SINGAPORE - Air quality improved on Thursday morning after it inched into the unhealthy range on Wednesday evening.

The three-hour Pollutant Standards Index (PSI), a measure of air quality here, was 57 at 7am.

Air quality started to deteriorate from Wednesday evening, reaching 107 at 9pm.

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.

When PSI readings are in the moderate range of 50-100, normal activities can be carried out.

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.

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