For a period earlier this month, the historic city of Chiang Mai in northern Thailand had the worst air of any city in the world.The city gained the same unwanted accolade last year. The practice of agricultural burning in the hills around Chiang Mai renders the air so toxic from February to April that it becomes unsafe to breathe. Respiratory problems and allergies caused by PM2.5, a type of pollution, led to more than 12,000 people being admitted to hospital in 2023.The bad air affects everyone, including the young and physically fit. In December 2023, Krittai Tanasombatkul, a 29-year-old doctor and basketball fanatic, succumbed to lung cancer. Like 40% of people with the disease in the city, he was not a smoker.