Haze is expected to choke Bangkok and northern parts of the country until Monday next week, due to changing atmospheric patterns and trans-boundary pollution from Cambodia.
"Bangkok and other parts of the country are now under threat from the trans-boundary haze caused by massive open burning outside the country," said Theerapat Prayurasiddhi, Centre for Air Pollution Mitigation (CAPM) member and permanent secretary of the Prime Minister's Office.
Mr Theerapat said the Pollution Control Department's (PCD) monitoring equipment had detected over 4,000 hotspots in Cambodia, and the department had written to the Asean Secretariat in Jakarta, Indonesia, asking the bloc to look into them.
Over 10,000 hotspots are detected in the Lower Mekong region annually, as farmers engage in open burning to prepare their land for the harvest and re-planting of maize, sugar and rice, Mr Theerapat said.
Since a low-pressure system formed above Thailand several days ago, at least 26 provinces have reported unhealthy levels of the ultra-fine PM2.5 dust particles, with 24-hour averages hovering between 46-82 microgrammes per cubic metre (µg/m³).
The PCD set the "safe" threshold for PM2.5 exposure at 50µg/m³.
Bangkok's air quality dropped on the first day of school reopening, with 67 out of the 73 air quality monitoring stations reporting higher-than-recommended PM2.5 readings. Yesterday, PM2.5 levels in the city hovered between 52 and 82µg/m³.
To help combat the open burning of farm waste, the government is offering soft loans of up to 1.5 billion baht for sugarcane farmers to purchase industrial shredders and sugarcane harvesting vehicles.
A total of 895 hotspots have been detected in Thailand since the start of the year, most of which were illegally lit in national forest reserves, as well as Sor Por Kor land, which was given to landless farmers.