The showers expected today and this week could bring some relief from the haze, which brought air quality to unhealthy levels yesterday.
The National Environment Agency (NEA) said in its daily advisory that with winds forecast to blow from the east-northeast or east-southeast today, "the shift in winds is expected to bring some showers over the region".
It said the "slight deterioration" in the air quality yesterday was due to smoke haze being blown in from Kalimantan by the prevailing winds.
Yesterday, 246 hot spots were detected in Sumatra and 474 hot spots in Kalimantan, which also affected parts of Peninsular Malaysia.
"For the next few days, an increase in rain showers can be expected over the region, including Sumatra and Kalimantan," said the NEA. "The showers may help to improve the hot spot and haze situation in Sumatra and Kalimantan."
The 24-hour Pollutant Standards Index (PSI) is forecast today to be between the high end of the moderate range and the low end of the unhealthy range, the NEA said.
It advised healthy people to reduce prolonged strenuous exertion outdoors today, and the elderly, pregnant women and children to minimise it, while people with chronic heart or lung disease should avoid it altogether.
At 8pm yesterday, the PSI for all regions was over 100, with the south recording the highest figure at 123, and the north the lowest at 111.
A PSI reading of zero to 50 indicates good air quality while a reading of 51 to 100 is in the moderate range. A reading of 101 to 200 is considered unhealthy.
Air quality is considered very unhealthy when the PSI ranges from 201 to 300, and hazardous when the PSI reading is more than 300.
In addition to the 24-hour PSI, the one-hour PM2.5 concentration is a good indicator of current air quality.
Measuring the average hourly concentration of PM2.5 particles, the dominant pollutant during haze episodes, it can help people gauge their level of immediate activity, such as whether they should go for a jog.
There are four bands for PM2.5 concentrations: zero to 55 for normal, 56 to 150 for elevated, 151 to 250 for high, and very high for above 250.
In the south, where the Singapore Grand Prix race was held yesterday, the one-hour PM2.5 concentration reading was in the "elevated" range of 78 at 8pm, though it did not deter the crowds who flocked to watch the race.
Stay-at-home mother Michelle d'Cruz, 48, said she and her two children, aged 10 and 13, were more concerned about the haze than her husband, a 47-year-old engineer.
She said: "The kids and I stayed in because the haze is a put-off and we didn't want to risk getting sick with exams around the corner. My husband, on the other hand, went to F1, regardless."
• Additional reporting by Goh Yan Han