The haze that hung over Singapore from Thursday night through to yesterday morning was probably caused by local pollution instead of forest fires overseas, experts said.

"Based on the latest satellite images, there were no significant hotspots or smoke haze detected in the nearby region," said a spokesman for the National Environment Agency (NEA).

"The haziness was due to the accumulation of particulate matter under light wind conditions," the spokesman added.

The 24-hour Pollutant Standards Index (PSI) reached a high of 95 in southern Singapore at 8am yesterday.

A PSI reading above 100 indicates unhealthy air quality, while a reading of between 51 and 100 is considered moderate.

Under moderate conditions, normal activities can be carried out.

The 24-hour PSI across Singapore hovered in this range for the rest of the day, although readings in southern Singapore leaned toward the higher end.

Experts told The Straits Times there were a negligible number of hotspots in Riau, Sumatra or Kalimantan - areas in Indonesia where fires are started that are usually the cause of smoke haze.

"This makes Indonesia unlikely to be the source of the bad air," noted volunteer group PM.Haze (People's Movement to Stop Haze), which monitors haze in Singapore.

"Another significant source of our poor air is exhaust from traffic," said PM.Haze executive director Zhang Wen.

Usually, domestic pollution is blown away by the wind, she said. "But as it was not windy yesterday, the pollutants could not disperse, making it look hazy."