SINGAPORE - Hazy conditions returned on Saturday as air pollutant levels crept back into the unhealthy range.

The 24-hour Pollutant Standards Index (PSI) was 120-148 as of 6pm, while the 3-hour PSI fell to 97 - in the upper end of the moderate zone.

After a brief respite over the last two days when air quality hovered in the moderate zone, conditions deteriorated overnight, with the 3-hour PSI peaking at 173 at 5am on Saturday.

An update by the National Environment Agency (NEA) at 9am indicated that this was due to haze from the surrounding regoin being blown in by weak prevailing winds.

"Some haze from Kalimantan has continued to spread westward to the sea areas south of Singapore. For the rest of the day, hazy conditions can still be expected," said the statement.

"The prevailing winds are forecast to shift to blow from the north-east and this may bring some improvement to the haze conditions later today."

NEA added that the 24-hour PSI for the next 12 hours is expected to be in the low and mid sections of the unhealthy range.

Thundery showers have been forecasted over the northern and western parts of Singapore in the afternoon, which could potentially provide some relief.

Given the current air quality, healthy persons 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.

Persons who are not feeling well, especially the elderly and children, and those with chronic heart or lung conditions, should seek medical attention.

The Singapore Grand Prix, which takes place this weekend, is expected to go ahead as planned unless the 24-hour PSI enters into the very unhealthy (201-300) range.

Race organiser Singapore GP told The Straits Times on Sept 15 that there were no plans to amend the racing or entertainment schedules based on current PSI levels.

A number of measures have been taken - the PSI reading and relevant health advisory will be broadcast at the race track, while N95 masks are available at cost price. Medical and first aid personnel are also on standby to attend to haze-related conditions.

Practice sessions for the race on Sunday went off without a hitch on Friday night.

For updates, go to the NEA website, NEA's Facebook page, Twitter account @NEAsg, or download the myENV app.