Chiang Mai burning season by month: when to get ready
When the Chiang Mai burning season starts, peaks and ends, how to read the PM2.5 number, and how to prepare a rented home before February.
Siamo editors
Late February. Doi Suthep, the mountain you see from many condos in Nimman, has disappeared behind grey haze. The air app on your phone shows a red number before breakfast, and on the worst days small flakes of ash land on the balcony table.
The Chiang Mai burning season comes at the same time every year, so you can prepare for it the way you prepare for the rainy season.
Why the air goes bad
In the dry season, farmers and others burn fields and forest across northern Thailand and the neighbouring countries. Chiang Mai sits in a valley with mountains on all sides. On days without wind, the smoke cannot leave the valley and stays over the city.
The Dr Artima clinic in Chiang Mai describes the season as running from mid-January to mid-April, with the peak in March. Rain ends it, but not on a fixed date. In some years storms around Songkran in mid-April clean the air. In other years the haze lasts until the end of April and only the May rains clear it.
Month by month
The months repeat every year, but the severity changes. A year with more rain can be mild, while a dry year without wind can be bad from early February. The table shows the usual pattern, so check the actual readings before you make plans.
| Month | What usually happens | What to do |
|---|---|---|
| October to December | Clean air, the best months of the year | Buy a purifier and spare filters while shops have the widest choice |
| January | Mostly fine, first hazy days late in the month | Book aircon cleaning, check that windows close tightly |
| February | Haze on most days, often worst at night and in the morning | Keep windows closed, run the purifier in the bedroom |
| March | Usually the worst weeks of the year | Keep one room closed and clean, or plan time away |
| April | Bad until the first heavy rain, then better | Watch the forecast for rain |
| May to September | Rainy season, clean air | Change the purifier filter, store the unit in a dry place |
How to read the number
Most apps show two things: PM2.5 in micrograms per cubic metre (µg/m³) and an AQI score. The µg/m³ figure is the more useful one.
Thailand's Pollution Control Department uses a 24-hour standard of 37.5 µg/m³, in force since June 2023. Its Air4Thai scale puts PM2.5 into five levels:
| PM2.5, 24-hour average | Thai rating |
|---|---|
| 0–15 | very good |
| 15.1–25 | good |
| 25.1–37.5 | moderate |
| 37.6–75 | starting to affect health |
| above 75 | affects health |
For comparison, the World Health Organization's 2021 guideline for a 24-hour average is 15 µg/m³. In March, Chiang Mai is often far above both.
Air4Thai, the government app, uses the Thai AQI scale. IQAir and most international apps use the American scale. The same amount of smoke gives a different AQI score on each scale. The apps also use different sensors: Air4Thai shows official stations, while IQAir adds cheaper sensors from other networks. One app may also show the last hour while the standard is a 24-hour average. As a result, two apps can show very different numbers for the same street at the same time. Pick one app, look at µg/m³, and watch how it changes from day to day.
Getting a rented home ready
The air in one closed room can be kept much cleaner than the air outside. Most people start with the bedroom.
- 1
Start with the bedroom. You spend about eight hours there without moving, so clean air there matters most.
- 2
Choose a purifier that fits the room size. The rule of thumb from AHAM, the US association that tests purifiers, is a clean air delivery rate (CADR) of at least two-thirds of the floor area in square feet. In metric this is about 12 m³/h of CADR for every square metre of floor. A 20 m² bedroom needs at least about 250 m³/h. The rule assumes a normal ceiling of about 2.4 m, and many purifiers sold in Thailand are tested under other standards, so treat the result as a rough minimum.
- 3
Close the gaps. Keep the windows closed. If there is a wide gap under the door, put a rolled towel there.
- 4
Use the aircon on recirculation. Most wall-mounted aircon units in Chiang Mai condos only cool the air that is already in the room, which is what you want. If your unit has a fresh-air vent, close it.
- 5
Get the aircon cleaned in January. A dirty filter blows dust back into the room you are trying to keep clean.
- 6
Buy spare filters before February. Filters clog faster when the air is dirty, so check the filter indicator more often in March.
If you are choosing a condo before the dry season, the viewing tells you a lot. Old sliding windows often leave a gap you can see when they are closed. Units on higher floors and in newer buildings usually seal better. Some condos come with a purifier from the owner, so it is worth checking whether one is there and whether it is big enough for the bedroom.
Where people get it wrong
The most common one is waiting until March to buy a purifier. In the clean months there is time to compare models and wait for a sale; in March you buy whatever is left.
Another is putting a small purifier in a big room. A unit made for a 15 m² bedroom cannot clean a 40 m² living room with an open kitchen, the number stays high, and people decide that purifiers do not work. A purifier of the right size in a closed room brings the number down.
Extractor fans undo a lot of sealing. The kitchen hood and the bathroom fan push air out of the condo, and outside air comes back in through every gap, so on bad days switch them off or keep the bedroom door closed while they run.
A sudden jump in the score can simply mean you opened a different app, for the reasons above.
Masks are the last one. Cloth masks and loose surgical masks leave gaps at the sides. Masks marked N95 or KN95 filter much more, but only if they fit closely around the nose and chin, and many ear-loop masks do not.
Leaving for the worst weeks
Some people who rent in Chiang Mai long term spend part of March in places outside the northern burning area, such as the southern islands. If your lease runs through the season, tell the landlord before you go. Many landlords are glad to know the condo is empty and locked, and you can ask them to check on it once while you are away.
What this does not fix
A purifier cleans the air in one closed room. It does not make outdoor exercise safe, and it does not replace medical advice. If you have asthma, a heart or lung condition, are pregnant or have small children, talk to a doctor before the season starts.
Chest pain or serious trouble breathing is an emergency. Call 1669, Thailand's emergency medical number, or go to a hospital emergency room immediately.
Questions people ask
When does the Chiang Mai burning season start?
Haze usually builds through February and peaks in March, and the season runs roughly from mid-January to mid-April. The exact start depends on the weather and on how much burning happens that year. Start checking the readings in late January instead of waiting for March.
Is it safe to stay in Chiang Mai in March?
Many people stay and spend most of their time indoors, with a closed bedroom and a purifier of the right size. People with asthma, heart or lung problems, small children or a pregnancy should ask a doctor before deciding. Some people who live here leave for the worst two or three weeks.
Do I need a purifier in every room?
No. Start with the bedroom, because you spend the most hours there in one place. Add a second unit for the room where you work if your budget allows. One closed room with a purifier of the right size does more than a whole condo with one weak unit.
A first step
Measure your bedroom and multiply the floor area in square metres by 12. Write the number down. That is the rough minimum CADR in m³/h to look for when you buy a purifier, and the clean months before February are the best time to buy.