THE SINGAPORE government has urged the Indonesian authorities to take urgent measures to halt transboundary haze, as the haze clouding Singapore's skies crossed into the unhealthy range on Monday.
At 10pm, the PSI was 155 - the highest since 1997, when the index peaked at 226.
Any reading above 100 is considered unhealthy.
Environment and Water Resources Minister Vivian Balakrishnan said he is "deeply concerned that the 113 hotspots detected over Sumatra (on 17 June 2013) have led to such a bad haze in Singapore".
He said the Government has been in touch with the Indonesian authorities "to register our concern, and renew our offer of assistance".
He will also speak to the Indonesian Environment Minister personally "to convey the seriousness of the situation".
The National Environment Agency (NEA) said it will continue to monitor the situation closely and provide further updates when necessary.
The haze was visible across the island, shrouding landmarks like Marina Bay Sands and the Singapore Flyer on Monday. NEA said hazy conditions are expected to persist for the next few days.
People with heart and lung disease; children and older adults are advised to reduce prolonged or heavy outdoor exertion.
Smoke from forest fires in Indonesia's Sumatra island, some deliberately started to clear forests for planting, has been carried by winds from the west and south-west to Singapore this week and to Malaysia, where it reached unhealthy levels over the weekend.
The number of Sumatran hot spots has been rising: on June 15, there were 101 hot spots, while on Sunday there were 138.