World-class Formula One drivers roared through the Marina Bay Street Circuit last night, as the air quality here worsened.
The three-hour Pollutant Standards Index (PSI) climbed throughout the day to hit the unhealthy level of 129 at 9pm, while the 24-hour reading was in the moderate range of 59-67.
At 11pm, the three-hour PSI went down to 102 and the 24-hour PSI rose to 64-70.
The hazy conditions were "due to an incoming haze cloud from Sumatra" in Indonesia, said Minister for the Environment and Water Resources Vivian Balakrishnan on his Facebook page.
In its advisory, the National Environment Agency (NEA) said satellite images showed 64 hot spots in Sumatra yesterday, while Kalimantan had 73. Widespread smoke was visible in southern Kalimantan.
Indonesia's National Disaster Mitigation Agency said in a statement that it had conducted cloud-seeding operations to induce rain in South Sumatra over the weekend, which resulted in a sharp drop in the number of hot spots to 31 yesterday afternoon. Four helicopters were deployed for water-bombing operations to put out fires in the province.
"Many of the fires were on plantations and land near settlements, which indicates they were deliberately burnt," agency spokesman Sutopo Purwo Nugroho said. "The combination of illegal logging and forest fires is common in South Sumatra."
There were no hot spots in Riau and Jambi provinces.
Dr Sutopo said the effort to tackle the haze as a result of forest fires was ongoing, through land and air operations, law enforcement and community education, and that law enforcement was a top priority.
Singapore's NEA said the haze may continue today, as prevailing winds are forecast to blow from the south-east or south. Overall air quality is expected to fluctuate between the high-end of the moderate band (51-100) and the low-end of the unhealthy range (100-200).
The agency reminded the public that health impact depends not only on the PSI level, but also on one's health status and the length and intensity of outdoor activity. It advised people with chronic lung or heart disease to avoid prolonged or strenuous outdoor physical exertion.