Singapore's Pollutant Standards Index (PSI) is creeping back up after a slight reprieve from Friday afternoon's peak of 401, the worst reading in the country's history.

An 8pm update on Saturday saw the PSI rising back up to 83 from a low of 73 at 5pm.

Air becomes "hazardous" when the index passes 300, a reading between 101 and 200 is "unhealthy". The previous worst reading here was 226, in 1997.

People can resume normal outdoor activities if the reading dips below 100.

Prime Minister Lee Hsien Loong posted a photo of him and a family on his Facebook page saying he took a walk at Labrador Park earlier.

"Met families out enjoying the blue skies," said PM Lee. "Hope the clear air lasts, but let's be prepared for the haze to come back."