Planning your trip · 28 September 2026 · 8 min read
Chiang Mai's smoke season: when it happens and how to plan around it
Every dry season, smoke collects in the valley around Chiang Mai. The shape of that season repeats from year to year. Its dates and its severity do not.
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Every year, smoke from forest and farm fires collects in the valley around Chiang Mai, usually from February into May and sometimes from January. The broad shape of the season repeats: a build-up through February, a peak that most often falls in March, and relief once sustained rain arrives, sometime between late April and May. The calendar does not repeat. The first days of poor air and the last ones move by weeks from one year to the next, and one season can be far worse than the one after it.
That decides how to plan. The seasonal pattern tells you which weeks carry the most risk, so it is what you choose dates by. Once you are in Chiang Mai, the air itself, on the day and over the last few days, tells you what to do with each day. This article covers both. Health questions are left to the public-health authorities whose guidance it cites.
When the smoke season actually happens
There is no official start date. Each year, Chiang Mai province announces a period during which open burning is banned or tightly controlled, and it is easy to read that period as the season itself. It is an administrative decision, and its dates move: it began on 15 February in 2023 and on 1 January in each year since, and it has ended anywhere from late April to the end of May. It tells you when the authorities expect fires. It does not measure the air.
The measurements come from the monitoring stations. The Pollution Control Department's regional office in Chiang Mai counts the days on which PM2.5, the fine particles used to track the smoke, averaged more than Thailand's 24-hour standard of 37.5 µg/m³, explained further down. In 2025 the first such day in the province came on 9 January and the last on 28 April. In 2024 they fell on 25 January and 9 May.
Across years, the pattern looks like this.
March is the usual peak. A study of ten years of monitoring data across upper northern Thailand, from 2013 to 2022, found PM2.5 at its highest in March. It is a tendency, not a rule. At the Chang Phueak station in the city, March clearly led in 2025, with 22 days above the standard against 14 in April. In 2024, April was as bad as March: 29 days in March, 30 in April.
Songkran comes after the usual peak, which makes it tempting to treat as the way out of the season. It is not a clean-air window to count on. In 2024, every day of April was above the standard at Chang Phueak, Songkran included.
| January | Usually still clear, but not reliably: the first days above the standard can come early in the month. |
|---|---|
| Early February | Often still reasonable. Days above the standard become more frequent as the month goes on. |
| Mid to late February | The season usually starts to build. |
| March | Most often the hardest month. |
| April | Not reliably better than March, and in some years just as bad. Songkran, the Thai New Year in mid-April, falls inside this window. |
| Late April to May | Relief usually comes with sustained rain, somewhere in this window. |
Overview of Chiang Mai's seasons
How bad will it be in your year?
Nobody can tell you in advance. In Chiang Mai province, the number of days above the national standard fell from 114 in 2023 to 87 in 2024 and 60 in 2025, on the regional office's count, and the office put 2025 at half the average of the five years before it. Two falls in a row are not a forecast. The office ties each season to how much burns and to the weather, and both change from year to year.
Those counts cover the whole province: a day counts if some stations are above the standard, even when others are not. In 2025, the Chang Phueak station in the city recorded 46 such days, against 60 for the province.
Within a season, the air also moves from day to day. Even in March 2025, the worst month at Chang Phueak, 9 days stayed within the standard. Rain can bring a real break: after rain in March 2024, Thailand's Department of Health reported that the air across the upper north had begun to improve. The break was short: at Chang Phueak, every day of the following April was above the standard. The season is not over until sustained rain arrives.
What actually changes for a visitor
The views. Distance is the first thing smoke takes. The view over the city from the terrace of Wat Phra That Doi Suthep, and the ranges you would otherwise see from Doi Inthanon, are what the smoke hides most reliably. The temples stay open and the mountain roads stay usable, but a trip built around views is the most exposed.
Outdoor exertion. When fine-particle levels are high, health authorities advise making long or intense outdoor activity shorter and less strenuous, and the advice applies at lower levels to sensitive groups. That weighs on hiking, cycling, running and long walking days far more than on a temple visit or a meal.
Heat. March and April are also the hottest months of the year. On any strenuous day, smoke and heat add up.
Forest restrictions. During the smoke season, the authorities can restrict entry to forest areas to prevent fires. Such orders change in dates and scope from year to year. If a walk through forest matters to your plans, check the current notices for that park before you go.
What still works. Most of the city carries on as usual: temples, markets, restaurants, cafés, massage, a long lunch. The smoke changes how you arrange a day more than whether the day is worth having.
Doi Suthep, and when the view is clearest
How to read the air in 30 seconds
Look at the PM2.5 figure, in micrograms per cubic metre (µg/m³), before the index number. Apps turn PM2.5 into an air quality index, but not all on the same scale: Thailand's index and the American one give the same reading a different number and a different colour. The concentration is the figure they share.
Two reference points help. Thailand's national standard for PM2.5, averaged over 24 hours, has been 37.5 µg/m³ since June 2023. The World Health Organization's 2021 guideline for the same 24-hour average is 15 µg/m³. Both are daily averages, so a single hourly figure above them is a warning sign, not yet a day above the standard. They help you read a number. Neither is a line at which we would tell you to cancel.
For the figures themselves, Air4Thai (website and app), run by the Pollution Control Department, is the official monitoring reference. IQAir is a practical second tool for day-to-day checks; it uses the American index and updates hourly.
Read the trend. One hourly figure can jump or fall with the wind. The last two or three days, and whether they are climbing or easing, say more about tomorrow than the number at breakfast.
Should you move your dates?
That depends less on the smoke than on what the trip is for.
If your dates are flexible. November to January are usually the clearest months, with January the least certain of them. If the trip has to fall later in the dry season, March carries the most risk in most years, and it is the month we would move first. Late February and April are a gamble: in some years they are clearly better than March, in others April is just as bad. May is usually clearer, though not reliably, and it is hot.
If your dates are fixed and the trip is about the outdoors. A trip built on views, treks or long rides by bicycle or motorbike is the one smoke can empty. Keep those days movable and prefer bookings you can shift. If one particular view is the reason for the whole journey, that is the case where we would still try to change the dates.
If your dates are fixed and the trip is about the city. The case for going anyway is strong. Temples, food, markets and cafés all work on a smoky day, and the adjustments are modest. The exception is anyone in a group that health authorities treat as sensitive, covered below.
If you move your dates: rain, heat and festivals in the other months
Living with a smoky week
Nothing below makes the smoke go away. It makes a smoky week easier to live with.
Plan each day the evening before. Look at the recent trend, decide the next day's outdoor plan from it, and give that plan an indoor fallback.
Choose the room for its air. The US Environmental Protection Agency advises staying indoors and filtering indoor air when particle levels are high, and Thailand's Department of Health advises reducing time outdoors and keeping doors and windows closed. In practice, ask whether the room has a working air purifier.
Carry a fitted respirator. The EPA says a well-fitted N95 respirator can help, and warns that a mask held by ear loops, or worn over a beard, will not seal properly. Thailand's Department of Health also advises wearing a mask when levels are high.
If you are in a sensitive group. Children, older adults, pregnant women and people with heart or lung disease are among the groups health authorities treat as sensitive to fine particles. If that includes you or someone travelling with you, follow the official guidance and ask a doctor before travelling in the season. This article is not medical advice.
The five-day itinerary, with an afternoon that can move indoors
Where to stay, and when to book
Why Chiang Mai fills with smoke
The smoke comes mostly from burning, in forests and on farmland, in northern Thailand and across its borders. The Pollution Control Department names burning in forest and farming areas as the main cause. A modelling study of the 2019 season estimated that about 29% of the PM2.5 over Chiang Mai province came from outside Thailand on average over the season, and at least half on the days when the air was unhealthy.
Geography does the rest. Chiang Mai sits in a basin ringed by mountains. In the dry season, a temperature inversion, a layer of warmer air lying over cooler air near the ground, can hold the smoke in the valley instead of letting it disperse.
The season ends when the weather turns. Storms in late April and May, and then the rainy season, clear the air and damp down the fires. NASA's Earth Observatory describes the fires in the region fading with the May rains.