Singapore was shrouded in a smoky-smelling haze on Monday as pollution from forest fires in Indonesia's Sumatra island spread to neighbouring countries.
The Pollutant Standards Index reading at noon stood at 80, in the higher range of the "moderate" band, according to the National Environment Agency (NEA) website. Any reading above 100 is considered unhealthy.
The haze was visible at street level in Singapore's central business district but has so far not affected business or air transport. Singapore schools are on holiday.
People with heart and lung disease, those over 65 and children are advised to "reduce prolonged or heavy outdoor exertion" even in moderate conditions.
"The hazy conditions are expected to persist for the next few days," the NEA said.
Malaysia has also been affected by the recurring haze problem, which occurs in the dry season as a result of forest fires in the sprawling Indonesian archipelago, some of them deliberately started to clear land for cultivation.
Haze reached unhealthy levels in Malaysia over the weekend.
On Monday, the Malaysian pollutant index showed unhealthy levels of between 102 and 121 in parts of the states of Pahang, Terengganu and Malacca.
In the capital Kuala Lumpur, the sky remained hazy with a reading of 82.
Southeast Asia's haze problem hit its worst level in 1997-1998, causing widespread health problems and costing the regional economy billions of dollars as a result of business and air transport disruptions.