After staying in the clear for several days, the three-hour Pollutant Standards Index (PSI) soared steadily from 86 at 1pm to hit 127 at 9pm yesterday.
Hazy conditions are expected to persist today with air quality, as measured by the 24-hour PSI, being in the almost or slightly unhealthy range, said the National Environment Agency (NEA) in an update yesterday. PSI is considered unhealthy when it enters the range of 101 to 200.
The levels of PM2.5, which are small, toxic particles, were "elevated" in most parts of the Republic, NEA noted.
The Meteorological Service Singapore had said last week that it was expecting the situation to improve with more rain and less haze as the south-west monsoon season has ended.
NEA said yesterday's condition was due to smoke haze from Sumatra blown in by prevailing southerly winds.
The agency detected 205 hot spots in Sumatra last Saturday. It said drier weather in south Sumatra over the past two days had led to a spike in hot spots.
So far, the highest three-hour PSI this year was 153 on Oct 6.
As the PSI climbed, some Singaporeans took to social media to lament missed opportunities for picnics and runs.
With the holidays around the corner, housewife Stephanie Soh, 47, who has three daughters, said she hopes the haze will dissipate.
Her 10-year-old daughter will be having her last exam paper today and is hoping to get out of the house for some time in the sun.
The other two, aged 16 and 18, are in the midst of exams. They mostly stay indoors, with the air-conditioner cranked up.
Said Mrs Soh: "We experience dry throat and our eyes tear sometimes. My youngest has requested to go to the park and Universal Studios so we hope the situation improves."
Still, the haze is expected to dissipate with the onset of the rainy season during the north-east monsoon early next month, according to the Meteorological Service Singapore's forecast last week.
NEA recommends that when the index hits the unhealthy band, healthy people should reduce prolonged or strenuous outdoor physical exertion, while the elderly, pregnant women and children should minimise such activities.
Those with chronic lung or heart diseases should avoid such activities completely.