It's 32 deg C in Chiang Mai as cars on the highway speed towards an invisible mountain. Doi Suthep, which usually looms over the city, has vanished into the ash-coloured horizon.
Welcome to smog season. From February to April every year, the mountainous region of northern Thailand turns from tourist magnet to an eye-watering hell as smoke from corn farms being cleared settles into the valleys.
Amid soaring temperatures, planes are occasionally grounded, while hospitals register a 50 per cent spike in patients with eye and skin allergies, as well as lung and heart ailments, say officials. Locals start watching air pollution readings as well as more homespun indicators.
"If I can't see Doi Suthep... I don't go anywhere," Chiang Mai's chief provincial health officer, Dr Paisan Thanyawinichkul, tells The Straits Times as he fiddles with his personal bottle of antihistamine eyedrops. "I close the doors and windows."
There are many culprits behind the filthy air. Private vehicles are one. Chiang Mai's provincial capital may be Thailand's second-largest outside Bangkok, but it still lacks a metro system or equivalent form of public transport. Between 2010 and last year, the number of private cars, lorries and vans registered in the province grew by 53 per cent. Urban sprawl chews up greenery that would otherwise act as a filter.
Homeowners in this vast, mountainous terrain find it easier to burn their rubbish rather than truck it to a disposal facility hours away.
Locals set fire to the forests to ferret out wildlife and pick an exotic mushroom, which sprouts from the charred ground. These dark, spherical hed thob are sold by foragers from makeshift roadside kiosks for as much as 1,000 baht (S$39) a kg.
But the biggest culprit by far is open burning on corn farms, which have steadily expanded in tandem with demand for animal feed.
According to the Asean Agricultural Commodity Outlook report, maize plantations in Thailand expanded by 77,880ha between 2008 and last year. Yet the 4.6 million tonnes of corn produced last year was only enough to meet about nine-tenths of local demand.
Corn is a major component of animal feed. It is mixed into fodder for chickens, ducks, cows and even fish either consumed in Asean's second-biggest economy or exported to the rest of the world.
While northern farmers have long burned their agricultural waste, experts say the haze became serious only over the past decade, when rising corn prices enticed highland smallholders to convert to the crop.
These farmers either sold their corn to middlemen or directly to Charoen Pokphand (CP) group, a Thai agricultural conglomerate, via contract farming arrangements.
Corn yielded quick cash because it could be harvested in four months. Plus, CP not only guaranteed the purchase price, but also reduced farmers' outlay by supplying seed and fertiliser, and then deducting their costs from the final payment.
The crop unfortunately also leaves the same amount of stubble, husk and other residue in weight as it does in corn - all of which is burnt at the end of each harvest.
Last year, when smog became so severe that the Singapore Armed Forces sent two Chinook helicopters to help fight the fires, CP - which also sources for corn in Laos, Myanmar and Vietnam - was singled out for criticism. It denied responsibility for the haze, but ended farming contracts in Mae Chaem, a particularly notorious "hot spot" in Chiang Mai province.
One of the farmers cut loose, Mr Chitnarong Chompootan, now sells his corn for 12 baht per kg instead of 16 baht.
"We were the scapegoats," he laments. "We were only responsible for 20 to 30 per cent of the smoke. The rest comes from elsewhere."
Indeed, satellite images during this period tend to show similar hot spots raging in Myanmar and Laos, which researchers also attribute to corn farms.
The price of animal feed corn dipped to an average 9.45 baht per kg last year from 10.51 baht per kg in 2012, according to data from the Thai Feed Mill Association. Yet some farmers have responded by clearing more land for corn.
"They have to keep expanding their plantation to get the same amount of money," says Dr Nion Sirimongkonlertkul from the Rajamangala University of Technology Lanna, who has been studying the issue for over a decade.
Air pollution continued to hit unsafe limits in the provinces of Chiang Mai, Chiang Rai, Phayao and Mae Hong Son this week despite a 60-day burning ban until April 15.
Medical experts warn of serious long-term health effects if no sustainable solution is found.
The incidence of cancer of the trachea, bronchus and lung in the north is the highest among all regions, a phenomenon which they suspect is linked to the annual smog.
Officials are cajoling farmers to turn corn stalks into fertiliser, while Warm Heart Foundation, a Chiang Mai-based, non-governmental organisation, is trying to promote charcoal made from corn residue.
The farmers, weary of being reproached annually, are open to suggestion.
"If we have good water supply, we can do longan," says Mr Chitnarong. "What I want is for the government to take good care of the farmers, not just throw us some seeds and go away."
Chiang Mai fights seasonal haze. Watch the video at http://str.sg/ZtwY and read the blog at http://www.straitstimes.com/asia/se-asia/where-burning-is-a-way-of-life