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.

FENG ZENGKUN