Pai district in Mae Hong Son province, one of the best known tourist destinations in the North, experienced the highest level of hazardous ultrafine dust in the country on Wednesday.
The highest level of PM2.5 -- 356 micrograms per cubic metre (µg/m3) -- was recorded in tambon Wiang Tai, according to a daily report of air quality data from the Pollution Control Department (PCD).
The PM2.5 levels nationwide ranged from 10 to 356 µg/m3 while the safe level is 50 µg/m3.
The PM2.5 levels in the North on Wednesday ranged from 35 to 356 µg/m3, while in Bangkok and the surrounding provinces, they were between 29 and 99 µg/m3, it said.
Levels in the Northeast ranged from 33 to 125 µg/m3, while those in the Central Plains recorded hovered between 32 and 63 µg/m3.
The air quality in the East and the South, meanwhile, was rated fair to very good on Wednesday.
Bangkok and its surrounding provinces, as well as the northern provinces, are being warned to brace for potentially worse haze from today till tomorrow.
Dr Suphat Chai-ngam, director of Khun Yuam Hospital in Khun Yuam district of Mae Hong Son, issued a health warning saying everyone in the district should wear a face mask capable of guarding against PM2.5.
Inhaling the polluted air in an open-air setting without proper protective equipment is equivalent to smoking 15 to 20 cigarettes a day, he said.
As the wildfire and haze pollution occurs for roughly two months a year, everyone should take pains to protect their health, he added.
"For those who keep on burning crops, you are destroying not only the forest but also the lungs of thousands of people, which is an inhumane act. Many of these people are now sick, and some have died prematurely," Dr Suphat said.
In Khun Yuam district alone, more than 40 rai of forest land was destroyed by two manmade wildfires this year, said Khongkrai Phonbunkamnoet, an officer with a wildfire control station in the district.
In Buri Ram, more than 200 tourists who gathered before dawn on Wednesday at Prasat Hin Phanom Rung in Phanom Rung Historical Park to observe a solar phenomenon in which the rising sun usually shines through all 15 doorways of an ancient temple on a hilltop ended up being disappointed as the sky was too hazy for them to see anything.