Changing wind directions are expected to bring some relief at the weekend to residents here, who had to cope with hazardous air quality yesterday.

Still, the National Environment Agency (NEA) warned that air quality is expected to remain in the unhealthy to very unhealthy range.

The haze worsened dramatically on Thursday because of a tropical storm brewing in the western Pacific Ocean, east of the Philippines.

The storm caused winds to blow from the south or south-west, sending haze from a dense haze cloud sitting slightly to the south of Singapore over the island.

This caused the 24-hour Pollutant Standards Index (PSI) to reach just under 300 - above which air quality is considered hazardous - on Thursday.

Air quality hit hazardous levels at 4am yesterday, staying so till 1pm.

The three-hour PSI yesterday peaked at 341 at 5am, before dropping to 80 at 2pm, and rising again to 154 at 9pm.

The NEA said in a notice on its website that these readings "can be volatile especially during periods of haze".

Yesterday, the Ministry of Education said primary and secondary schools are scheduled to re-open on Monday.

But if the NEA's 24-hour PSI forecast tomorrow evening indicates that air quality will reach hazardous levels, schools will be shut again.

Audrey Tan