People in Nan on Friday began to suffer from hazardous microdust as city areas became enshrouded by haze emanating from 91 hotspots.
A satellite, operated by the Geo-Informatics and Space Technology Development Agency (Gistda), found the hotspots mainly coincide with bushfires and farm burning over the last week.
The haze, filled with PM2.5 and PM10 particles, was noticeable over parts of Muang district.
Tourists visiting the scenic Wat Phrathat Khao Noi hilltop temple could not enjoy the usually clear panoramic view of the city as the high dust levels hindered visibility.
A dust detector in Nan municipality indicated yesterday that levels of both pollutants had risen to levels that could significantly impact public health.
According to Gistda, the pollution largely originated in 25 protected forests, 59 forest reserves, three crop fields and four Sor Por Kor land plots the government granted to landless farmers.
The situation added to concerns that the authorities are struggling to curb air pollution in many provinces.
The Air Quality Index, which scores poor air quality, was still highest in Khon Kaen. The AQI in the northeastern province reached 173, with the level of PM2.5 spiking to 75 microgrammes a cubic metre, according to the Pollution Control Department.
The safety level of particulate matter (PM), with 2.5 micrometres in diameter, has been set at 50 µg/m³ in Thailand.
Many residents in Khon Kaen's Muang district have taken to wearing masks while officials continue to spray water in many areas in a bid to reduce dust levels.
"Dust in Khon Kaen comes from sugarcane burning," Khon Kaen governor Somsak Changtrakun said.
"Many farmers are speeding up their harvest, so they resort to burning instead of hiring workers for the job," he added.
Mr Somsak earlier said the province is holding talks with plantation owners to find ways to reduce pollution caused by their practices.
In Kalasin, factories have been blamed for polluting the environment, according to an inspection carried out yesterday.
A roadside asphalt-producing plant near Don Chan district was "filled with a huge amount of dust and lacked standard protection", said Col Manop Khaikhunthot, chief of Kalasin's Internal Security Operations Command.
His team also told the operators of rubber-processing plants and tapioca flour factories to improve air filters and wastewater treatment facilities.