SAMOENG, Thailand (AP) -- When the haze season comes, village chief Nanthawat Tiengtrongsakun and his tribesmen start preparing the land for fire.

They cut shrubs and trees on their small parcels of land, then set controlled burns that will clear their fields for planting -- and send up plumes of smoke that add to some of the worst air in the world. It's a slightly sweet gray haze that blurs the mountains in this part of northern Thailand to a faint outline, makes the air itself feel solid and turns breathing and swallowing painful for some.

The Pakanyo, who have carried out the practice as long as they have lived in these hills about 90 minutes from Chiang Mai, a top tourist destination, say they get blamed by city dwellers for fouling the air and damaging forest land.

"We are the ethnic group that preserves the forest, but other people have the concern that we are destroying the forest," Tiengtrongsakun said. "My argument to them is that we have been living (here) for generations. If we are the cause of the damage, the forest around us would have to be all gone."

The Pakanyo are just a small part of a cluster of factors that show how deeply fire is ingrained in local practices and why Thailand's air pollution issue is so intractable.

During the haze season, from February to April, Chiang Mai city regularly tops the list of the world's worst cities for air pollution. In March and April, its levels of fine particulate matter -- things like dust, dirt, soot and smoke that get into lungs and even bloodstreams -- are on average about 20 times the World Health Organization's recommended limit for exposure.

The city is at the forefront of the air pollution fight in Thailand in part because of its toxic air quality index readings, but also because it is home to a strong civil society and a meaningful local government effort to tackle the issue. And that effort has been reinforced by Prime Minister Srettha Thavisin, who has visited Chiang Mai four times since taking office last summer. He's called Chiang Mai a "model" that other places in Thailand should learn from, and last fall pledged to push through clean air legislation to "ensure that access to clean air is a basic human right for all."

Dirty air has become a fixture for public discussion in Thailand over the past two decades, but despite significant research and advocacy, the problem persists.

Air pollution in northern Thailand has been traditionally blamed on farmers who grow corn and sell it to big agro-food companies like CP Foods to be used as animal feed. There are other ways to deal with stubble, like biochar, which involves burning in a low-oxygen environment that means lower particulate emissions. But that requires significant labor in highland areas. And tilling the stubble into the soil, even if the mostly subsistence farmers had the equipment, would be difficult on the mostly hilly terrain.

CP said in March it had set up a tracing system to avoid buying corn produced on deforested or burned land. The same month, Srettha said he plans to ban imports of corn grown on land cleared by burning.

But the problem is wider than northern Thailand. Researchers say corn cultivation has mostly shifted into neighboring Myanmar and Laos, where stubble-burning is also practiced. Srettha has set up a working group with those countries aimed at cutting down the practice, and invited Cambodia to join, too.

Researchers at Chiang Mai University traced the sources of air pollution affecting the city, and in a paper published in April in the journal Atmospheric Environment, reported that more than 51% of the haze came from biomass burning -- material like leaves or crop stubble. The second-largest share, about 23%, was long-range pollution from other countries, most likely India, they said.

Fire is a deep part of the culture in northern Thailand, featured in local sayings that signal the coming of a period of growth and renewal. In people's day-to-day lives, it's used in clearing the forest floor to manage wildfires, or to clear space for an expensive mushroom to grow that will bring in better income, or to clear the ground of noisy leaves as part of hunting practices.

Fire is often used as a form of protest in Thailand, too, said Olivier Evrard, a Thailand-based senior researcher at French National Research Institute for Sustainable Development (IRD). In 2018, a national controversy erupted when locals discovered that a branch of the judiciary had built a housing and residential complex in protected forest land at the base of a sacred mountain outside Chiang Mai. They eventually vacated the premises owing to the backlash, but fires are still set near the site every year, likely in protest.

There's been no shortage of policies attempting to regulate the burning. Thailand issued a national zero-burning directive starting in 2013, with different provinces implementing a blanket ban on burning at different times.

But people responded by burning before and after the zero-burn period, extending the haze season's duration, said Mary Mostafanezhad, a professor at the University of Hawaii who has studied air pollution in Chiang Mai. After seeing that the zero-burning policy did not work, Chiang Mai province adopted a newer policy: Fires could be set, as long as you applied beforehand.

The fires are to be submitted via FireD, an app developed by Chiang Mai university professor Chakrit Chotamonsak. The app uses weather and satellite data to predict if a fire on a particular day will cause more pollution or whether conditions will blow away the smoke and pollutants.

The researchers estimate that as many as half the fires in the province aren't registered, but they still consider the app a positive step. Even the decision to use FireD, which is the transliteration of "good fire" in Thai, in 2021 was an important shift, said Chaya Vaddhanaphuti, an independent researcher who had worked with the FireD team.

"This changes the perspective that fire was seen as a bad image, fire was seen as savage," he said, noting that many in rural northern Thailand depend on fire.

Yet, to the villagers, it's strange being asked to fill out paperwork for permission to do what they already know how to do -- set a prescribed burn in good weather so they can clear a patch of land for the coming year's crops.

Tiengtrongsakun, the Pakanyo chief in the village of Ban Mae Lan Kham, this year did paperwork for 100 households who needed to start a fire to clear their fields. Not everyone in the village speaks Thai or understands the latest government policy.

"If we hand them the document or registration form, they don't know what to do with it," he said. "Often they just throw the papers away."

Researchers say policymakers need to look at the conditions of people's lives and consider the details of which land is being burned and why. But it's difficult because of politics and economics. Many of the people living in the hills surrounding Chiang Mai belong to various tribal groups not formally recognized by the Thai government. Others struggle with access to good education and jobs. For now, burning remains the most efficient and cheapest way for people to do what they need to do, whether it's farming, hunting or clearing the forest floor.

"If burning is the easiest and most cost-efficient way to grow your crops, or to make a living, until that is not true, it's going to continue to happen," said Mostafanezhad.

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AP reporter Napat Kongsawad and producer Vasapa Wanichwethin contributed to this report.

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