SINGAPORE - The three-hour Pollutant Standards Index (PSI) hit a high of 237 at midnight on Wednesday (Sept 23), as hazy conditions persisted and the 24-hour reading neared the mid-point of the unhealthy range.

As of midnight, the 24-hour PSI was between 121 and 145. It crossed entirely into the unhealthy range for the first time on Wednesday at 9pm with a reading of 101-122. At 8pm, it had been between 95 and 116. The 24-hour PSI had hovered between the high end of the moderate and low end of the unhealthy range for most of the day.

The three-hour PSI, which is not tied to a health advisory, had been rising throughout Wednesday. It was up to 214 at 11pm.

The hazy conditions are expected to persist throughout the rest of Wednesday, and through Thursday as well, the National Environment Agency said in an update on Wednesday evening.

This is due to haze from Sumatra being blown in by prevailing winds from the south and south-west.

The 24-hour PSI for the next 24 hours is expected to be in the low to mid sections of the unhealthy range. It may enter the high end of the unhealthy range if denser haze is blown in from the surrounding region, the NEA added.

It also said that only one hotspot was detected in Sumatra today, due to partial satellite pass and cloud cover over parts of the island.

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.

For updates, visit the NEA website, the haze microsite, the NEA Facebook page and Twitter, or download the myENV app.