Planes sprayed water over the world's most polluted city today to alleviate toxic smog clouding the air.

The aircraft flew across Chiang Mai and saturated the air with tiny water droplets believed to disperse PM2.5 dust particles.

Chiang Mai was given a ranking of 223 by Swiss air quality monitor IQAir today. They recorded 173 micrograms of PM2.5 particles per cubic metre of air - more than 34 times the safe levels advised by the World Health Organization (WHO).

Supit Phithakthum, director-general of the Department of Royal Rainmaking and Agricultural Aviation which deployed the planes, blamed the persistent smog on the growing farm sector, rapid economic development, temperature inversion, and transboundary haze.

He cited forest fires and agricultural burning both within Thailand and in neighbouring countries as the primary reasons for the pollution, adding the haze is expected to worsen next year.

He said: 'Several organisations in Thailand, including our department, are continuously searching for solutions to the air pollution, such as spraying water from buildings or using chemicals to reduce atmospheric temperatures.'

He added that they are designing new equipment for the planes to better spray high-pressure water streams from the sky.

The anti-air polltion operations will continue throughout the Songkran Festival, he said.

Scientists found that smog is at its worst in Thailand 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.