SINGAPORE - The hazy conditions in Singapore worsened on Thursday (Sept 24), as denser haze from Sumatra was blown in by the prevailing southerly winds.

At 11pm on Thursday, the 24-hour Pollutant Standards Index (PSI) was 226-279, moving entirely into the very unhealthy range for the first time.

The three-hour PSI, which is not tied to a health advisory, spiked at 262 at noon but dropped to 225 at 3pm. It then climbed to 317 - in the hazardous range - at 11pm.

A check on the National Environment Agency's (NEA) MyEnv App revealed that the hourly PM2.5 in the south was 319. Readings for the other parts of Singapore ranged between 221 and 267.

PM2.5 are small, toxic particles that are associated with vehicle emissions and the haze from forest fires.

In a Facebook post at 7.37pm, Minister for Social and Family Development Tan Chuan-Jin urged people to keep track of news and updates on the haze situation, as there will be important updates on developments.

To provide some air-conditioned respite from the haze, several MPs - including Nee Soon GRC's Lee Bee Wah and MacPherson SMC's Tin Pei Ling - announced on Facebook that RC centres and community clubs in their wards will be opened to residents, starting from Thursday night.

The current hazy conditions are expected to persist, and there could be a further deterioration in the air quality later in the day, said the NEA.

The 24-hour PSI for the next 12 hours is expected to be in the high end of the unhealthy range and the low end of the very unhealthy range (201-300), and may enter the mid-section of the very unhealthy range if denser haze from Sumatra continues to be blown in.

Some Singaporeans who were planning to head out on Thursday, a public holiday, had to cancel their plans or opt for other activities because of the haze.

Public officer Allyne Han, 29, cancelled her tennis game. "The haze is more manageable for me this time compared to two years ago, but the unhealthy PSI range recently has made me rethink some of my planned outdoor activities," said Ms Han.

Pre-school teacher Joanne Lim, 23, went ahead with plans to visit the old Tanjong Pagar Railway Station which was open to the public for Hari Raya Haji - but only after some persuasion.

"The haze was really quite bad. There didn't seem to be any significant crowd at the station as well - my friends had to convince me to tag along after lunch," she said.

Engineer Brian Sim, 33, said he would stay indoors as far as possible. Mr Sim, who has sinus problem, said: "I've been nursing a cold for the past two weeks, and my throat has been feeling itchy because of the haze. I thought the haze conditions had turned for the better over the F1 weekend but it came back with a vengeance on Monday. I've had to take MC yesterday."

Businesses expecting the public holiday crowd were disappointed.

Ms Dioa Jovolin, supervisor at Trapizza, an Italian eatery at Siloso Beach in Sentosa, said: "We usually see at least half the restaurant filled on weekends and public holidays, but the haze was so terrible today that we only got three tables of customers at lunch. It has definitely affected our business as we are an outdoor restaurant."

Fabulous Baker Boy, a bakery cafe at Fort Canning Hill, also said business has been affected. The cafe has 60 seats, half indoor and the rest outdoor.

Owner Juwanda Hashim, 42, said: "My business has fallen by about 30-40 per cent. It's also due to the cafe's location being out in the open on the side of the hill - people are less inclined to come by as we are the only place around this area."

It was a slow start for the newly opened Karting Arena at Turf City, which has an electric karting track. Owner Yuey Tan, 33, said: "It's only our first week but certainly, the traffic today has been less than we expected, especially since it's a public holiday. It's unfortunate that the haze has worsened in the past few days, but what's important is to stay healthy during this period."

Given the current air quality, 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.

People 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.