SINGAPORE - The three-hour Pollutant Standards Index (PSI) hit 113 at 9am after it crossed into the unhealthy range at the 8am reading when it was 106.
The day started poorly with a reading of 97 at 7am, which is in the high end of the moderate range.
The 24-hour PSI stands at 83 to 103, which straddles the moderate and unhealthy range.
There might be some reprieve later in the morning and early afternoon as the National Environment Agency said to expect thundery showers over northern, eastern and western Singapore.
The agency said in an update on Sunday, that 14 and 126 hotspots had been detected in Sumatra and Kalimantan respectively.
On Sunday, the three-hour PSI peaked at 113 at 3pm before dipping to 89 four hours later.
When the reading is above 100, it is recommended that people cut down on prolonged or strenuous outdoor physical activity.