Air pollution in northern Thailand worsened as smoke from forest fires and agricultural burning spread across the 'disaster zone' this morning.

Footage shows gray mist engulfing roads in Chiang Mai City on April 6 morning as the province continued to struggle with wildfires.

A strong 'burnt' odour permeated the air, causing eye, nose, and skin irritation among residents.

Chiang Mai was the worst city in the world for air pollution, according to Swiss monitor IQAir. They gave the city a ranking of 371 with sensors across the region detecting 321 micrograms of harmful PM2.5 particles in the air for every single cubic metre - more than 64 times the safe levels set by the World Health Organisation (WHO).

Thailand's state-controlled Pollution Control Department also reported that the PM2.5 readings at all stations were in an 'unhealthy' range, from 98 to 250 micrograms per cubic metre.

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 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.

Data scientist engineer Worasom Kundhikanjana said: 'PM 2.5 pollution has a seasonal trend in Bangkok and the northern provinces, but has only recently received public attention. During this winter haze, the harmful particle pollution level is high throughout the day, including on weekends.

'Unfortunately, spraying water into the air does not appear to be effective, since the volume of water is minuscule compared to actual rain.

'With the start of the monsoon season, the weather in Bangkok will get better, and public attention on this issue will likely fade. However, without immediate policy measures, the problem will come back again next winter.'