CHIANG MAI -- This 700-year-old city nestled in a green valley of northern Thailand has long enjoyed a story-book reputation for its myriad Buddhist monasteries and charming wooden homes, the delicate manners of its citizens and a tranquillity hardly ruffled by its temple bells.

More than 10 million tourists visited the city and its surroundings in 2018, according to official data, while local expatriate groups estimate that the area has attracted up to 60,000 retirees from various countries.

But recent visitors to the "Rose of the North,'' as it has often been called, may have wondered what the fuss was about.

On some days between February and early May, Chiang Mai was ranked as the world's most polluted city, with air quality below notorious pollution hot spots such as New Delhi, Beijing and Dhaka, according to official measurements.

At such times, only faint outlines of the mountain that looms behind the city could be seen, while locals and tourists donned face masks and shunned the many outdoor cafes. Some people and pets coughed blood, and hundreds were treated at hospitals for respiratory diseases. Chiang Mai already records the country's highest incidence of lung cancer.

A worshipper stands before a temple in the Wat Inthakhin Sadue Muang, one of Chiang Mai's ancient monasteries. (Photo by Denis D. Gray)

The main cause of this pollution horror was a regional inferno of burning fields and forests. Every year, despite dire warnings and penalties, farmers across northern Thailand set post-harvest crops alight while villagers ignite mountainsides to harvest mushrooms, ants' eggs and other forest products for commercial sale. Similar burning occurs in neighboring Myanmar, Laos and Cambodia.

The grounds of Wat Phra Singh, one of the finest monasteries in the city. (Photo by Denis D. Gray)

With some 70% of municipal income derived from tourism, a sharp drop in Thai and foreign visitors this year has bruised Chiang Mai's economy, with hotels reporting bookings down by up to 20%. Moreover, the annual "burning season," widely regarded this year as the worst ever, is not the only challenge facing Thailand's northern hub.

An older generation that grew up riding in trishaws now endures rush-hour traffic jams in a burgeoning metropolitan area of 12 million people with virtually no public transport. Toxic exhaust gases fume from city traffic, while haphazard development mars a once graceful urbanscape.

Privately owned vehicles make up more than 90% of traffic. A study by Chiang Mai University forecast in 2017 that 2.6 million vehicles will cram the city's roads by 2026, double the level a decade earlier. For years, local operators have maintained a tight hold on the songthaew, adapted pickup trucks that are the closest thing to public transport.

Construction of a light rail system was due to start in 2020, delivering a 12-km line after five years. But Chiang Mai Mayor Tassanai Buranupakorn now says building work is likely to be delayed.

Nawit Ongsavangchai, an architecture expert at the University of Chiang Mai, said plans for appropriate and sustainable development have been hobbled by widely disparate views of the city's future among locals, incoming Bangkok businessmen, foreign residents, tourists and ethnic minority migrants.

Achitect Nawit Ongsavangchai explains a student project at Chiang Mai University. (Photo by Denis D. Gray)

Above all, Nawit said, city officials have failed to deal with urgent issues in a timely, comprehensive manner, allowing uncontrolled development. "In the past we didn't care much about preserving our history," he said. "We built, built, built, so the old houses disappeared from our urbanscape.''

Chiang Mai's economic rise began during a boom in the 1980s that saw Bangkok investors flocking to build condominiums, high-rises and hotels. Some violated regulations, while others took advantage of the country's porous planning acts, which usually expire after seven years. Gaps before each new act is approved allow developers to exploit loopholes.

"Chiang Mai, like Bangkok, is suffering from the disease of rapid and unregulated development. The grand dame is sick," political scientist Tanet Charoenmuang said in 1995, as the city celebrated the 700th anniversary of its founding.

The patient has worsened since then, although the city remains a draw for many, retaining nooks and alleys of abiding charm and blessed by stunning natural surroundings. Tourists still come, led by visitors from China, and the city continues to attract artists, designers, writers and digital nomads. In 2017 Chiang Mai was designated a UNESCO Creative City for Crafts and Folks Arts, and the city fathers are preparing a bid for UNESCO World Heritage site status.

It will not be easy. As Suwaree Wongkongkaew, head of the Chiang Mai City Arts and Culture Center, noted: "There are some people who think that Chiang Mai is too bruised to be considered a World Heritage site. They don't see how we can push for the status within the current mechanisms of the city management plan.''

The core of the UNESCO proposal will highlight Chiang Mai's Old City, a 2.2-sq-km area flanked by walls and a moat, as well as Doi Suthep, the mountain that rises 1,676 meters over the city. Both are under pressure.

The Old City, the seat of bygone kings, is packed with restaurants, boutique hotels, souvenir shops and tattoo parlors. No building can rise above 12 meters, and should incorporate some ''local features." But many structures are clearly inappropriate, such as a concrete blockhouse serving as a Toyota dealership.

The statue of an elephant, an icon of old Chiang Mai (Photo by Denis D. Gray)

Many inhabitants regard Doi Suthep as sacred, harking back to the legend of a white elephant believed to have carried part of Buddha's shoulder bone to its summit. Nevertheless, the mountain has been much abused. Although it is part of a national park, sizable segments have been excised for a botanical garden, a safari park and official institutions.

The latest incursion sliced 23.5 hectares from the foot of the mountain to build bungalows, condominiums and vacation homes for judges and court officials. Despite strong protests by local environmental groups, a recent court ruling favored the developers. Activists have also decried a forest of multistory city billboards advertising everything from moisturizing creams to pickup trucks that block formerly panoramic views of the mountain.

"The loss of a jewel of Thai culture, the loss of a unique repository of an older Southeast Asian Buddhist way of life to commerce, tourism and agribusiness has come about more or less by lack of design," said James Stent, a former banker who serves as a consultant on Asian heritage projects. He first came to Chiang Mai in 1967.

Stent said the cultural destruction of Chiang Mai has been "more pronounced than anywhere else" in Thailand. "The charm and culture of half a century ago is now to be found packaged for tourists," he said. "Businesses have prospered from the mass onslaught of tourism, but at the cost of making the city into a theme park, a parody of old Chiang Mai."

This kind of criticism is unlikely to stem the tourist tide, but authorities are clearly worried that publicity over Chiang Mai's pollution crisis this year will deflate visitor arrivals. Official statistics are not yet available, but by one estimate -- from guesthouse owner Annette Brady -- arrivals plunged by 30% for the Thai New Year festival, the high point of Chiang Mai's annual tourism calendar.

Some Thais and foreign residents also left, heading to beaches or abroad to escape the pollution. Businesses complained about lack of customers and even such sites as Thapae Gate along the Old City wall, particularly popular with Chinese tourists, were nearly empty on some days.

"Will people remember the pollution this year and not book next year during the smoky season? Yes, I think they will," said Brady, a longtime resident.

Even if traffic and construction pressures are eventually countered, there appears to be no effective answer to the annual burning ritual. "We have done everything we know how to reduce the problem," provincial governor Supachai Iamsuwan told the local magazine Citylife. "How to take fire away from the forests? How to change attitudes? It comes back to awareness and education."

Even so, the governor refused calls to declare the province a disaster zone, fearing that would tarnish Chiang Mai's image as a tourist destination.