Chiang Mai in northern Thailand has again today ranked as the worst city in the world for air pollution today.

The once-popular Thai holiday destination topped the list of most polluted cities on April 4, according to Swiss air quality company IQAir, which has live daily air ratings monitored 24/7.

Chiang Mai had reached a level of 210 or 'very unhealthy' according to US AQI. It was far ahead of China's Shenyang City which was at 179 'unhealthy' US AQI.

Footage shows apocalyptic gray smog hovering over houses in the city, where locals have been grappling with respiratory diseases caused by the persistent toxic haze.

Business owners and residents have been urging the local government to declare northern Thailand a 'disaster zone'. Tens of thousands of people across the region have been admitted to hospitals with pollution-induced illnesses.

However, defiant Chiang Rai governor Puttipong Sirimat and Interior Minister Anupong Paochinda refused to declare the issue a national disaster, fearing the 'negative impact' of the label on tourism.

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.'