When is Bangkok's air worst?
January and February are the worst months and June to September the cleanest. In more detail: from the end of the rainy season to the end of winter. A Bangkok city spokesperson, quoted in the Thai press, said PM2.5 typically worsens between November and March, when the air is still and closed in, and a UN ESCAP analysis puts the peak at November to February. Our own history is from the CAMS air-quality model through Open-Meteo at central Bangkok, averaged over 2023, 2024 and 2025. The strip puts the three coastal bases under it, so you can compare as you read:
Average PM2.5 by month, µg/m³ (model, 2023-2025)
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Bangkok | |||||||||||
| January: 54 | February: 50 | March: 41 | April: 30 | May: 24 | June: 13 | July: 10 | August: 11 | September: 15 | October: 37 | November: 30 | December: 31 |
| Krabi | |||||||||||
| January: 12 | February: 12 | March: 14 | April: 12 | May: 8 | June: 9 | July: 10 | August: 7 | September: 7 | October: 6 | November: 7 | December: 8 |
| Phuket | |||||||||||
| January: 13 | February: 12 | March: 14 | April: 12 | May: 9 | June: 9 | July: 11 | August: 8 | September: 8 | October: 6 | November: 8 | December: 9 |
| Koh Samui | |||||||||||
| January: 15 | February: 14 | March: 15 | April: 11 | May: 7 | June: 7 | July: 8 | August: 6 | September: 5 | October: 5 | November: 8 | December: 10 |
Model values on a grid of about 45 km, mean of 2023-2025 · How we measure
Read Bangkok's row first. January (54 µg/m³) and February (50) are the worst months, March (41) is still high, and the average is still about 30 in April. The rainy season then clears the air: 13 in June, 10 in July, 11 in August and 15 in September. October jumps to 37, earlier than the usual "November to March" window. The reason is two of the three years: October averaged 39 in 2023 and 47 in 2024, and only 25 in 2025.
Where to be next: your month-by-month route, $19
A route for your months with a leave-by date for each stop, a backup, and the seasonal traps on your path. Drafted from 11 years of weather data, every number checked against it, in your inbox within 48 hours. Not included: visas, insurance or bookings.
Why does Bangkok get a pollution season?
The sources we read agree on the broad answer and disagree about the shares. In the cool, dry months air movement weakens and a temperature inversion keeps city emissions near the ground. Three things pile onto it:
- Traffic and construction, including the building work around the BTS and MRT lines.
- Factories and power generation in and around the city.
- Crop and forest burning in surrounding provinces and further afield, carried in on the wind. The UN ESCAP study found PM2.5 in Bangkok moving with the count of open agricultural fires within about 400 km.
So the season is a weather event on top of a steady local supply, which is why it ends when the rains start and not when the traffic changes.
How much does it differ from year to year?
More than the three-year average hides. February averaged 83 µg/m³ in 2023, 26 in 2024 and 41 in 2025, and March went from 68 to 22 to 33. If you plan around "February is bad", you are right on average and sometimes wrong by a factor of three.
The model also reads low on bad days, because it averages an area about 45 km across. It is more reliable for when than for how much: real readings on a bad winter morning are higher than these averages.
Where is the air cleaner than Bangkok from December to March?
On the coasts, as the strip shows. The wind blows across open water and there are few big sources. Krabi and Phuket read 8 to 14 from December to March, and Koh Samui 10 to 15, against Bangkok's 31 to 54.
The trade-off is weather, not air. The two coasts have their wet seasons at different times: our three-coasts guide shows which coast is dry in which month, and where is warm in January ranks the whole region. Chiang Mai is not an escape from Bangkok's season, since it has its own from about February (see Chiang Mai burning season). For the cool season, the April heat and the rains in the city, see Bangkok weather by month.
Where to go instead: three coastal bases
Krabi
31° high in Jan79 mm in Jan
12 µg/m³ in January and February, a quarter of Bangkok's level. Dry from December to March on the Andaman coast.
Suits: people who put air first and want sun in the dry season
Phuket
30° high in Jan73 mm in Jan
13 in January and 12 in February, with the most services of the three. Same dry months as Krabi.
Suits: people who want clean winter air and a nomad scene
Koh Samui
28° high in Jan152 mm in Jan
15 in January and 14 in February, the highest of the three but a quarter of Bangkok's. Gulf coast, so its wet months come at the end of the year, not in summer.
Suits: people who want an island base for the first months of the year
How would you plan around Bangkok's season?
- Treat June to September as the clean months, and expect rain.
- Avoid January and February if you can choose, and treat October as a warning month in some years.
- If you must be in Bangkok in the season, check live readings on Air4Thai, AQICN or IQAir by district, and note how old the reading is. The WHO guideline is 15 µg/m³ for 24 hours and 5 for a year (2021); older pages still quote 25, the 2005 value, and Thailand's own daily limit is looser than the WHO's, so check which limit a number is compared with.
- Split the winter: a month in Bangkok for work, the rest on a coast.
- Keep a margin for a heavy year. In 2023 it was bad until April (47 µg/m³ that April).
We give no health advice here. If you are sensitive to polluted air, speak to a doctor before planning a long stay.
About this data
Daily PM2.5 averaged by month from the CAMS global air-quality model through Open-Meteo, mean of 2023 to 2025, one point per city. A model cannot see a single road or a bad hour, and reads low on bad days. Use it to compare months and places with each other.
Frequently asked questions
When is Bangkok's air quality worst?
Officials and local media say November to March. In our model data for 2023 to 2025 the highest monthly averages were in January (54 µg/m³) and February (50), with October (37) already high in two of the three years.
What are the cleanest months in Bangkok?
June to September. The monthly average was 13 µg/m³ in June, 10 in July, 11 in August and 15 in September, close to the WHO daily guideline of 15. That is also Thailand's rainy season.
Is Bangkok's air better than Chiang Mai's?
They peak at different times and for different reasons. Bangkok's model average is highest in January and February, Chiang Mai's in March and April, and Bangkok is clean in the rainy season. Our Chiang Mai guide has its weekly chart.
Does the pollution season change from year to year?
A lot. In our model, February averaged 83 µg/m³ in 2023, 26 in 2024 and 41 in 2025. A three-year average is a guide to when, not a forecast of how bad.
Where in Thailand is the air cleaner in January?
In our model data the Andaman coast and the Gulf islands read far lower than Bangkok: Krabi 12, Phuket 13 and Koh Samui 15 µg/m³ in January, against 54 in Bangkok. Pick on weather too, since the coasts have their rainy seasons at different times.
Not sure where yet? Pick your months and what to avoid, and get your 3 best places → Where should I go?
Weather data: Open-Meteo (CC BY 4.0), open-meteo.com; rain totals: CHIRPS (Climate Hazards Center, UC Santa Barbara). Processed by NomadWeather from stored copies of the data.