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 PM2.5 exceeded 91 microgrammes per cubic metre of air (μg/m³) 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 μg/m³ in 24 hours.

The PM2.5 level in Mae Sai district in Chiang Rai reached 480 μg/m³ 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. Long-term effects include cancer.