The haze that shrouded Singapore yesterday was the worst in 16 years, and is expected to last for most of this week.
The three-hour Pollutant Standards Index (PSI), which measures air quality, climbed steadily throughout yesterday, reaching a high of 155 by 10pm, crossing into the unhealthy range.
The haze, which first hit the island last Friday and is likely to persist for the next few days, has worried Environment and Water Resources Minister Vivian Balakrishnan, who was "deeply concerned" that the 113 hot spots detected over Sumatra yesterday had caused air quality to plummet.
The last time Singapore saw such bad smog was in 1997, when the PSI topped 226.
Dr Balakrishnan said he would personally speak to his Indonesian counterpart to convey the seriousness of the situation, as well as renew Singapore's offer of assistance.
In a Facebook post last night, he said that "commercial interests in Indonesia have been allowed to override environmental concerns" for too long.
"We need the Indonesians to enforce their own laws," he wrote.
With the hazy conditions likely to continue, the National Environment Agency has advised people with heart and lung disease, children and older adults to reduce prolonged or heavy outdoor exertion.
The Ministry of Manpower yesterday also told employers to issue protective devices such as N95 masks, which filter out 95 per cent of very fine particles, to employees with heart or respiratory illnesses who are working outdoors when the PSI surpasses the healthy threshold of 100.
The Singapore Armed Forces, too, has "reduced physical and outdoor training accordingly".
Yesterday, the acrid-smelling haze was visible across the island, shrouding landmarks like Marina Bay Sands and the Singapore Flyer.
The number of Sumatran hot spots detected by satellites rose from 101 on Saturday to 138 on Sunday, before dropping to 100 yesterday.
Indonesian officials say the smog is from huge tracts of peat lands around the coastal city of Dumai - located on the coast of the Strait of Malacca that faces Singapore - that have caught fire due to the early hot season.
Farmers are also burning plantations to clear land for the next planting season.
The dry season is expected to last until monsoon rains start in Sumatra around October, said Mr Okta Irawan, a weather forecaster based in Jambi, South Sumatra.
Meanwhile, "we are doing all we can to contain the fires", Mr Ilyas As'ad, a deputy environment minister, told The Straits Times by telephone.
Indonesia has deployed dozens of firefighters in the area to douse the flames.
Officials say they have managed to put out several fires near Dumai, 250km north-west of Singapore.
But there are many more fires in Bengkalis and Rokan Hilir, two regencies in Riau that border Dumai.
Water-dropping aircraft would be deployed only if local governors made a request, which they have yet to do, said the Forestry Ministry.
The lack of rain also means the pollution is not washed out of the air.
Then, winds from the west and south-west blow the smoke over to Singapore and Peninsular Malaysia.
Meanwhile, organisers of outdoor events this week are watching the skies closely.
Mr Ben Swee, race organiser of Running Guild, which is holding the Banana Relay fun run at Punggol this Sunday, said the race could be cancelled if the PSI that morning exceeds 100.