“Often there is so much smog that I cannot breathe,” says Yupin Moosooloi, an ethnic Karen woman.

The hills of northern Thailand used to be known for the lush forests that covered them, the flamboyantly attired hill tribes that inhabited them, and the bracing fresh air that permeated them.

By now many of those verdant forests have been thinned but they still remain, as do the hill tribes, who draw busloads of tourists daily. These days, though, the air is far from bracing and fresh.

“Often there is so much smog that I cannot breathe,” says Yupin Moosooloi, an ethnic Karen woman who lives in a hamlet of subsistence farmers near Chiang Mai, a historic city nestled in the foothills of undulating mountain ranges in northern Thailand. “[But] at least here on the hills we can still see trees from a distance,” she adds. “In the city the smog is a lot thicker. There you can barely see anything at all. The smog covers everything.”

For several months Chiang Mai and its hilly environs have been blanketed in a thick haze. In March air pollution levels soared to fine particulate matter concentrations of up to 532 µg/m3 (one millionth of a gram per cubic meter of air), which was by far the worst level of air pollution anywhere in the world during the period. It was five times the level considered safe by the World Health Organization.

“The worst affected in my village are the children,” Yupin says. “Some of them have even started coughing up blood.”

Children are especially at risk of suffering from the debilitating effects of air pollution. Longer exposure to airborne toxins can cause or exacerbate a variety of lasting physical and mental conditions from pulmonary diseases to learning disabilities. Complicating matters for people in remote and impoverished areas is a lack of adequate health care facilities. In Yupin’s village tribespeople tend to rely largely on traditional forms of healing, which are routinely ineffective.

“Air pollution is stunting our children’s brains, affecting their health in more ways than we suspected,” says Dr. Maria Neira, director of the Department of Public Health, Environmental and Social Determinants of Health at the WHO.

Yet even as tribespeople in Chiang Mai are suffering from air pollution, many of them are also contributing to it. Local farmers often clear land by burning dry vegetation on it. They also dispose of accumulated waste by burning it. Fires started this way can spiral out of control and result in flashfires that tear through nearby forests.

“Smog is caused by wildfires,” Yupin says. “Some farmers leave cigarette butts burning while they look after their grazing animals. The dry vegetation then catches fire.”

Such detrimental practices are hardly limited to northern Thailand. They are also common in neighboring countries from Myanmar and Laos to Cambodia and Malaysia. Periodically, land clearing through slash-and-burn triggers massive environmental calamities, as has just happened in northern Thailand.

In 2015 a massive haze affected numerous countries in Southeast Asia, particularly Indonesia, Malaysia, Singapore, Thailand and the Philippines. Lasting from June to the end of October, the haze swathed a vast tract of territory in toxic fumes for weeks on end, triggering an environmental calamity and a health crisis. It was caused by fires set by farmers on the Indonesian islands of Sumatra and Kalimantan to burn dry vegetation.

According to the WHO, as many as 7 million people die each year of causes that can be linked to air pollution. More than 90% of children worldwide breathe toxic air every day, the WHO says.

“Polluted air is poisoning millions of children and ruining their lives,” stresses Dr. Tedros Adhanom Ghebreyesus, WHO’s Director-General. “This is inexcusable. Every child should be able to breathe clean air so they can grow and fulfil their full potential.”