SINGAPORE - Haze shrouded the Republic on Wednesday, with the three-hour Pollutant Standards Index reading 102 at 8pm.

A PSI reading over 100 refers to unhealthy air quality, while a reading of between 50 and 100 is moderate.

The National Environment Agency (NEA) said in its daily haze advisory the haze was due to smoke being blown to Singapore by prevailing winds.

There were 23 hot spots detected in Sumatra today, though the low number was due to cloud cover over parts of the island, NEA said. In Kalimantan, 139 hotspots were detected. Widespread thick smoke was observed in Kalimantan's southern and western parts.

"Some of the haze has spread to the southern parts of the South China Sea," the agency noted.

The haze is expected to persist on Wednesday night, the NEA said in its forecast.

Thursday could bring some relief, however, as it said to expect thundery showers in the late morning and early afternoon. But occasional slight haze may still be experienced during the day.

The overall air quality from Wednesday to Thursday night is expected to be in the high-end of the moderate range but may creep into the low end of the unhealthy range from time to time.