Hazy conditions persisted overnight with the three-hour Pollutant Standards Index (PSI) reaching 94 at 9am on Sunday.
The haziness was due to smoke haze from Singapore's surrounding region, blown in by the prevailing winds, said the National Environment Agency (NEA).
At 7am on Sunday, the one-hour concentrations of PM2.5 was 47 - 54 ug/m3, and the 24-hr PSI was 90 -104, in the Moderate to Unhealthy range.
For Sunday, the prevailing winds are forecast to be light and variable in direction. Thundery showers are expected in the afternoon, said NEA.
The current hazy conditions are expected to persist, and the overall 24-hour PSI is expected to fluctuate between the high-end of the Moderate range and the low-end of the Unhealthy range.
Given the air quality forecast for the next 24 hours, healthy persons should reduce prolonged or strenuous outdoor physical exertion, said NEA.
The elderly, pregnant women and children should minimise prolonged or strenuous outdoor physical exertion, while those with chronic lung or heart disease should avoid prolonged or strenuous outdoor physical exertion.