SINGAPORE - The haze has reached low, unhealthy levels as of 11am on Sunday and is "likely to persist" for the rest of the day, according to the National Environment Agency (NEA).

The 3-hour PSI, which has been creeping upwards throughout the morning, reached a reading of 104 and hit 110 at 1pm. When the reading is above 100, it is recommended that people cut down on prolonged or strenuous outdoor physical activity.

The numbers for the 24-hour PSI had been increasing though those numbers have yet to cross into the unhealthy range.

The southern and eastern parts of Singapore reported the highest levels - 94. The overall 24-hour PSI for Singapore is expected to be in the high-end of the moderate range and low-end of the unhealthy range on Sunday.

According to the NEA's haze microsite, Sunday's worsening air quality was likely due to smoke from Sumatra, blown in by the prevailing winds. But thundery showers forecasted in the afternoon may help dispel some of the haze.

The PSI measures soot, dust particles, sulphur dioxide, nitrogen dioxide and ground level ozone in the air.