Forest fires have sent air quality in nine provinces to a level considered harmful to people's health, as cities in the North registered air quality that was among the world's worst.

The air quality index (AQI) in Chiang Mai as of 4am 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.

Information from the Pollution Control Department (PCD) confirmed the haze in the North had reached worrying levels.

PM2.5 levels ranged from 70-124 microgrammes per cubic metre (µg/m³), exceeding the safety standard of 50 µg/m³ or lower.

Prime Minister Prayut Chan-o-cha is scheduled to visit Chiang Rai this Saturday, eight days before the national election.

Dr Rangsrit Kanjanavanit, a Chiang Mai resident who serves as a lecturer and cardiologist at the Faculty of Medicine at Chiang Mai University, said the smoggy skies show how the government's campaign to eradicate forest fires and open burning has failed.

Some of the haze has been attributed to neighbouring countries like Laos and Myanmar.

Critics say Thailand should work more with other governments to tackle the widespread practice of open burning.

Haze has been a seasonal problem in the North for over a decade. It usually appears from January to April but peaks in March as the extremely dry conditions increase the magnitude of forest fires.

This is compounded by farmers burning waste to clear land for the next harvest season.