Smoke from farmers burning crop stubble combines with vehicle and factory emissions to send air pollution in Bangkok and other cities soaring in the early months of the year.

On Friday morning, the sprawling Thai capital was seventh on the list of the world's most polluted cities run by air monitoring company IQAir.

The level of PM2.5 pollutants -- cancer-causing microparticles small enough to enter the bloodstream through the lungs -- hit 86 micrograms per cubic metre, according to IQAir.

A reading above 15 in a 24-hour period is considered unhealthy by the World Health Organization (WHO).

High levels of PM2.5 were also recorded Friday in the northern cities of Chiang Mai and Udon Thani.

The government on Thursday ordered provincial authorities to enforce a ban on burning crop stubble, requiring them to report how many farmers they have arrested for breaching the rule.

"In every province, if you allow crop burning or fail to implement preventive measures, you will be punished," the Thai government said in a statement on Thursday.

More than 1.1 million pollution-protection masks have been distributed around the kingdom, and the health ministry is to monitor vulnerable groups, including children and pregnant women.

The government has also told drivers to ensure their vehicles comply with emissions limits.

Pollution is expected to spike between Friday and Wednesday as cool, stable weather conditions hamper the dispersal of pollutants.

Last week, Bangkok authorities closed more than 350 schools as pollution soared, but no such order was given Friday.

The city's Skytrain, metro, light rail system and bus services have been free to use all week in a bid to reduce emissions from vehicles.

Air pollution has closed schools across other parts of Asia recently, including in Pakistan and India.

Nearly two million students around New Delhi were told to stay home in November after authorities ordered schools shut because of worsening air pollution.