Air pollution across the upper North remains at critical levels with authorities monitoring almost 400 active hotspots in Chiang Mai alone on Sunday.

The air quality did not improve on Monday morning, with IQAir recording "very unhealthy" air quality and hazardous, PM2.5 dust levels at 170.1 microgrammes per cubic metre (μg/m³) at 10.30am in Chiang Mai.

Government spokeswoman Narumon Pinyosinwat said fires have undoubtedly affected air quality, with 17 provinces in the North reporting an increase in levels of PM2.5 pollutants on Sunday.

Chiang Dao district in Chiang Mai reported the highest PM2.5 reading in the North on Sunday at 360μg/m³.

Ms Narumon said the worsening pollution is caused by a combination of factors which include arson, drought and winds which trap pollutants right above Thailand.

A combination screenshot from https://aqicn.org/city/chiang-mai/ and https://www.iqair.com/thailand/chiang-mai at 10.30am on Monday.

Most of the local fires were related to an ongoing bushfire in Doi Suthep-Pui National Park in Chiang Mai's Muang district, which suddenly raged out of control late on Saturday after burning for about a week.

Authorities used four helicopters to fight the fire, while drones and paragliders were sent to help detect hotspots and suspicious activities among villagers, who have been blamed for starting the forest fires to clear land for agriculture.

A source said that after several fires near Hmong villages within the park and in nearby Hang Dong district were found to have been deliberately set, local villagers in the area have been instructed not to engage in slash-and-burn farming until April 30.