Singapore may have some haze today and the air may even become unhealthy, the National Environment Agency (NEA) warned yesterday evening.

It said that healthy people should reduce prolonged or strenuous outdoor activities, while the elderly, pregnant women and children should minimise doing such activities.

Those with chronic lung or heart disease should avoid such exertion, and people who do not feel well should seek medical attention, the NEA added.

The agency said that 120 hot spots were detected in Sumatra, Indonesia, yesterday, mainly in the southern half of the island, and smoke plumes with slight to moderate haze were visible from some of the hot spots.

"With the prevailing winds forecast to blow from the south-west, occasional hazy conditions can be expected," said NEA.

It added that the 24-hour Pollutant Standards Index (PSI), which indicates the air quality, is expected to be in the high end of the moderate range of 51 to 100, and may even enter the low end of the unhealthy range.

The 24-hour PSI, which is published online hourly, averages air quality readings in the previous 24 hours. Air is unhealthy when the index crosses 100.

The 24-hour index was in the moderate range across the island from morning to night yesterday, and was slightly higher in the west. The NEA highlighted that the haze was blown in from fires in Sumatra.

Mr David Law, 18, a full-time national serviceman, said the smell of the haze had lingered throughout the day at his training grounds near Clementi. "It was serious to the extent that my throat felt uncomfortable when I was outside," he said, adding that he spent most of the day indoors for training.

Separately, Dr Sutopo Purwo Nugroho, spokesman for Indonesia's disaster mitigation agency (BNPB), told The Straits Times that efforts to contain the fires are ongoing, and they involve BNPB, the forest ministry, the military, the police and local residents.

BNPB has deployed three aircraft to do cloud seedings and six helicopters for water bombing in Riau. Cloud seeding in Riau has also been carried out since June 22 and more than 100,000kg of chemicals have been used to induce rain. "The threats of forest and land fires will increase as the weather will get hotter," Dr Sutopo said.