# Chiang Mai Air Quality by Month: Cleanest & Worst Months

> Chiang Mai air quality by month: the cleanest and worst months, how PM2.5 levels shift through the year, and when burning-season haze peaks and clears.

URL: https://chiangmaibrief.com/air-quality-and-burning-season/air-quality-by-month/
Last-Modified: 2026-10-17
Author: Daniel Calleja

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Chiang Mai has one of the most pronounced seasonal air-quality patterns of any city in Thailand, and the clean and polluted periods are not spread evenly through the year. Broadly speaking, February to April carries the greatest pollution risk, with March historically the worst single month. The rainy-season months, roughly June to September, are generally much cleaner, sometimes dramatically so. The rest of the year sits on the slopes between those two extremes.

Two cautions matter. First, this pattern is a planning guide, not a forecast: timing and severity vary considerably from year to year with rainfall, drought, fire activity and regional smoke. Second, monthly history tells you what to expect in general, while live AQI and PM2.5 readings tell you what is happening today. Use the first to choose your months and the second to plan your days.

![Hazy Chiang Mai air quality in the dry season, with Doi Suthep barely visible through burning-season smoke](/images/featured/hazy-chiang-mai-skyline-with-doi-suthep-barely-vis.webp)

## What Are the Best and Worst Months for Air Quality in Chiang Mai?

The worst months are February, March and April, with March usually the peak of the burning season. In some years, April is as bad or worse: in 2023, Chiang Mai's April monthly mean PM2.5 concentration reached 104.2 µg/m³, exceeding even that year's March figure of 97.8 µg/m³, according to IQAir city-level data. On 1 April 2023, IQAir's live ranking placed Chiang Mai as the most polluted major city in the world at US AQI 216 (reported by the Bangkok Post).

The cleanest months are June to September, when monsoon rains wash particulate matter out of the air. In IQAir's city-level data for 2020 to 2024, Chiang Mai's July monthly mean PM2.5 stayed between 5.3 and 6.3 µg/m³ every year, close to the World Health Organization's 2021 annual guideline of 5 µg/m³. For context, the March or April monthly mean is typically three to ten times the July to September mean, and in a severe year such as 2023 the swing approached twentyfold.

The shoulder months matter too. December and January are usually moderate but deteriorating, October and November are transitional, and May is the turning point when sustained rains arrive. All PM2.5 figures in this article are concentrations in micrograms per cubic metre (µg/m³), not AQI numbers, unless a scale is explicitly named.

## How Does Chiang Mai Air Quality Change Month by Month?

The table below summarises the typical monthly pattern using qualified relative descriptions rather than false precision, because no month has a fixed air-quality level. A data table beneath it gives substantiated monthly mean PM2.5 figures from two separate datasets, showing the real magnitude of the seasonal swing.

![Chiang Mai air quality month by month timeline showing clear winter air, worsening haze through March, and cleaner rainy-season months](/images/content/chiang-mai-burning-season-when-months-timeline-720.webp)

Typical Chiang Mai burning-season pattern through the year. Timing varies with rainfall, wind and fire activity.

### Table 1: Chiang Mai air quality month by month (planning summary)

Month
Typical air-quality pattern
Relative PM2.5 risk
What usually drives conditions
Planning takeaway

January
Moderate but deteriorating
Rising; moderate to high by late month
Cool, dry weather; burning begins; inversions
Generally workable; check daily readings

February
Often poor
High
Agricultural and forest burning; basin traps smoke
Sensitive groups should plan around haze

March
Usually the worst month
Very high; historical annual peak
Peak burning plus transboundary smoke; stagnant air
Highest-risk month; expect unhealthy days

April
Often poor, sometimes the worst
Very high
Burning continues until first sustained rains
Do not assume improvement before May

May
Transitional, improving
Falling from high to moderate
Monsoon onset, typically mid-May
Conditions usually turn by late May

June
Generally clean
Low
Rainy-season washout
Among the best months to be in Chiang Mai

July
Typically the cleanest month
Very low
Peak monsoon rainfall
Cleanest air of the year

August
Generally clean
Very low
Wettest period
Excellent for outdoor plans

September
Generally clean
Low
Continued monsoon
Excellent, with rain rather than smoke

October
Transitional, mostly clean
Low, rising late in the month
Rains taper off around mid-October
Largely fine; haze risk returns later

November
Deteriorating
Moderate and rising
Burning resumes; cool season begins
Watch readings from late November

December
Moderate; haze building
Moderate
Dry air, inversions, early burning
Usually acceptable, but the season has turned

### Table 2: Monthly mean PM2.5 concentration (µg/m³), Chiang Mai

Month
IQAir city aggregate, 2020–2024 range
PCD stations 35T/36T, 2011–2020 mean

January
19.5–43.2
32.6

February
24.8–57.5
49.5

March
39.9–97.8
80.9

April
34.4–104.2
57.6

May
13.2–27.3
25.1

June
6.1–8.9
14.7

July
5.3–6.3
14.6

August
5.3–10.8
14.6

September
6.2–10.2
14.2

October
9.0–14.8
18.1

November
15.8–18.5
20.9

December
24.6–26.3
28.6

Sources and methodology: the first column is the range of monthly mean PM2.5 concentrations (µg/m³) for IQAir's Chiang Mai city aggregate in the 2020 to 2024 World Air Quality Reports (government plus low-cost sensors; the station mix varies by year). The second column is a ten-year monthly climatology (2011 to 2020) from two Pollution Control Department stations in Mueang Chiang Mai district (35T at City Hall and 36T at Yupparaj Wittayalai School), published in the peer-reviewed journal Environments (Intara et al., 2022). The datasets cover different periods and sensor mixes, so treat them as two independent views rather than directly comparable figures. Both show the same shape: a sharp March peak, a deep rainy-season trough and a steady build-up from October.

## What Is Chiang Mai Air Quality Like From January to March?

### January

January sits in the cool, dry season. The air is usually moderate early in the month, but this is when deterioration begins: levels typically start rising around mid-January before peaking in March. In IQAir's 2020 to 2024 city data, January monthly means ranged from 19.5 to 43.2 µg/m³, so some Januarys are already unhealthy for sensitive groups while others are relatively mild. Burning begins around now: sugarcane pre-harvest fires run from November to December, and satellite-detected fire activity builds across the region from January.

### February

February is when the haze becomes hard to ignore. Monthly means in the IQAir city data ranged from 24.8 to 57.5 µg/m³ across 2020 to 2024, and the ten-year PCD climatology puts February at 49.5 µg/m³, well above Thailand's 24-hour standard of 37.5 µg/m³ (in force since 1 June 2023). The hot season begins around mid-February, winds are often calm, and cool-season temperature inversions in the Chiang Mai–Lamphun basin trap smoke close to the ground. Plan on regular unhealthy days in a typical February.

### March

March is historically Chiang Mai's worst month for air quality, confirmed by multiple independent datasets: the PCD climatology peaks at 80.9 µg/m³ in March, academic studies of the 2019 to 2021 seasons name March as the highest-concentration month, and IQAir's data puts March on top in three of the five years from 2020 to 2024, with April the highest month in 2023 and 2024. Individual days can be far worse than the monthly mean. Hourly PM2.5 concentrations during the burning season have been recorded climbing as high as roughly 400 µg/m³ (IQAir's 2021 burning-season analysis), and in March 2019, when the monthly mean hit 98.7 µg/m³, visibility was at times poor enough to disrupt flights. Even March varies considerably, though: the monthly mean was 39.9 µg/m³ in the mild, wetter year of 2022 against 97.8 µg/m³ in 2023. For week-by-week and year-by-year March detail, see [Chiang Mai air quality in March](/air-quality-and-burning-season/air-quality-by-month/march/).

![Infographic on why Chiang Mai haze in March and the burning season traps smoke over the valley](/images/content/why-chiang-mai-gets-smoky-burning-season-720.webp)

## What Is Chiang Mai Air Quality Like From April to June?

### April

April often surprises people who assume the haze ends in March. It does not. The PCD climatology shows April as the second-worst month at 57.6 µg/m³, and in IQAir's data April 2023 (104.2 µg/m³) was the single worst month of the entire 2020 to 2024 record. Burning continues until the first substantial rains arrive, which can be as late as the end of the month. In 2024, PCD regional offices reported PM2.5 above the national threshold into late April and early May. Treat April as a burning-season month.

### May

May is the turning point. The Thai Meteorological Department defines the rainy season as beginning around mid-May, and as sustained rains establish themselves, PM2.5 falls sharply. Across 2020 to 2024, Chiang Mai's May monthly means ranged from 13.2 to 27.3 µg/m³: still elevated at the start of the month in bad years, but clearly improving. Early May can still be hazy, so treat May as a transition rather than an instant reset.

### June

June is firmly in the clean season. IQAir's city data shows June monthly means of 6.1 to 8.9 µg/m³ between 2020 and 2024, close to the WHO annual guideline of 5 µg/m³. The PCD climatology records a higher but still modest 14.7 µg/m³. Regular monsoon rain keeps particulate matter washed out of the air. For visitors who want to be outdoors, June is one of the most reliable months.

![Monthly mean PM2.5 in Chiang Mai compared across seasons, from clean rainy months to worst months for air quality](/images/content/chiang-mai-aqi-snapshot-comparison-720.webp)

## What Is Chiang Mai Air Quality Like From July to September?

### July

July is typically Chiang Mai's cleanest month. In the IQAir city data, July monthly mean PM2.5 never left the narrow band of 5.3 to 6.3 µg/m³ across five consecutive years (2020 to 2024), an unusually stable result given how variable the rest of the year is. If you are choosing a month purely on air quality, July is the strongest candidate, with the trade-off that it is also one of the wetter months.

### August

August is similarly clean, with IQAir monthly means of 5.3 to 10.8 µg/m³ across 2020 to 2024 and a PCD climatology value of 14.6 µg/m³. August is usually the wettest or co-wettest month of the year in Chiang Mai, which is precisely why the air is so clean: smoke is effectively a non-issue.

### September

September closes out the reliable clean window. IQAir's data shows September monthly means of 6.2 to 10.2 µg/m³ for 2020 to 2024, and the PCD ten-year climatology actually records September as its lowest month at 14.2 µg/m³. Chiang Mai's monthly PM2.5 across June to September typically sits within roughly one to three times the WHO annual guideline, the only sustained period that comes close to international clean-air benchmarks.

![Lush green-season rice fields outside Chiang Mai after monsoon rain, when Chiang Mai rainy season clean air is at its best](/images/content/lush-green-season-rice-fields-outside-chiang-mai-a.webp)

## What Is Chiang Mai Air Quality Like From October to December?

### October

October is transitional. The rainy season usually ends around mid-October, and air quality remains good for most of the month: IQAir monthly means ranged from 9.0 to 14.8 µg/m³ across 2020 to 2024, with the PCD climatology at 18.1 µg/m³. Late October can see the first slight uptick as the rains taper off, but for practical purposes October is still a clean-air month.

### November

November is when the seasonal decline begins. Monthly means in the IQAir data sat between 15.8 and 18.5 µg/m³ across 2020 to 2024, noticeably higher than September or October. The Pollution Control Department's official haze-season monitoring window for the north runs from 1 November to 15 May, which reflects how early the risk period starts. Burning resumes in the wider region from around this time, and by late November the air is often visibly worse than in October.

### December

December brings cooler, drier air and a clearly building haze. IQAir's December monthly means were consistent at 24.6 to 26.3 µg/m³ across 2020 to 2024, and the PCD climatology records 28.6 µg/m³. The ten-year PCD study found that PM2.5 exceeded Thailand's annual standard (then 25 µg/m³) from December to May, roughly half the year. Official bulletins have recorded Chiang Mai readings above the national 24-hour threshold as early as the first week of December (58.2 µg/m³ on 6 December 2025), but December is the start of the slide, not the depths of it.

## Why Does Chiang Mai Air Quality Change So Much Through the Year?

The monthly swing comes from the interaction of fire, weather and geography. During the dry months, agricultural residue burning (particularly maize grown for animal feed, plus rice and sugarcane residue), forest fires and transboundary smoke from Myanmar and Laos pour particulate matter into the air. Studies consistently attribute the majority of northern Thailand's haze-season PM2.5 to biomass burning; one NASA Earth Observatory estimate puts roughly 70 per cent of Chiang Mai's April PM2.5 down to burning, and hotspot counts show Myanmar and Laos typically record more fires than Thailand itself.

Geography then makes it worse. Chiang Mai sits in a mountain-ringed basin, and the calm winds and temperature inversions of the cool and hot seasons trap pollutants near the surface instead of dispersing them. When the southwest monsoon arrives around mid-May, frequent rain physically washes particles out of the atmosphere, which is why the rainy-season months are so consistently clean.

Year-to-year variability sits on top of this cycle. Wet La Niña periods suppress fire: 2022 was the mildest recent year, with an annual mean of 18.4 µg/m³ and a March monthly mean of 39.9 µg/m³. Dry El Niño periods do the opposite: 2023 was the worst year in the recent IQAir record, with an annual mean of 33.4 µg/m³ and northern Thailand's exceedance days jumping from 70 in 2022 to 112 in 2023 (PCD end-of-season figures). Even COVID-era lockdowns made no dent: a 2020 study found Chiang Mai was the exception among Thai cities, its burning-related pollution unaffected by restrictions. For the broader context behind the smoke itself, see our guide to [Chiang Mai air quality and burning season](/air-quality-and-burning-season/).

![Chiang Mai burning season smoke at sunset, when PM2.5 levels are often highest in the worst months for air quality](/images/content/chiang-mai-burning-season-hazy-sunset.webp)

## FAQs About Chiang Mai Air Quality by Month

### Which months are worst for smoke in Chiang Mai?

February, March and April are the worst months for smoke, with March historically the peak. Some years the worst conditions shift into April, as happened in 2023, and in severe years the haze can begin building as early as December.

### When does the haze clear in Chiang Mai?

The haze usually clears with sustained monsoon rains, typically from late April into May; the rainy season officially begins around mid-May. The exact clearing date moves year to year: in 2024, meaningful improvement only came after several consecutive days of rain in May. Do not count on clean air before June, and treat any visit in April or early May as a gamble on the timing of the rains.

### When is the best time to visit Chiang Mai for cleaner air?

June to September offers the cleanest air of the year, with July typically the single cleanest month. October is also generally good. For cooler weather with reasonable air, October and early November are the usual compromise before the haze builds from late November.

### Is Chiang Mai's air quality getting better?

The honest answer is that there is no clear long-term improvement in the haze season. Annual mean PM2.5 for Chiang Mai city has fluctuated between 18.2 and 33.4 µg/m³ over 2017 to 2025 (IQAir data), with the bad years tracking drought and El Niño conditions more than any steady trend. A review of PCD measurements from 1996 to 2019 found no long-term deteriorating trend either, and Thai remote-sensing experts report only a slight, hard-to-notice decline in fire activity. Both 2022 and 2025 were relatively clean years, but 2023 was the worst on the recent record. Variability, not progress, is the defining feature.

### Where should you check Chiang Mai's air quality day to day?

Three sources cover most needs. Air4Thai (air4thai.pcd.go.th) is the Thai Pollution Control Department's official platform, reporting the Thai AQI from government monitoring stations. IQAir (iqair.com) reports the US AQI and runs the widely quoted live ranking of the world's most polluted major cities. AQICN (aqicn.org) also converts readings to US AQI. Be aware that Thai AQI and US AQI use different calculations, so the same air can produce different index numbers on different platforms; comparing the raw PM2.5 concentration in µg/m³ is the safest way to compare like with like.

## Sources

- [IQAir — World Air Quality Report 2020 (Chiang Mai city monthly and annual PM2.5)](https://www.iqair.com/dl/pdf-reports/world-air-quality-report-2020-en.pdf)

- [IQAir — World Air Quality Report 2021](https://www.iqair.com/dl/pdf-reports/world-air-quality-report-2021-en.pdf)

- [IQAir — World Air Quality Report 2022 (Greenpeace Thailand mirror)](https://www.greenpeace.org/static/planet4-thailand-stateless/2023/03/d1d69c24-2022_world_air_quality_report_en.pdf)

- [IQAir — World Air Quality Report 2024](https://www.greenpeace.org/static/planet4-chile-stateless/2025/03/edf90b7a-2024_world_air_quality_report_vf.pdf)

- [IQAir — Chiang Mai city page (2019 Feb/Mar/Apr monthly means; 2017–2019 annual series)](https://www.iqair.com/air-quality/thailand/chiang-mai/chiang-mai)

- [IQAir — New data exposes Thailand's 2021 burning season (hourly peaks up to ~400 µg/m³)](https://www.iqair.com/in-en/newsroom/thailand-2021-burning-season)

- [Intara et al. (2022), Environments (MDPI) — PCD stations 35T/36T monthly PM2.5 climatology 2011–2020, Mueang Chiang Mai](https://pmc.ncbi.nlm.nih.gov/articles/PMC9696662/)

- [Jainontee et al. (2023), Aerosol and Air Quality Research — March confirmed as highest-PM2.5 month, 2019–2021 episodes](https://aaqr.org/articles/aaqr-22-11-aac22-0432)

- [AAQR (2020) — COVID-19 measures and Thai air quality (Chiang Mai exception)](https://aaqr.org/articles/aaqr-20-06-covid-0344)

- [Bangkok Post, 1 April 2023 — Chiang Mai world's most polluted city, US AQI 216](https://www.bangkokpost.com/thailand/general/2541069/chiang-mai-hospitals-overflow-with-pollution-sufferers)

- [Bangkok Tribune, May 2024 — PCD regional exceedances into late April/early May 2024; CMU biomass attribution](https://bkktribune.com/pm2-5-spikes-in-upper-north-despite-rainy-season-approaching/)

- [Bangkok Tribune — Under the Haze I (PCD end-of-season 2023 statistics: 112 vs 70 exceedance days; hotspot counts)](https://bkktribune.com/special-report-under-the-haze-i-demystifying-the-flames/)

- [Prachatai English, September 2024 — May 2024 rain relief; station 36T exceedance-day counts](https://prachataienglish.com/node/11115)

- [NASA Earth Observatory — Smoke Shrouds Northern Thailand (~70% April PM2.5 from biomass burning; basin inversions)](https://science.nasa.gov/earth/earth-observatory/smoke-shrouds-northern-thailand/)

- [Land Development Department (Thailand) — Seasons (TMD season definitions: rainy mid-May to mid-October etc.)](http://www1.ldd.go.th/ldd_en/en-US/seasons/)

- [Nation Thailand, 14 July 2022 — Thai PM2.5 standard tightened to 37.5 µg/m³ 24-h from 1 June 2023](https://www.nationthailand.com/in-focus/40017734)

- [Evrard & Mostafanezhad, CSEAS Journal (2023) — no long-term deteriorating trend in PCD data, 1996–2019](https://englishkyoto-seas.org/2023/08/vol-12-no-2-olivier-evrard-and-mary-mostafanezhad/)

- [Pattaya Mail, December 2025 — PCD haze-season bulletin; Chiang Mai 58.2 µg/m³ on 6 December 2025](https://www.pattayamail.com/thailandnews/dust-danger-returns-as-thailand-tightens-grip-on-cane-burning-and-biomass-emissions-528653)

- [Air4Thai — Pollution Control Department official monitoring (Thai AQI)](https://air4thai.pcd.go.th/)

- [AQICN / World Air Quality Index project — Chiang Mai (US AQI)](https://aqicn.org/city/chiang-mai/)

- [WHO — Ambient (outdoor) air quality and health (PM2.5 annual 5 µg/m³, 24-h 15 µg/m³ guidelines, as cited in IQAir reports)](https://www.who.int/news-room/fact-sheets/detail/ambient-(outdoor)-air-quality-and-health)

## Frequently asked questions

### Which months are worst for smoke in Chiang Mai?

February, March and April are the worst months for smoke, with March historically the peak. Some years the worst conditions shift into April, as happened in 2023, and in severe years the haze can begin building as early as December.

### When does the haze clear in Chiang Mai?

The haze usually clears with sustained monsoon rains, typically from late April into May; the rainy season officially begins around mid-May. The exact clearing date moves year to year: in 2024, meaningful improvement only came after several consecutive days of rain in May. Do not count on clean air before June, and treat any visit in April or early May as a gamble on the timing of the rains.

### When is the best time to visit Chiang Mai for cleaner air?

June to September offers the cleanest air of the year, with July typically the single cleanest month. October is also generally good. For cooler weather with reasonable air, October and early November are the usual compromise before the haze builds from late November.

### Is Chiang Mai's air quality getting better?

The honest answer is that there is no clear long-term improvement in the haze season. Annual mean PM2.5 for Chiang Mai city has fluctuated between 18.2 and 33.4 µg/m³ over 2017 to 2025 (IQAir data), with the bad years tracking drought and El Niño conditions more than any steady trend. A review of PCD measurements from 1996 to 2019 found no long-term deteriorating trend either, and Thai remote-sensing experts report only a slight, hard-to-notice decline in fire activity. Both 2022 and 2025 were relatively clean years, but 2023 was the worst on the recent record. Variability, not progress, is the defining feature.

### Where should you check Chiang Mai's air quality day to day?

Three sources cover most needs. Air4Thai (air4thai.pcd.go.th) is the Thai Pollution Control Department's official platform, reporting the Thai AQI from government monitoring stations. IQAir (iqair.com) reports the US AQI and runs the widely quoted live ranking of the world's most polluted major cities. AQICN (aqicn.org) also converts readings to US AQI. Be aware that Thai AQI and US AQI use different calculations, so the same air can produce different index numbers on different platforms; comparing the raw PM2.5 concentration in µg/m³ is the safest way to compare like with like.