Singapore haze: What the numbers mean

SINGAPORE - Singapore is blanketed by its worst haze in 16 years. But what goes into air quality measures?

What is the PSI?

The Pollutant Standards Index, or PSI, is Singapore's main indicator of air quality.

It measures five key air pollutants: sulphur dioxide, nitrogen dioxide, ozone, carbon monoxide and PM10, which is particulate matter 10 microns or smaller in size - about one-seventh the diameter of a strand of human hair.

The reading reflects the pollutant with the highest concentration.

Any reading above 100 is considered unhealthy, and anything higher than 300 is hazardous.

The National Environment Agency (NEA) has been publishing hourly updates of its three-hour average PSI reading on its website. For example, the three-hour average at 3pm is taken from readings from noon to 3pm.

This means that the PSI at, say 3pm, is not strictly speaking a reflection of the haze you might see around you at that time, but an average of the situation over the previous three hours at various recording stations around the island.

Each hour, the NEA also publishes the 24-hour average PSI for the past 24 hours. The 24-hour PSI is from readings over the past 24 hours, averaged out. This reflects the sustained exposure levels that a person might face, and is said to be a better indicator of the health impact of the haze.

What is PM2.5?

PM stands for particulate matter, while PM2.5 refers to very fine particles that are less than 2.5 microns in size - a thirtieth of the diameter of a human hair.

The NEA also publishes hourly PM2.5 readings, averaged over 24 hours, which measure micrograms of PM2.5 per cubic metre.

Readings above 40 trigger health advisories. At 9pm yesterday, the 24-hour PM2.5 reading was 199-242 µg/m3, varying by geographical area.

Where does particulate matter come from?

Fine particulate matter, or PM2.5, can come from many sources all around us, such as burning plants, automobiles and natural sources like volcanic eruptions. It can even be formed in the atmosphere by chemical reactions. In this latest case, much of it comes from burning plant matter in Sumatra.

What are the health effects of PM2.5?

Because these very fine particles can get farther into the lungs and pass more easily into the bloodstream, long- and short-term exposure to PM2.5 can cause cardiovascular problems such as heart attacks and strokes. PM2.5 exposure is linked to increased hospital admissions and emergency department visits for respiratory effects.

These include asthma attacks as well as increased respiratory symptoms such as coughing, wheezing and shortness of breath.

Concerns about the health effects of PM2.5 levels have led governments to push for the use of higher quality diesel fuels in vehicles since these produce exhaust fumes with less of these tiny particles. Efforts to raise fuel quality have been ongoing for many years, quite separately from concerns over the haze that comes and goes.

Why is the PM2.5 reading better than the PSI in some regions and worse in others?

Climate researcher Matthias Roth, an associate professor of geography at the National University of Singapore, said the distribution of PM10 and PM2.5 depends on the burning source.

Different areas in Singapore may have been affected by haze from different spots in Indonesia, which may vary in their vegetation or soil composition.

Why does the PSI peak in the evening and drop overnight?

The wind tends to drop in the evening, meaning smoke accumulates then, said the NEA. But calmer breezes do not bring as much haze from Sumatra, so the PSI falls some hours later.

Professor Roth said this could partly be because the sun's heat helps the air to rise and mix. Without this, pollutants could be trapped near the surface.

Dr Grandey from Censam suggested a few other possibilities. Some pollutants are produced or destroyed by sunlight. So the amount of sunlight may affect the pollution to some extent. Or the daily pattern could just be spurious. But more research and analysis would have to be done to test these theories, he said.

What filters out PM2.5?

Ministry of Manpower guidelines say employees who work outdoors for sustained periods and have health conditions that make them susceptible should be issued with N95 masks or other protective devices when the PSI goes above 100.

And high-efficiency particulate air filters (Hepa filters) can remove particles that are as small as 0.3 micron in size.

For more haze updates from AsiaOne, click here:


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