Levels of PM2.5 have reached the "unhealthy" red zone throughout the North and the upper Northeast, as people in Chiang Mai have been urged to stay indoors and work from home.

- North -

The Pollution Control Department (PCD) reported that PM2.5 levels in the North ranged from 94 to 416 µg/m³ as of 11am on Friday.

Elsewhere in the North, PM2.5 readings were 384 μg/m³ in Pai; 340 μg/m³ in Chiang Rai's Mae Sai district; and 321 μg/m³ in Chiang Rai's Chiang Khong district.

- Upper Northeast -

In the upper Northeast, the highest level at 220μg/m³ was reported in Muang district of Nong Khai province.

Air quality map of Thailand on Friday morning. (Screenshot: IQAir).

- Work from home -

Late Thursday night, Chiang Mai governor Nirat Pongsitthithaworn urged people in the northern province to stay indoors and work from home to protect themselves and reduce the health impact from PM2.5 particles.

Over the past 24 hours, Chiang Mai's Muang district reported a PM2.5 reading of 233 µg/m3.

The PCD attributed the air pollution to local forest fires, hotspots in neighbouring countries and stagnant air.

Thick smog is expected to remain in the North, especially in border provinces, until April 14, it said.

On Friday morning, the air monitoring website IQAir ranked Chiang Mai as the most polluted major city in the world.

Levels of the most dangerous PM2.5 particles -- so tiny they can enter the bloodstream -- were more than 66 times the World Health Organization's annual guideline, according to IQAir.

Thailand has been choking on air pollution since the start of the year, caused in part by seasonal agricultural burning.

Nearly two million people have needed hospital treatment for respiratory conditions caused by air pollution this year, according to the Public Health Ministry.

Officials previously warned Bangkok residents to stay indoors and work from home in February as the capital was covered with harmful haze.