Chiang Mai Air Quality: Burning Season, PM2.5 and What to Do

Chiang Mai's air is clean from June to October and smoky from February to April, when forest and farm fires across the region fill the basin. In March, nearly nine days in ten break Thailand's daily limit at the Old City station. Below: today's official reading, the season month by month, why it happens, what helps at home, and whether it matters where you live. Every figure is dated and sourced at the end.

Chiang Mai's air quality today#

These are the only two official stations in Chiang Mai, both run by Thailand's Pollution Control Department: one at City Hall in Chang Phueak, north of the Old City, and one at Yupparaj Wittayalai School inside the Old City. Each number is the average of the last 24 hours, the same averaging as Thailand's daily limit, and the colour is Thailand's own scale. On smoky days the two often differ by tens of micrograms, which is why we show both.

International air apps usually show the US Air Quality Index, which turns the same smoke into a different number. At 20.2 µg/m³ of PM2.5, Thailand's index reads 38 (green) and the US index about 72 (yellow); at 37.5, Thailand's limit, they read 100 and about 106; at 75.1, 201 and about 165 (our calculation from both governments' published tables). Compare micrograms, not index numbers: a US reading of 200 is not a Thai reading of 200.

When is the burning season in Chiang Mai?#

From February to April. The smoke usually builds in February, peaks in March or April and clears when the rains arrive in May. January and May are shoulder months, and from June to October the official stations almost never record a day above Thailand's limit. On the ten-year average March is the worst month; in two of the last three years April was.

A peer-reviewed study of the same station's data, published in 2025, found the same figures. The City Hall station reads a little higher in most months and follows the same shape. No year from 2015 to 2025 met Thailand's annual standard of 15 µg/m³ at the Old City station; even mild 2025 averaged 17.0 (our calculation from Pollution Control Department data).

The last three seasons#

Seasons do not keep to the calendar. In 2025 the worst came in March and April was mild; in 2024 and 2026 April was worse than March, and at City Hall April 2026 averaged 85.8 µg/m³. A local paper reported PM2.5 above the national standard in the city by 3 February 2026. If you are planning a visit or a move, treat late February to mid-April as the risky window and May as uncertain until the rains arrive.

How bad is the burning season, year by year?#

It changes sharply from year to year. At the Old City station the smokiest recent year was 2023 and the cleanest were 2022 and 2025. 2026 brought the biggest fires in years, with 24-hour readings above 300 µg/m³ in forested Chiang Dao, yet inside the city 2024 had more smoky days.

2026 needs two numbers. The province counted 11,023 fire hotspots from January to May, 2.3 times as many as in 2025 and mostly in forest, and 1,468,289 rai (about 2,350 km²) burned between January and April. On 31 March the department's regional office recorded a 24-hour average of 330.2 µg/m³ at Mueang Na in Chiang Dao, and on 4 April emergency disaster-assistance zones were declared in 17 districts of Chiang Mai, Lamphun and Phayao, seven of them in Chiang Mai for wildfires, Mae Rim and Doi Saket among them.

Yet the province counted 53 days above the standard from January to May 2026, against 60 in 2025, and in the city 2024 was the harder season: the Old City station had 33 days above 75 µg/m³ in 2024 against 8 in the first half of 2026, and its worst day reached 163.9 against 106.9. The cleanest years need care too: 2022 was a wet year as well as a year of tighter burning control, so not all of its drop was policy.

Why is Chiang Mai so smoky?#

Because of fires, above all in the forested hills, and on farmland, both in Thailand and across the border in Myanmar and Laos. Chiang Mai sits in a basin ringed by mountains over 1,000 m high, and in the dry season cold-morning inversions keep the smoke in the valley until the May rains wash it out.

• Burning makes the peak. A 2026 peer-reviewed study puts burning at nearly 49% of Chiang Mai's PM2.5 over the year and more than 70% in April.
• Forest more than farms. In 2024 about 90% of the province's burning emissions came from forest fires (62,421 tonnes, against 6,631 from farmland), and maize-stubble burning adds sharp peaks in March and April. How many forest fires people light, for foraging, hunting or clearing, no source we read has measured.
• Smoke from abroad. One modelling study of the 2019 season found that smoke from outside Thailand made up about 29% of Chiang Mai's PM2.5 from January to May, and about 45% in March.
• The basin. In March the layer of air that mixes during the day is often only about 500 m deep, and the department's regional forecast puts the poorest ventilation between 7 pm and 10 am. That is why mornings are often the haziest, and why windows are best kept shut overnight on smoky days.

What is being done about the burning?#

Chiang Mai bans open burning each dry season, in 2026 from 1 January to 31 May, and prosecutes those who break the ban under existing laws. A national Clean Air Management Act is still not law: the two houses of parliament passed different versions, and since September 2026 the bill has sat with a joint committee.

• The yearly ban: in 2026, no open burning outside fuel-management zones and no burning of rubbish or leaves in communities. Unavoidable burns need a permit through the Fire-D system and the district command centre's approval. The 2027 ban had not been announced on 24 October 2026; earlier ones were signed around New Year.
• Penalties under existing laws: up to three months in prison and a ฿25,000 fine for a public nuisance, and 4 to 20 years for fires in a national park. In the 2026 season the province brought 245 cases against 262 suspects.
• Farmers who burn lose disaster relief and farm support from 1 April 2026 to 31 March 2028, and since 1 January 2026 imported feed corn must come from burn-free production.
• Chiang Mai, Chiang Rai, Lamphun and Mae Hong Son form a pollution control zone from February to May every year, and Thailand, Laos and Myanmar share fire-risk maps and PM2.5 data under their CLEAR Sky plan for 2024–2030.

For the coming season, Thailand's Meteorological Department said on 15 October 2026 that it expects El Niño to strengthen towards the end of 2026 and possibly continue into early 2027. It gave no haze forecast, but El Niño years have brought heavier smoke before: El Niño 2019 was far smokier at the Old City station than La Niña 2017, with 97 days above the limit against 64 (our calculation from Pollution Control Department data).

Is Chiang Mai's air bad for your health?#

In the smoky months, yes. Fine particles (PM2.5) reach deep into the lungs and the bloodstream and mainly harm the heart and lungs, according to the World Health Organization. Thailand's daily limit of 37.5 µg/m³ is two and a half times WHO's guideline of 15, so a day within the Thai standard is not a safe day by WHO's measure.

The effect shows in Chiang Mai's hospitals. From January to March 2024, Maharaj Nakorn Chiang Mai Hospital treated 30,339 patients for illnesses linked to PM2.5, against 12,671 a year earlier, and in the 2025 season the province recorded 9,234 hospital admissions for chronic obstructive lung disease (COPD). If you have a heart or lung condition, the US Centers for Disease Control and Prevention advises making a plan with your doctor before the season and keeping 7 to 10 days of prescription medicines at hand.

The hospitals foreigners use

Where each one is, what it does well and how to reach it at 2 am.

Read the hospitals guide

How to protect yourself at home#

Make one room clean: close its windows and door, run the air conditioner on recirculation, and run a HEPA purifier sized for the room on its highest setting all the time, ideally in a bedroom. Outdoors on smoky days, wear an N95, KN95 or FFP2 mask that seals around your face; surgical and cloth masks do not stop smoke particles.

What size of air purifier do you need?#

Look for the purifier's clean air delivery rate (CADR) for smoke. The US Environmental Protection Agency's rule is a CADR, in cubic feet per minute, of at least two-thirds of the floor area in square feet; the appliance makers' association suggests a CADR equal to the square footage, which cleans faster. Purifiers sold in Thailand usually state CADR in m³/h, as the table does.

• One correctly sized purifier for each room you use, the bedroom first: a purifier meets the rule only for the room it stands in, with the door shut. Some Thai guidance asks for less, three times the room's volume per hour, about 125 m³/h for a 16 m² bedroom with a 2.6 m ceiling; that is below the EPA minimum.
• Choose a HEPA model without an ozone-producing ioniser or plasma feature; the EPA warns against them.
• Keep windows shut overnight on smoky days, when the valley's air moves least, and air the room when the outdoor reading falls: a sealed room builds up carbon dioxide.
• Masks: N95, KN95 or FFP2, sealed around the face; a beard breaks the seal. Respirators are not made for small children, so a filtered room is their main protection.
• On bad days, avoid adding to the air indoors: frying, candles, incense, and vacuuming without a HEPA filter.

Does it matter where you live in Chiang Mai?#

Less than people hope. On the smoky days every station in and around the city breaks the limit together, and no measurement shows a district, or the hills, staying clean. What changes your exposure most is the home: windows that close tightly, a purifier in each room you use, and ideally a filtered fresh-air system.

• On 31 March 2026 every Chiang Mai station in the department's regional summary was above the limit, from 66.9 to 330.2 µg/m³. On the 32 worst days between 4 March and 25 April 2026, every comparable station exceeded it on every day: City Hall, the Old City, Suthep, Hot and Lamphun (our calculation from the department's regional summaries).
• The largest gap measured lies inside the city. On those 32 days the City Hall station averaged 93.5 µg/m³ against 64.9 in the Old City, 5.9 km away: 44% higher (our calculation). In March 2024 the Old City read higher, so neither side of town is reliably cleaner.
• Higher ground is no refuge in any measurement we found. The department's Suthep station averaged 68.5 µg/m³ on the 32 days, against 64.9 in the Old City, and in April 2026 fires on Doi Suthep came within about 500 m of the temple.
• Six of the seven districts our listings cover, Hang Dong, San Kamphaeng, Saraphi, San Sai, Doi Saket and Mae Rim, have no official station; only low-cost university sensors cover them. Watch the labels: the department's reading named Hang Dong comes from Hang Dong subdistrict in Hot district, about 79 km south-west, not from Hang Dong district.

What to ask about a condo or house#

A closed home alone is not enough. In nine Chiang Mai houses during the 2019 haze, the air indoors averaged 91% of the PM2.5 outside. Filtration is what makes the difference: in one Chiang Mai school, classrooms with a filtered fresh-air system that kept them under slight positive pressure measured 3.4 to 5.0 µg/m³, rooms with purifiers 17.4 to 24.2, and the air outside 58.7 to 87.8, according to the system's maker, which names no school. Some developers advertise filtered or dust-free homes, but none we found has published a test result. Ask four things:

1. Which filter grade does it use: HEPA H13, or which MERV rating?
2. How much air does it deliver to each room?
3. Does it keep the home under slight positive pressure, so outside air cannot leak in?
4. How often are the filters changed, and who pays for them?

When renting, check that the windows and balcony doors close tightly before the season. Renting first also lets you live through one smoky season before you buy.

Schools, and leaving for the season#

Thai state schools may close on red days. Since January 2025 a principal can close a school at once when the 24-hour PM2.5 reaches 75.1 µg/m³, for up to seven days or until the air improves, and teach online instead; outdoor activities stop at very high or red levels.

Of the international schools, Prem Tinsulanonda publishes the most: filtered fresh-air systems under positive pressure, over 100 sensors and an air quality committee, though no thresholds for outdoor play, sport or closure. We found no published policy for the others, which does not mean they have none. Parents can ask four things: the school's indoor PM2.5 target, the reading at which play and sport move indoors, whether classrooms are filtered or pressurised, and whether carbon dioxide is measured.

Many residents leave for part of March and April. It is widely reported, but no one has counted it. If you plan to, a lease that allows a few weeks away, or a shorter one, makes it easier.