Forest fires have sent air quality in nine provinces to a level considered harmful to people's health, as cities in the North like Chiang Mai registered air quality that was among the world's worst.
The air quality index (AQI) in Chiang Mai as of 4pm Tuesday stood at 230, compared with 170 in the Bangladesh capital Dhaka and 164 in Kabul, Afghanistan, according to Air Visual, an app that monitors air quality.
At midnight Tuesday, in slightly cooler night-time temperatures, Chiang Mai still was listed as the world's worst, with air quality in the high-danger zone for all residents but below 200 on the AQI scale.
But air pollution soared once more on Wednesday, the AQI hitting a staggering 296 at Chiang Mai International School close to downtown at 1pm, according to Air Visual.
The AQI measures a broad spectrum of air pollutants including PM2.5, PM10 and carbon dioxide.
Information from the Pollution Control Department (PCD) confirmed the haze in the North had reached worrying levels.
PM2.5 refers to atmospheric particulate matter (PM) with a diameter of less than 2.5 micrometres, which is about 3% the diameter of a human hair. It can lodge in the lungs and enter blood vessels, leading to respiratory and cardiovascular disease.
According to PCD-supplied information taken from 17 air-quality-monitoring stations in nine provinces as of 9am Tuesday, the level of PM2.5 ranged from 70-124 microgrammes per cubic metre (µg/m³), exceeding the safety standard of 50 µg/m³ or lower.
Chiang Rai's Mae Sai district had the worst recorded level (163 µg/m³), followed by Phrae, Lampang, Nan and Chiang Mai.