TL;DR: Burning season in northern Thailand is the roughly February-through-April stretch when farmers burn crop residue, wildfires spread through dry forest, and smoke drifts in from Laos and Myanmar, pushing Chiang Mai’s air to hazardous, sometimes world’s-worst levels. On March 29, 2026, IQAir recorded an AQI of 263 and PM2.5 of 188 µg/m³ in the city, more than 12 times the WHO 24-hour guideline, and Chiang Mai ranked among the world’s most polluted major cities repeatedly that spring. If you’re sensitive to smoke, have young kids, or are pregnant, avoid the north from February to April and head to the islands instead.
Most Chiang Mai travel content either skips this entirely or buries it in a single vague line about “haze season.” That’s a disservice, because burning season is the single biggest reason a trip to northern Thailand can go from spectacular to genuinely unpleasant, or unsafe for some travellers, and the honest details, real AQI numbers, which weeks are worst, who should actually skip it, rarely make it into the glossy version. This guide gives you that, sourced to IQAir, the WHO, and Thai news reporting from the 2026 season, as a companion to outthailand.com’s best time to visit Chiang Mai guide.
Where prices are mentioned (masks, air purifiers), they’re in Thai baht (THB) with US dollars in parentheses at ฿33 = US$1 (July 2026).
Table of Contents
- What causes burning season?
- When exactly does it happen?
- How bad does the air actually get?
- Who should skip northern Thailand during this window?
- How do locals and expats cope?
- What does this mean for your travel plans?
- Is the south a better bet during these months?
- Is burning season getting better?
- FAQ
What causes burning season?
Northern Thailand’s burning season is driven by three overlapping sources, not a single cause. Agricultural burning comes first: farmers burn rice stubble, corn husks, and sugarcane residue after harvest because it’s the cheapest, fastest way to clear a field before the next planting cycle. Forest fires add to it, spreading through tinder-dry hillsides in the dry season, sometimes started deliberately to clear land or encourage the growth of edible wild mushrooms. And it’s genuinely regional: smoke drifts across the border from agricultural and forest burning in Myanmar and Laos, so what Chiang Mai breathes isn’t purely a Thai problem. Chiang Mai University researchers describe the upper North as sitting in a basin ringed by mountains that traps pollution rather than letting it disperse, which is a large part of why Chiang Mai city, specifically, tends to fare worse than flatter parts of the country during the same weeks.
When exactly does it happen?
Haze typically creeps in through January, builds through February, and peaks in March into early April, before easing once the first heavy pre-monsoon rains arrive, usually somewhere between mid-April and May. The table below shows how that plays out month by month, based on 2025-2026 reporting; treat it as a pattern, not a guarantee, since the exact timing shifts every year.
| Month | Typical severity | What’s happening |
|---|---|---|
| January | Low-moderate, rising | Early creep as dry-season burning begins; usually the calmest month of the window |
| February | Moderate-high | Official 2026 agricultural burning ban began Feb 1; burning still increases regardless |
| March | Peak, often hazardous | Worst month most years; Chiang Mai hit AQI 263 / PM2.5 188 µg/m³ on March 29, 2026, and ranked world’s most polluted major city on March 26, 2026 |
| April | Still severe, easing late | 2026 burning ban technically ended March 31, but wildfire activity and haze continued; Chiang Mai hit AQI 231 (Apr 2) and 209 (Apr 7); Songkran (Apr 13-15) falls in this window |
| May | Clearing | First sustained rains typically arrive, clearing particulates fast; by July, Chiang Mai’s PM2.5 AQI is back to the “good” range |
Figures compiled from IQAir’s 2026 newsroom reports and Nation Thailand’s 2026 season coverage; see Sources. Individual years vary, 2026 itself started later than usual, with clear skies through February and early March before conditions deteriorated sharply.
How bad does the air actually get?
Genuinely hazardous, and this isn’t tourism-board hedging. On March 29, 2026, IQAir recorded an AQI of 263 in Chiang Mai with PM2.5 at 188 micrograms per cubic meter (µg/m³), more than 12 times the WHO’s 24-hour PM2.5 guideline of 15 µg/m³. IQAir ranked Chiang Mai the world’s most polluted major city on the evening of March 26, 2026, its second most polluted at an AQI of 231 on April 2, and its most polluted again at an AQI of 209 on April 7. Those are individually verified, ranked days, not a single cherry-picked headline.
The full-year picture is more mixed than the worst days suggest: Chiang Mai’s 2025 annual average PM2.5 was 18.2 µg/m³ (a “moderate” AQI of 63), still 3.6 times the WHO’s annual guideline of 5 µg/m³, but far below the peak-week extremes. In other words, burning season is a concentrated, brutal few weeks rather than a pollution problem spread evenly across the calendar, which is exactly why the timing matters so much for trip planning.
Who should skip northern Thailand during this window?
If you’re in a higher-risk group, this is the honest answer: skip Chiang Mai, Chiang Rai, and Mae Hong Son from February to April, or at least build serious flexibility into your dates. Prolonged PM2.5 exposure worsens asthma and bronchitis, strains the heart, and hits children, pregnant women, and older travellers hardest, and hospitals in Chiang Mai and Chiang Rai reported a real rise in respiratory-related admissions during the worst weeks of the 2026 season. If you or a travel companion has a respiratory or cardiac condition, is pregnant, or is travelling with young kids, that risk is concrete, not theoretical. See outthailand.com’s Thailand vaccinations and staying healthy guide for broader health-prep context, and note that this guide is informational, not medical advice, check with a doctor if you’re in a higher-risk group and your travel dates aren’t flexible.
For everyone else, it’s a judgment call: some travellers simply accept a few smoky days for cheaper shoulder-season rates and thinner crowds; others reroute entirely. Neither is wrong, but go in with real numbers, not hope.
How do locals and expats cope?
People who live through it every year rely on a fairly consistent toolkit. N95 or KN95 masks are the baseline, widely available at pharmacies (Boots, Watsons, and local chemists) for roughly ฿20-150 (US$0.60-4.50) per mask; surgical and cloth masks don’t filter PM2.5 effectively, so they’re not a substitute. Air purifiers with HEPA filters are the second layer, small units start around ฿5,000 (US$150), and many long-termers just ask hotels or serviced apartments directly whether rooms have one before booking. AQI apps (IQAir’s AirVisual, or Thailand’s own Air4Thai) get checked every morning, since pollution often peaks around midday, and outdoor exercise typically gets skipped entirely on days above AQI 150. Some residents build a fixed “escape trip” south into their calendar every year, timing a few weeks in Phuket, Krabi, or the Gulf islands to coincide with the worst of March.
What does this mean for your travel plans?
Two things collide here that most itineraries don’t plan for. First, Songkran (Thailand’s water festival, April 13-15, 2026 nationally, with Chiang Mai’s own celebration extended across 12 days, April 6-17) falls squarely inside burning season’s tail end. Some years the festival gets a clean-air reprieve if the first rains arrive early; other years, revelers are throwing water under a genuine haze. There’s no way to guarantee which in advance, check air quality forecasts as your dates approach rather than assuming a festival means clean air.
Second, mountain viewpoints lose their entire reason for existing on bad days. Doi Suthep, the temple-topped mountain overlooking Chiang Mai city, can vanish into a pale, featureless haze, and Doi Inthanon National Park’s layered valley views, the main draw of the trip, flatten into a murky, yellow-brown blur in photos. If those views are the point of your visit, that’s a legitimate reason to shift your dates outside late February through early April, or to hold your day-trip plans loose and chase whichever day the AQI forecast looks best.
Is the south a better bet during these months?
For most travellers with flexible dates, yes. The Gulf islands (Koh Samui, Koh Phangan, Koh Tao) and the Andaman coast (Phuket, Krabi, Koh Lanta) sit outside the mountain-ringed basin where northern Thailand’s smoke collects, and they’re not part of the agricultural burning belt feeding it, so they’re largely unaffected by the same haze. February-April also lands solidly within the south’s dry season, making it genuinely one of the better windows to visit the islands rather than a consolation prize. See outthailand.com’s best time to visit Thailand guide for how the north’s smoke season and the south’s dry season line up against each other across the full year, and outthailand.com’s Thailand weather by month guide for the region-by-region detail.
Is burning season getting better?
Genuinely mixed, not a clean trend either direction, and any source claiming otherwise is oversimplifying. Chiang Mai’s annual average PM2.5 actually improved, from 26.4 µg/m³ in 2024 to 18.2 µg/m³ in 2025, per IQAir, yet 2026 still produced some of the worst single-day readings on record, including the March 29 spike to AQI 263. On the policy side, Thailand’s official 2026 agricultural burning ban ran February 1-March 31, but wildfire activity and hazardous air continued well into April, after the ban had technically ended, land-clearing pressure ahead of the May planting season doesn’t stop just because a ban expires. More encouragingly, Thailand, Laos, and Myanmar launched a joint 2026-2027 action plan in 2026, sharing satellite fire-hotspot monitoring under a “Clear Sky” strategy to tackle the cross-border haze at its regional source, rather than treating it as a purely domestic problem. Whether that cooperation moves the needle on the ground is still an open question; expect a bumpy, non-linear few years rather than steady improvement.
Where to next
For the full seasonal picture across northern Thailand, start with outthailand.com’s best time to visit Chiang Mai guide, which covers the cool, hot, and rainy seasons alongside this one. The Thailand weather by month guide lays out the national calendar so you can see exactly where burning season sits against everywhere else, and best time to visit Thailand gives the country-wide overview if your dates are still flexible. If safety concerns beyond air quality are on your mind, see is Chiang Mai safe, and for broader health prep, see Thailand vaccinations and staying healthy. Whatever the air quality does, check outthailand.com’s live Chiang Mai events listings, indoor markets, workshops, and community events are a solid way to fill a smoky afternoon without breathing more of it than you have to.
Sources
- IQAir: Chiang Mai among top 10 most polluted cities during Thailand’s burning season, March 4, 2026: early-season AQI and PM2.5 readings.
- IQAir: Chiang Mai among top 10 most polluted cities in the world, March 27, 2026: world-ranking data, 2025 annual PM2.5 average, WHO guideline comparison.
- IQAir: Chiang Mai among top 10 most polluted cities in the world, April 16, 2026: April conditions, long-term average context.
- Open The Magazine: Chiang Mai Air Crisis 2026: March 29 peak AQI/PM2.5 reading, hospital admissions, wildfire hotspot counts.
- Nation Thailand: Northern wildfire surge pushes Thailand hotspots to 2026 high: 2026 agricultural burning ban dates, hotspot counts, Chiang Mai University basin description.
- Nation Thailand: Chiang Mai ranks world’s second most polluted city as AQI hits 231: April 2, 2026 ranking data.
- Khaosod English: Thailand, Laos step up cooperation on cross-border haze under Clear Sky strategy: regional policy cooperation, 2026-2027 action plan.
- World Health Organization: WHO Global Air Quality Guidelines: PM2.5 annual and 24-hour guideline thresholds.
- outthailand.com Best Time to Visit Chiang Mai: seasonal overview and smoke-season context.
- Xe.com: USD/THB Currency Converter: exchange rate reference, July 2026.