Stubborn farmers are continuing to set alight their fields to clear waste crops, adding to Thailand's worsening air pollution problem. Aerial footage shows large blackened swathes of a rice field in Nakhon Ratchasima province smoldering after workers set fire to it on February 8. The illegal agricultural practice called stubble burning, or slash and burn, is still prevalent because it is cheaper and easier than other field-clearing methods. Government officials have raised fines for anyone caught using fires to clear their fields but many continue. The smoke then drifts down in the air currents and envelops the capital Bangkok leading to soaring numbers of hospital admission due to respiratory illnesses and infections. Phimai district chief Jirattachai Jeephimai said: 'We have already warned farmers to stop burning their crops, but the issue persists despite the threat of legal action.' He added that village officers have been enlisted to deter the farmers, but they continue to start the fires. Authorities have warned locals to stay indoors as air pollution in Bangkok and neighbouring areas reached unsafe levels. Thailand's pollution control department blamed the rise in PM2.5 or hazardous inhalable particles on 'stagnant weather conditions' combined with agricultural burning in the north, northeast and in neighboring Laos and Cambodia. 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. This is combined with high numbers of polluting cars on the road and large construction projects across the capital. 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 – prevent the pollution from being dispersed naturally. The Thai government has been criticized 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."