SINGAPORE - Singaporeans should focus on the 24-hour Pollutant Standards Index (PSI) reading rather than the three-hour PSI, as the latter is not the best indicator of the health impact of the haze.
Making this point on Friday was Second Minister for the Environment and Water Resources Grace Fu, who said: "The health impact of the air pollutants is actually determined both by the concentration and also the duration of exposure."
She added: "The 24-hour measurements are a better reflection of the total (length of) exposure of an individual to the particulate matter.
"It is important for the public... not to be too overly concerned with the three-hour PSI, which may show spikes and drops from time to time."
Her comments came after the three-hour PSI reading hit a new record of 401 at noon on Friday.
The three-hour PSI, which is averaged from readings taken in the past three hours, was introduced in 1997 to give people more current information about air quality.
However, there is very little data showing the health effects of short-term exposure to pollutants, said Defence Minister Ng Eng Hen, who is leading an inter-ministerial committee to tackle the haze problem.
"The Environment Ministry has repeatedly pointed out that studies co-relating exposure to pollutants (to health) have been related to 24-hour measurements. For this reason, our health guidance is based on 24-hour PSI," he said.
The 24-hour PSI is averaged from readings taken over the past 24 hours. The National Environment Agency started to report a rolling 24-hour PSI reading every hour on its website on Friday, to give a better sense of the likely impact on people's health.
Turning to another measurement of pollution - PM2.5, which measures very small particles that can penetrate deep into the lungs - Dr Ng said the Government will not issue a separate daily health advisory for it.
"It is confusing to give the public two tables," he said, but promised that the authorities will track 24-hour PM2.5 levels.
The daily health advisory will still be based on the PSI.
But if the PM2.5 levels rise to a level that triggers a stronger health advisory, that will be issued instead.
"It is very confusing for the public to have three figures in their heads - the 24-hour PSI, the three-hour PSI and the PM2.5 - so we decided to simplify this, and medical professionals have said this is a sensible way to do it," he said.
PSI: What and why
What is the Pollutant Standards Index (PSI) based on?
During a haze, it is usually based on the concentration of PM10, which are very small particles that are 10 microns or smaller.
What is the difference between the 24-hour PSI and three-hour PSI?
The 24-hour PSI is based on readings taken over the past 24 hours. The three-hour PSI is based on readings taken in the past three hours.
Why are Singapore's health advisories based on the 24-hour PSI and not the three-hour PSI?
The haze's health impact depends on both the pollutants' concentration and how long people are exposed to them.
Minister for Defence Ng Eng Hen said studies on the haze's effect on health have been mostly related to 24-hour measurements, so Singapore takes its cue from the 24-hour PSI.
What is PM2.5 and why are people worried about it?
PM2.5 refers to particles that are 2.5 microns or smaller.
This makes PM2.5 a subset of PM10.
Because PM2.5 particles are so small, they can penetrate deep into the lungs.
This makes them more toxic.