Chiang Mai burning season

For most of the year Chiang Mai's air is fine, and from June to November it's often lovely. Then, somewhere around February, the mountains around the city start to fade into a grey-brown haze, the sun goes orange in the afternoon and you can smell smoke in the evening. That's burning season. It comes back every year, and if you live here you learn to plan around it. If you're visiting, it's worth knowing about before you book.

Below you'll find why it happens, when it tends to peak, how bad it can get, and what the health officials say to do. There's also a calculator: pick any date and it shows you what PM2.5 looked like on that date in past years.

The short version

When

Roughly February to April. Fires pick up from February, usually peak in March and fade by May once the rain comes. In our data, the smokiest stretch of the year runs from early March to around 10 April.

Why

Mostly small agricultural fires and forest fires across northern Thailand and the region, trapped in Chiang Mai's valley by the mountains around it.

How bad

On bad days it's genuinely bad. City monitoring stations averaged 79.7 µg/m³ of PM2.5 across the whole of April 2024. That's more than five times the WHO's 24-hour guideline of 15.

What to do

Check a live air app every day, cut back on outdoor activity when PM2.5 goes over 37.5, wear an N95 when it's bad, and run a purifier in a closed room.

Air quality calculator

Pick a day and a month. The calculator shows the average PM2.5 for that date across the past few years, the best and worst year, and what that level means on Thailand's air quality bands. Each year's number is the average of that date and the 3 days either side, so one odd day doesn't throw it off. The chart shows the whole year, with your date marked.

This is based on past years, not a forecast. Smoke depends on fires and weather, and one year can be very different from the next. For the air today, check Air4Thai, CMU CCDC, IQAir or AQICN.

Where the numbers come from

I wanted real station readings for this, but I couldn't get Thailand's official station history in a form I could use here. So the calculator uses estimates from the CAMS global model run by the EU's Copernicus programme, for the grid square that covers Chiang Mai city. The model's history for Chiang Mai starts in August 2022, which gives four burning seasons, 2023 to 2026, and four or five years for every date. Each grid square is about 45 km across, so it's an average over the whole valley rather than your street.

The model gets the shape of the year right: clean from June to December, rising from February, worst in March and early April. It's good for answering "is this a smoky time of year?" It's not good at the very worst days, which brings me to the next part.

How far off the model is

On smoky days the model reads low, sometimes a lot lower than the city's monitoring stations. Here's how its monthly averages compare with measured station averages published by SmogRank, which collects data from Thailand's official stations:

April 2024

Stations averaged 79.7 µg/m³. The model said 29.9.

April 2026

Stations averaged 68.1. The model said 54.3.

March 2025

Stations averaged 51.0. The model said 44.3.

March 2026

Stations averaged 34.6, and here the model was a little high at 38.3.

The gap is biggest on the worst days. The highest day in the model data is 99.4 µg/m³, in 2026. The worst day in the station data was 223.0 µg/m³, on 7 April 2023, followed by 195.0 on 1 April and 188.0 on 8 April that year. So if the calculator says a date is orange, treat that as "often orange, sometimes much worse". The plus side: in the clean months the model and the stations agree well. The cleanest month in the station data recently was September 2025, at 10.1.

What burning season is and why it happens

The smoke comes from a lot of small fires rather than one big one. NASA's Earth Observatory describes it as mostly small agricultural fires and forest fires burning across northern Thailand and the region. The upper North's dry season runs from January to April. Researchers looking at fires in northern Thailand say forest fires dominate in the upper North and peak in March and April, that the forest fires are set by people, and that fires lit to clear corn fields often spread into the forest. Burning leftover crops matters more further south, in the lower North.

It's regional too. On 29 March 2026, Thai officials counted 480 fire hotspots in Chiang Mai province alone and 1,312 across the North, even though open burning had been banned in the province since 1 January. Smoke from fires in neighbouring countries drifts in as well, which is why the ASEAN haze centre raised its regional alert to the highest level, level 3, on 27 March 2026.

Then there's the geography. Chiang Mai sits in a valley with mountains on most sides. NASA explains that the mountains block the air from moving and encourage temperature inversions, where a layer of warmer air sits on top of cooler air near the ground like a lid. Smoke from local fires and haze from further away get trapped under it. That's why the view of Doi Suthep from town can disappear completely. NASA also notes that roughly 70% of the PM2.5 in Chiang Mai in April comes from burning plants and crops.

When it peaks and how bad it gets

In the model data, the smoothed average climbs above Thailand's standard of 37.5 µg/m³ on 27 days a year, from 5 March to 10 April, and peaks around 1 April at 46.7. The cleanest time is late July, at around 8.6. Mid-March averages about 40, and mid-August about 12.

Years vary a lot. Counting days over 37.5 between January and May, the model had 40 in 2023, only 8 in 2024, 29 in 2025 and 45 in 2026. But remember the model reads low: the stations put April 2024 among the worst months anywhere, sixth worst of 2,929 cities SmogRank tracked that month.

Some recent bad spells, from the news at the time:

24 March 2025

Chiang Mai University's sensors read 611 µg/m³ for one hour at Ban Pang Fueang, 256 in Chiang Dao and 177 in the city centre, and IQAir ranked Chiang Mai fifth worst in the world.

1 April 2025

150 hotspots were counted across 15 districts, and Chiang Mai was ranked the third most polluted city in the world.

5 April 2025

Chiang Mai's regional environment office counted 44 days over the standard since 1 January.

31 March 2026

Eight northern provinces, Chiang Mai among them, were in the red band. The highest reading was 330.2 µg/m³ at Mueang Na in Chiang Dao. The next day officials ran artificial rain operations.

Notice that some of the worst numbers come from outside the city, in places like Chiang Dao and Mae Chaem, closer to the forests that burn. The mountains aren't an escape in March. On 5 April 2025, Mae Chaem read 107 µg/m³ and Mae Hong Son 172.2, and a local paper reported tourist arrivals in Pai down 90% during the 2026 smoke.

What the numbers mean

PM2.5 is the amount of very fine dust in the air, in micrograms per cubic metre (µg/m³), averaged over 24 hours. Thailand tightened its 24-hour standard from 50 to 37.5 on 1 June 2023 and changed its colour bands to match. These are the bands the calculator uses:

Blue, very good

0 to 15.

Green, good

15.1 to 25.

Yellow, moderate

25.1 to 37.5.

Orange, starting to affect health

37.6 to 75.

Red, affects health

Over 75.

The World Health Organization is stricter. Its 2021 guidelines say PM2.5 shouldn't go over 15 on more than a few days a year, and should average no more than 5 over a year. Apps like IQAir and AQICN show a US-style AQI number by default, which is a different scale from µg/m³, so a reading of "150" in one app and "55" in another can be the same air. Check which unit you're looking at.

Health advice

This is what Thailand's Department of Health advises during smoke season. It's general advice, not medical advice, so if you have a condition, talk to your doctor.

Check every day

Track PM2.5 on Air4Thai or the Life Dee app, and plan your day around it.

Over 37.5

Cut back on outdoor activity, especially long or hard exercise outside.

Over 75

Stay mostly in a clean-air room, and wear an N95 mask when you go outside.

Risk groups

Young children, older people, pregnant women and people with heart or lung disease should avoid outdoor activity in the red band.

Masks

Use a mask that fits snugly, and replace it if it gets discoloured or torn.

Get help

If you have severe symptoms, see a doctor.

Practical tips for living with it

Make one clean-air room. This is the Department of Health's main advice and it's what most locals do: close the windows, seal the gaps around doors and windows, run an air purifier, change the filter about every six months, and wipe surfaces with a damp cloth rather than sweeping dust back into the air. A bedroom is the obvious choice, since that's where you spend the most hours.

Find public clean-air rooms. The Department of Health runs a finder for certified clean-air rooms at podfoon.anamai.moph.go.th. It lists schools, clinics, hotels, restaurants and cafes that have one.

Stock up on N95s before you need them. They cost more than ordinary masks, and Thai PBS reported in 2025 that the cost is why few people in the area were wearing them. Having a box at home means you're not deciding on the worst morning.

Move your plans indoors. On the bad days, I'd swap the hike for something under a roof: the Chiang Mai City Arts and Cultural Centre, Art in Paradise, a mall like Maya or Central Chiangmai, a cafe or a coworking space. I can't tell you how clean the air is inside any particular place, so if it matters to you, ask whether they run purifiers, or use the clean-air room finder above.

Check before you head out. The air can change a lot from one day to the next, so look at the live reading before a run or a day trip rather than going by yesterday.

What it means for visitors

If you can choose, come between June and November, when the air is usually clean. December and January are usually in the good to moderate range. February is often moderate, and the smoke is starting. From early March to mid-April, expect hazy views, some days in the orange band and some much worse, and a real chance that a trip planned around hiking, viewpoints or outdoor things to do ends up mostly indoors.

If you're already booked for March or April, don't panic. Plenty of people visit then and enjoy it. Pack N95 masks, pick a hotel room with air conditioning (or ask whether it has a purifier), keep a few indoor plans in your back pocket, and check the live air each morning before deciding the day. If you have asthma or a heart or lung condition, or you're travelling with small children, I'd think hard about another time of year.