Cancer-causing air pollution choked Bangkok today as annual crop burning began in the country.

Video shows the thick haze blanketing the capital city on Tuesday December 12 morning as the seasonal smog covered the country's central plains and northeast region.

Bangkok was ranked the 11th most polluted city in the world, according to data from Swiss air quality tech company IQAir. The city had a PM2.5 level of 74.1 microgrammes per cubic metre - nearly 15 times the safe limit set by the World Health Organization.

Researchers found that smog - caused by high levels of harmful PM 2.5 particles in the air - is at its worst in the country when farmers illegally burn waste crops to cheaply clear their land between late October and April. They are pressured to meet the rising demand for food in the region - while keeping costs low to boost their profits.

Academics have found that smoke is combined with stagnant atmospheric conditions during those months - with little wind or rain - causing the toxic pollutants to linger in the air without being naturally dispersed by the weather.

In March this year, the Public Health Ministry of Thailand reported that around 1.3 million people in the country were had been unwell due to the rising levels of air pollution. Officials said 200,000 residents were hospitalised in a single week.

It came as researchers from Peking University in Beijing, China, also discovered that long-term exposure to particulate matter PM.25 air pollution is associated with an increased risk of lung cancer.

While earlier this month, Dr Krittai Thanasombatkul from Chiang Mai, Thailand, died of lung cancer. He had previously blamed PM2.5 microdust particles for his condition.

Thai politicians have failed to tackle the problem, which came to prominence several years ago. However, a draft 'Clean Air Bill' has been introduced which would provide subsidies to encourage farmers to switch to environmentally friendly methods of crop clearing instead of burning.