SINGAPORE - Hazy conditions in Singapore took a turn for the worse on Tuesday (Sept 29), with air quality reaching very unhealthy levels. On Wednesday too, the air quality is expected to remain between the unhealthy and very unhealthy levels, the National Environment Agency (NEA) said in an update.

The update sent at around 7.30pm on Tuesday said the 24-hour Pollutant Standards Index (PSI) for the next 24 hours is expected to be in the high-end of the unhealthy range and the low-end of the very unhealthy range.

"For tomorrow, the prevailing winds are forecast to continue blowing from the southeast, and hazy conditions can still be expected," it said.

On Tuesday, the 24-hour PSI entered the very unhealthy band of 142-203 at 4am, and has crept upwards since.

At 11pm the 24-hour PSI reading was between 163 and 202, while the three-hour PSI reading, which is not tied to a health advisory, was 140, down from a high of 250 at 2pm.

For the rest of Tuesday, the 24-hour PSI for the next 12 hours is expected to be in the low to mid sections of the very unhealthy range, NEA said in an earlier update at 2pm.

"The prevailing winds are forecast to blow from the south or southeast. The current hazy conditions are expected to persist, and they could improve slightly later in the day if the winds strengthen and blow from the southeast," NEA said.

A total of 49 hotspots were detected in Sumatra on Tuesday. The low hotspot count was due to cloud cover over parts of the island. Moderate to dense haze continued to be observed in Sumatra.

The haze had deteriorated on Monday after a brief reprieve over the weekend, as a shift in the prevailing winds blowing from the south-southwest brought denser haze from Sumatra, NEA said.

Heavy rain over most parts of Singapore on Monday evening brought some temporary respite.

Several schools that The Straits Times spoke to said they have put in place precautionary measures such as shifting some classes to air-conditioned classrooms.

These measures follow the unprecedented closure of primary and secondary schools last Friday when the haze hit hazardous levels.

Given the air quality forecast, healthy people should reduce prolonged or strenuous outdoor physical exertion. 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.

Those with chronic heart or lung conditions should seek medical attention if they are feeling unwell.

For updates, visit the NEA website and the haze microsite, and follow NEA's Facebook page or Twitter account, or download the myEnv app.