SINGAPORE - Air quality deteriorated over the course of Wednesday, and inched into the unhealthy band at 6pm.

Then, the three-hour Pollutant Standards Index (PSI), a measure of air quality here, had a reading of 102. By 9pm, the reading had crept up to 107.

At such levels, when PSI readings range from 101 to 200, air quality is considered unhealthy. Prolonged or strenuous outdoor physical exertion should be reduced or avoided under such circumstances.

The National Environment Agency (NEA) said in its daily haze advisory on Wednesday that the hazy conditions were due to smoke haze blown in from southern Sumatra by prevailing southerly winds.

There were 110 hotspots detected in Sumatra, and 58 detected in Kalimantan on Wednesday.

The high readings come after the three-hour PSI showed moderate readings for most of Wednesday. When PSI readings are 100 and below, normal activities can be carried out.

At 11am, for instance, the reading was a moderate 71. This slowly increased to 82 at 3pm, 94 at 5pm, before breaching unhealthy levels at 6pm.

Hazy conditions are expected to persist for the rest of Wednesday night and Thursday, despite thundery showers forecasted for Thursday afternoon, the NEA said.

The overall air quality for Thursday is expected to stay at the high-end of moderate range and low-end of the unhealthy range.