From a rooftop bar on Nimmanhaemin in the second week of March, Doi Suthep is not there. The mountain rises to the west of the old city, more than a kilometre above it. For roughly 10 weeks each year it disappears behind a grey-brown screen of smoke. Visitors photograph the absence, residents check an air quality app before deciding whether to cycle to work, and hoteliers watch the same apps because the number on the screen now has a documented relationship with the number of guests in the building.
The smoke itself is not new. Farmers across northern Thailand, Myanmar and Laos burn crop residue and forest floor litter between February and April, a practice tied to maize contracting cycles, mushroom foraging and land clearance that predates any tourism economy. What has changed is that researchers have now identified a specific concentration of fine particulate matter at which Chiang Mai begins to lose visitors, and that figure belongs in every operator’s planning calendar.
The number that turns haze into economics
The threshold is 36.835 micrograms of PM2.5 per cubic metre of air. Chiang Mai University economists reached that figure using kink regression on quarterly data from 2012 to 2018, a finding carried in the GeoJournal tourism study, whose own panel analysis across seven Thai provinces independently confirmed that rising particulate matter reduces international tourist numbers. Below that concentration, higher PM2.5 tracks alongside rising visitor numbers, because pollution climbs steadily through the dry season that overlaps Chiang Mai’s busiest tourism months. Above it, the relationship inverts, and each further increase in PM2.5 cuts the number of tourists arriving in the province. The threshold sits at Chiang Mai’s own quarterly average, which means the city spends part of every year on the wrong side of its own tipping point.
PM2.5 refers to airborne particles smaller than 2.5 micrometres, fine enough to pass from the lungs into the bloodstream. The World Health Organization sets its annual guideline at five micrograms per cubic metre. During peak burning weeks, Chiang Mai’s readings routinely run above 150, and the city has topped the global pollution rankings on multiple mornings in recent seasons.
The estimate rests on pre-pandemic arrivals data, so the threshold describes visitor behaviour before COVID reshaped booking patterns and app-checking habits. The direction of the effect holds across the wider Thai research, and the exact tipping point warrants fresh calculation on post-reopening numbers. The nonlinear shape matters as much as the figure itself, because haze does not shave a fixed percentage off arrivals every day it lingers. It behaves more like a switch, tolerated up to a point and punished sharply beyond it, which is how travellers making discretionary decisions behave.
Inside the guest room
The damage does not stop at the hotel entrance. An indoor air study of homes in Bangkok and Chiang Mai measured PM2.5 inside Chiang Mai homes at a mean of 106.80 micrograms per cubic metre during haze episodes, against 5.52 micrograms in non-haze periods. Indoor air in the north swings by a factor of roughly 19 between seasons, a range far wider than anything the same research team recorded in Bangkok.
For accommodation providers, that finding redefines the product. A guest who retreats indoors during a bad air week has not escaped the problem unless the building itself is sealed and filtered, and most of Chiang Mai’s charming older guesthouses are neither. An indoor reading below the outdoor level is small comfort when the outdoor air sits at 150 and the room still carries a heavy share of it. Properties that invest in filtration, publish indoor readings and treat clean air as a listed amenity are selling something the destination cannot otherwise supply in March.
The health ledger behind the booking data
Visitors read no research papers before they rebook, yet they breathe the same air that residents do, and repeat visits follow the experience rather than the marketing, an experience the medical literature documents in detail. A student respiratory study of 406 university students in Chiang Mai found 32 per cent reporting respiratory allergies, with quality of life scores falling across respiratory, sleep and emotional measures during the high PM2.5 season. A separate outpatient visits study worked through 216,000 hospital visits in Chiang Mai between 2011 and 2014, linking respiratory caseloads to particulate concentrations. Travellers experience the same air and adjust their plans accordingly.
What the smoke costs a second city
Chiang Mai carries more weight in this conversation than its size suggests. The Thailand Economic Monitor for July 2024 records that Bangkok’s urban agglomeration holds a population 29 times larger than Chiang Mai, the country’s next largest city, and argues that unlocking secondary cities is central to Thailand’s growth model. National tourism strategy leans on the same logic, pushing visitors and revenue beyond the capital and the beach corridors. A secondary city cannot carry that role while its worst air of the year settles over March and April, months the tourism calendar needs working hard.
Policy has started to acknowledge the stakes. Thailand’s updated National Adaptation Plan names tourism as one of only six priority sectors for climate resilience, a designation recorded in the Thailand climate report from the World Bank. Tourism accounts for 8.8 per cent of greenhouse gas emissions worldwide according to a Nature Communications study, so the sector sits within the climate agenda as both affected party and contributor. Chiang Mai faces the question at close range, because the air that clouds its dry season crosses borders and farming calendars that reach well beyond the province.
What travellers are telling the market
Demand-side data points in one direction. A DASTA presentation to the Stock Exchange of Thailand, citing Expedia Group research, reported that 95 per cent of Asia-Pacific travellers want sustainable options when booking, the highest share of any world region. The Booking.com sustainability report for 2025 adds the resident view, finding that only 16 per cent of locals worldwide believe capping visitor numbers is the answer to tourism pressure, while their leading demands are investment in transport at 38 per cent, waste management at 37 per cent and environmental conservation at 32 per cent. Chiang Mai’s residents and Chiang Mai’s guests want the same purchase, and it is cleaner air rather than fewer people.
The operator’s response
Two things have to happen at once, and confusing them is how the haze debate stalls every year. The source of the smoke has to be addressed, through the slow, cross-border work of changing how land is cleared across the region, and until that lands the businesses that depend on Chiang Mai’s first quarter have to run their year around a problem they did not create and cannot individually fix. Planning around the season is not a substitute for solving it, because it is what keeps operators solvent long enough for the solution to arrive.
The threshold number gives them a planning constant the debate has lacked. Operators who treat the burning season as a known, quantified feature of the market rather than an annual surprise hold three advantages. They price and staff the affected weeks realistically instead of discounting in panic, they invest in the one product attribute the season rewards through verified indoor air quality and say so on the booking page, and they publish honest seasonal guidance, because a guest steered toward November leaves better reviews than a guest ambushed in March, and trust compounds across visits in a way a single booking never does.
The smoke settles over March and April, the tail of the high season and the Songkran weeks that rank among the busiest of the year. A destination does not get to lose commercially important months to smoke indefinitely, and that is the case for fixing the source, made in the language of revenue rather than grievance. In the meantime, the businesses that build their year around the number, rather than hoping each burning season will be different, are the ones that will still be trading when the air finally clears.
