Northern provinces, especially Chiang Mai and Chiang Rai, were blanketed by unsafe levels of the carcinogenic PM2.5 on Sunday, according to the Health Department.
Dr Suwannachai Wattanayingcharoenchai, director-general of the department, said the levels of particulate matter 2.5 micrometres and less in diameter (PM2.5) exceeded 91 microgrammes per cubic metre of air in 24 hours in eight northern provinces – Chiang Mai, Chiang Rai, Lamphun, Lampang, Mae Hong Son, Nan, Phayao and Phrae.
Toxic dust was also afflicting the three northeastern provinces of Bung Kan, Nakhon Phanom and Nong Khai as of late Sunday morning, he said.
The safe threshold for PM2.5 in Thailand is set at 50 microgrammes per cubic metre of air in 24 hours.
The PM2.5 level in tambon Wiang Pang Kham of Mae Sai district in the northernmost province of Chiang Rai reached 480 microgrammes per cubic metre of air in 24 hours as of 11am on Sunday, Dr Suwannachai said.
The level exceeded the local safe threshold by nine times – and the World Health Organization's (WHO) standard by 32 times, he said.
At 10.30am IQAir, the air pollution monitoring website, reported Chiang Mai was the city with the worst air pollution in the world. The level of PM2.5 in the northern province was 33.4 times as much as the safe threshold set by the WHO.
Dr Suwannachai expected the air pollution to continue in the North and the Northeast, especially on border provinces, this week because of field burning in Thailand and neighbouring countries, along with stagnant air.
The high levels of PM2.5 would cause eye and skin irritation, breathing difficulties and chest pain, he said. Breathing difficulties could be acute in people with heart and respiratory tract illnesses, he said.
"Exposure to long-term and excessive PM2.5 will finally cause internal inflammation, damage to lung cells, cardiovascular diseases and cancer," Dr Suwannachai said.