The index soared to 321 at 10:00 pm (1400 GMT), according the the website of the National Environment Agency (NEA), well past the 300-mark considered "hazardous".
Before Wednesday, Singapore's most severe haze reading was in September 1997 when the number peaked at 226.
Anything above 200 is considered "very unhealthy", particularly to the elderly, young children and people with heart and lung disease.
When levels hit hazardous, vulnerable people can experience "early onset of certain diseases in addition to significant aggravation of (already present) symptoms," the NEA said. Healthy people also find it harder to exercise in such conditions, the agency added.
Now that the pollution levels have topped the 300 threshold, elderly people with existing heart or lung diseases are advised to stay indoors and the general population to "avoid vigorous outdoor activity", according to government guidelines.
The smog, which had been worsening since Monday, triggered a run on medical masks and angry complaints from foreign tourists and locals.
"We are going to leave Singapore two days early because we are having trouble breathing," said Zac Kot, 40, a business owner from the United States who was on holiday with his wife and two young girls.
Indignant Singaporeans attacked their government on the web for its handling of the problem. Disposable medical masks flew off drugstores' shelves as consumers and companies bought them in bulk and placed orders for more.
Even tourists from Indonesia -- traditionally the largest source of visitors to Singapore -- protested about the smoky haze from Sumatra island, where some farmers and plantations deliberately set off fires to clear land for cultivation.
"It's not very good, and it's getting harder to breathe. I just don't know where to go," said Rangga Adisapoetra, 30, a risk management executive from Indonesia's main island Java attending a mobile communications and broadcasting expo.
"Maybe tomorrow, I'll visit Universal Studios," he told AFP, temporarily lifting a grey face mask to speak.
The pollution problem peaks during the June-September dry season, when monsoon winds transport thick clouds of smoke from Sumatra to neighboring Singapore and Malaysia.
source: www.abs-cbnnews.com
