PM2.5 Season in Thailand: When and Where to Find Cleaner Air
Monthly PM2.5 figures for Chiang Mai, Bangkok and Phuket, what changed in 2026, and the places where no monthly data exists.
On 1 April 2026, Chiang Mai’s air quality index reached 235 on IQAir, the worst of any major city in the world for the fourth day running, according to the Thailand Starter Kit. The day before, all 111 Chiang Mai University monitoring stations in the province were in the red zone.
For a visitor or resident the practical question is when the air is clean, where, and how much of that is predictable. This feature sets out the monthly figures we could find for Chiang Mai, Bangkok and Phuket, what changed in 2026, and the gaps in the data.
How we compiled this
We combined published monthly averages from IQAir and from research on Pollution Control Department stations, a NASA-linked field study report on Bangkok, and IQAir’s city page for Phuket. These datasets cover different years and use different methods, so we show them side by side and do not merge them. For Thai cities outside these three, such as Hua Hin and Koh Samui, we found only live snapshots and no monthly series.
The standards
Thailand’s daily PM2.5 standard is 37.5 µg/m³, set in June 2023. The World Health Organization’s 24-hour guideline is 15 µg/m³ and its annual guideline is 5 µg/m³. IQAir put Thailand’s 2025 national average at 17.8 µg/m³, 3.6 times the WHO annual figure.
Chiang Mai: a six-month swing
Chiang Mai has the sharpest seasonal curve. The table shows two sources. The first is IQAir’s city average for 2020 to 2024, given as the lowest and highest annual value for each month. The second is a ten-year mean from two Pollution Control Department stations in the city, 2011 to 2020, published in the journal Environments.
| Month | IQAir range, 2020 to 2024 (µg/m³) | PCD station mean, 2011 to 2020 (µg/m³) |
|---|---|---|
| January | 19.5 to 43.2 | 32.6 |
| February | 24.8 to 57.5 | 49.5 |
| March | 39.9 to 97.8 | 80.9 |
| April | 34.4 to 104.2 | 57.6 |
| May | 13.2 to 27.3 | 25.1 |
| June | 6.1 to 8.9 | 14.7 |
| July | 5.3 to 6.3 | 14.6 |
| August | 5.3 to 10.8 | 14.6 |
| September | 6.2 to 10.2 | 14.2 |
| October | 9.0 to 14.8 | 18.1 |
| November | 15.8 to 18.5 | 20.9 |
| December | 24.6 to 26.3 | 28.6 |
March is the peak in the long station record, and April was the worst month in IQAir’s data in 2023 and 2024. June to September is the cleanest stretch, with July the lowest in IQAir’s series. The IQAir ranges show how much a single year can differ: March has ranged from 39.9 to 97.8 in five years.
Biomass burning dominates the peak. A field study report from NASA’s ASIA-AQ campaign says peak PM2.5 in the north matches the crop burning and wildfire season. It says peak pollution was lower in 2022, when heavier rain reduced burning, and higher in 2023 and 2024. An IQAir analysis found that in 2021 Chiang Mai’s average was 83.3 µg/m³ in March, and that March was the most polluted month from 2017 to 2021.
Bangkok: earlier and flatter
The same ASIA-AQ report says Bangkok’s PM2.5 peaks earlier than the north, and swings less, because urban sources are steadier. Monitors exceed the 15 µg/m³ annual goal from November to April, with monthly peaks in January or February rather than during the main burning season.
| Period | Bangkok average (µg/m³) |
|---|---|
| January to February | About 40 |
| November to April | 29 |
| May to October | 12 |
| Full-year average | 20.6 |
The report gives annual averages of 23.5 for 2023 and 21.2 for 2024. It attributes Bangkok’s pollution mainly to urban and industrial emissions, with some smoke from burning.
Phuket and the south: cleaner, with exceptions
IQAir’s Phuket page gives a 2019 annual average of 11.4 µg/m³, the only annual figure on the page. March 2019 was the worst month at 17.6 µg/m³. May, August and October were within WHO guideline levels, though the page lists October among both its best and worst months, so treat that month with caution.
The south can still see haze. Pattaya Mail reported in August 2026 that smoke from hotspots in Indonesia pushed PM2.5 into the moderate range in several southern provinces, including Phuket. Hourly readings in Hat Yai rose to about 30 µg/m³. We found no monthly series for Koh Samui, Koh Phangan or Hua Hin.
Limits of the data
The figures above come from different sensor networks. The Chiang Mai page we used notes that IQAir’s city average mixes government and low-cost sensors, and that the station mix varies by year. The two Chiang Mai series cover different periods, so the gap between them is not a trend. Bangkok’s figures come from monitoring sites cited in a research report. Phuket’s come from a single year. Use them for the shape of the season, not for exact predictions.
What changed in 2026
The Thailand Starter Kit, citing Thai Rath, reports 5,090 hotspots by 31 March 2026, about 39 per cent more than in 2025. Its guide says the open-burning ban ran from 1 January to 31 May. IQAir reported that Chiang Mai’s AQI exceeded 150 on 16 April. The Starter Kit’s guide also says expats often leave Chiang Mai for several weeks in this period, with some going south to Phuket, Koh Samui and Koh Phangan. It says forum users describe Pai and Chiang Rai as just as smoky, though it cites no measurements for that.
What to do with the data
If you are choosing travel dates, the figures point to June to September for Chiang Mai and May to October for Bangkok. For the north, February to April is the period to check closely. These are not guarantees, since individual years vary.
The Pollution Control Department advises children, older adults, pregnant women and others at risk to wear masks outdoors during bad episodes. It points readers to the Air4Thai and Air BKK apps for 1-hour and 24-hour readings. If you have asthma, heart disease or another chronic condition, ask your doctor what to do before a trip in the burning season.
Check a live reading for your destination the week you book. A seasonal table says what usually happens, not what is happening.
Jonas. Jonas covers medical travel, fitness, costs and the rules that affect wellness travellers in Thailand. A pragmatist who treats health as a long project, he wants to know what something costs, what the evidence says, and who it’s really for.
RestRobin is editorial content, not medical advice. Prices, programmes and opening details change, so check with the provider before you book, and talk to a qualified doctor before starting a treatment, fast or detox programme.
