SINGAPORE — The island was engulfed by unhealthy levels of smog yesterday, prompting the National Environment Agency (NEA) to request the Indonesian authorities to take immediate action to suppress the forest fires in Sumatra and provide urgent updates on the situation.

The latest bout of haze could see the recently-enacted Transboundary Haze Pollution Act come into force. Under the Act, which commenced on Sept 25, an offence would be deemed to have occurred if the Pollutant Standards Index (PSI) in any part of Singapore hits 101 or more — venturing into the unhealthy range and beyond — for a 24-hour period or longer.

The hazy conditions were expected to persist today. In a haze situation update issued at 8pm yesterday, the NEA said overall air quality over the next 24 hours is expected to be in the unhealthy range of between 101 and 200. As of 11pm, the 24-hour PSI reading was 91 to 107.

Throughout yesterday, residents across the island complained of a burning smell. Visibility was also affected. The three-hour PSI reading rose steadily in the morning and reached the unhealthy range at 1pm. Air quality then worsened in the evening, with the three-hour PSI hitting a high of 153 at 7pm — the highest reading since the air quality reporting was tweaked in April to incorporate levels of fine particulate matter, or PM2.5.

The NEA attributed the hazy conditions to smoke haze blown in from central Sumatra by the prevailing south-westerly winds.

Widespread smoke haze was also visible in the southern and eastern parts of Kalimantan. The number of hot spots detected in Sumatra and Kalimantan yesterday was 97 and 74, respectively.

The agency said the Republic had been affected by haze since late morning yesterday, with elevated PM2.5 levels in all parts of the island.

At one point, the one-hour concentrations of PM2.5 rose to 131 microgrammes per cubic metre in the southern part of Singapore, way past its normal levels of between 20 and 40 microgrammes per cubic metre.

The NEA said given the air-quality forecast for today, healthy people should reduce prolonged or strenuous outdoor physical exertion, while the elderly, pregnant women and children should minimise prolonged or strenuous outdoor physical exertion.

Those with chronic lung or heart disease should avoid prolonged or strenuous outdoor physical exertion. Those who do not feel well, especially the elderly and children, and those with chronic heart or lung conditions, should seek medical attention.

Singapore was hit by its worst bout of haze in June last year, when the PSI levels skyrocketed to an unprecedented 401.

Over the past weeks, the haze has returned on several occasions, with air quality reaching unhealthy levels — notably during the Singapore Grand Prix weekend last month.