The 24-hour Pollutant Standards Index (PSI) reading is expected to be in the "moderate" range from 6pm Sunday to 6pm Monday, said the National Environment Agency (NEA) in an advisory.

"The showers this morning helped to improve the air quality over Singapore," said the agency, adding that the prevailing southwesterly winds are expected to persist for the next few days.

But the 24-hour average readings of PM2.5 concentration - referring to particles which are 2.5 microns or smaller - will remain slightly elevated. Studies have shown that rain does not fully wash the PM2.5 particles from the air.

The three-hour PSI reading at 6pm stood at 28, after dipping to 17 at 2pm and 3pm - the lowest reading since the haze blanketed Singapore two weeks ago. The 24-hour average PSI at 5pm was 55-61, down from 56-61 at 4pm.

Air in the 51-100 PSI range is considered "moderate". A reading of between 101 and 200 is considered "unhealthy", between 201 and 300 is "very unhealthy", and anything above 300 is hazardous.

However, the 24-hour average readings of PM2.5 concentration remain elevated at 42-57 at 5pm. Health advisories kick in when that figure breaches 40.