 concentration levels), as scientific and epidemiological studies on the health effects of particulate matter have been based on this duration of exposure.

According to the NEA, the 1-hour PM2.5 concentration readings are only an indicative measure of the current air quality. There are currently few studies on the health effects of short-term exposure to PM concentration levels, it noted. 

In general, if the 24-hour PSI forecast is in the “moderate” range, normal activities can be conducted.

But if you’re already geared up for an outdoor activity but wonder if it is safe to go ahead in the next few hours, you should be referring to the one-hour PM2.5 concentration readings instead.

During haze periods, PM2.5 is the key pollutant that is factored into the PSI.

The one-hour PM2.5 bands and the accompanying descriptions of their health implications are intended to provide information on short-term fluctuations in PM2.5 concentrations through the day during haze periods, based on local conditions.

The NEA states that the one-hour PM2.5 is a “good indicator of current air quality”.

HEALTH CONDITION OF INDIVIDUAL ALSO A FACTOR

The NEA said that the health impact of haze is dependent on one’s health status, the PSI level, and the length and intensity of outdoor activity.

On Friday, the NEA said that given the air quality forecast for the next 24 hours, healthy people should reduce prolonged or strenuous outdoor physical exertion.

The elderly, pregnant women and children should minimise outdoor activities, while those with chronic lung or heart disease should avoid such activities, it added.

People who are not feeling well, especially the elderly and children, and those with chronic heart or lung conditions, should seek medical attention.

WHY IS PM2.5 SO IMPORTANT?

Because they are so tiny, PM2.5 particles are able to penetrate deep into the lungs and bloodstream. These particles can also be present during non-haze periods.

According to a 2013 World Health Organisation (WHO) report, it is estimated that about 3 per cent of cardiopulmonary and 5 per cent of lung cancer deaths globally are attributable to particulate matter, including PM2.5.

The report said that even at relatively low concentrations, the effect of inhalation of particulate matter on health is significant.

Exposure to particulate matter in the short and long term can cause aggravation of asthma and other respiratory diseases, even triggering heart attacks.

Those who are more vulnerable include those with pre-existing lung or heart disease, as well as elderly people and children. The report said exposure to particulate matter affects lung development in children, among other effects.