Air pollution shrouded the sky in a thick haze as agricultural fires burned out of control on Saturday morning.

Footage shows the toxic soup of PM2.5 particles between Chiang Rai and Chiang Mai in the north of Thailand on April 8.

The two cities have been ranked as the worst in the world for several weeks by Swiss air quality monitor IQAir. Levels of the dangerous PM2.5 particles in the atmosphere reached 200 micrograms per cubic meter - 40 times the levels advised by the World Health Organisation (WHO).

Chiang Mai Governor Nirat Pongsitthaworn, after initial reservations, finally declared several villages in Chiang Mai 'disaster zones'. The designation would allow resources to be mobilised quickly to address the hazardous PM2.5 dust levels, which have been responsible for thousands of hospital admissions.

The Thai Meteorological Department said that northern regions in Thailand will experience thunderstorms from April 6 to 9, with residents hopeful the rains will help dissipate pollutants.

Scientists found that smog is at its worst in the country when farmers burn waste crops to cheaply clear their land between late October and April. It has been worsened in recent years by rising demand for food and powerful agro-business lobbyists that can stifle the government's control over farmers.

The seasonal lack of wind and rain, combined with a winter low-pressure system in which cold air is trapped closer to the ground by warm air above – hampers the pollution from being dispersed naturally by wind.

The Thai government has been criticised for failing to tackle the country's air pollution crisis, with bizarre measures such as spraying water into the air and giant purifiers having no impact.