Deadly PM 2.5 smog caused by endemic agricultural burning shrouded Bangkok, Thailand, on Tuesday morning.

The killer pollutant - responsible for an estimated five million deaths worldwide annually - envelops large parts of the country each year from January to April when landowners burn off crop husks left in their fields after harvesting.

Smoke has surged across Southeast Asia in recent years amid rising demand for food from a rapidly developing population with an increasing GDP.

The World Health Organisation puts the safe level of PM 2.5 at 5 microgrammes per cubic metre of air but in the Thai capital on January 23, the level was 75 microgrammes - more than 15 times the guidance.

Swiss monitor IQAir has rated Bangkok as 'unhealthy' every day for the previous two weeks - causing concern for residents living in the semi-developed city.

Despite paying lip service for several years, politicians have failed to stop individuals from burning their land due to it being cheaper and easier than modern farming methods.

In March 2023, the Public Health Ministry of Thailand reported that around 1.3 million people in the country had been unwell due to the rising levels of air pollution. Officials said 200,000 residents were hospitalised in a single week.

They warned hospitals in December 2023 to prepare for an influx of patients.

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.

In December, Dr Krittai Thanasombatkul from Chiang Mai, Thailand, died of lung cancer. He had previously blamed PM 2.5 microdust particles for his condition.

The New York State Department of Health say PM 2.5 increases the risk of heart disease, asthma, and low birth weight.

While medics from South Korea published research in the National Institute of Health in the United States that said the pollutant increases the risk of stroke, ischemic heart disease, chronic obstructive pulmonary disease (COPD), lung cancer, and other illnesses such as immune deficiency and even obesity.