Thailand must ban stubble burning by farmers to improve air quality, the head of a leading agricultural body said on Monday, after a spike in dangerous pollution left millions needing medical treatment.

The Southeast Asian kingdom suffered dire air quality earlier in the year, with the capital Bangkok and the northern city of Chiang Mai ranked among the most polluted cities in the world on some days in April.

Authorities urged people to stay indoors and work from home as concentrations of the most dangerous PM2.5 particles — so tiny they can enter the bloodstream — reached unsafe levels.

One of the leading sources of the particles is farmers torching stubble to prepare land for the next crop, with “burning season” peaking between December and April.

The head of the country’s top agricultural body said the practice must be halted.

“The government should ban cropburning, the method widely used among farmers in Thailand,” said Pornsil Patcharintanakul, president of Thai Feed Mill Association, which operates under the Commerce Ministry.

He urged the government of new Prime Minister Srettha Thavisin to take action swiftly because of the urgency of the matter.

More than 2 million people sought medical treatment in Thailand this year because of problems linked to air pollution.

Pornsil, whose organization monitors the quality of animal feed, said Thailand should follow international farming guidelines that forbid stubble-burning.

He said three years was a realistic timeframe in which to ban the practice.

His call comes as the European Union begins to enforce its “Carbon Border Adjustment Mechanism,” charging for emissions linked to the production of imported goods.

Srettha’s government has said that tackling PM2.5 is one of its priorities, though it has not given details of what it plans to do.