What Size Air Purifier for Thailand's PM2.5 and Burning Season?
Figures reviewed 14 October 2026
When the air gets bad
Fine-particle pollution (PM2.5) in Thailand is seasonal. Bangkok and central Thailand usually see their worst air in the cooler, drier months, roughly December to March. Northern provinces, including Chiang Mai and Chiang Rai, are typically hit hardest during the burning season, roughly February to April. Timing varies from year to year.
Check daily readings on the Thai Pollution Control Department's Air4Thai service, or on IQAir. Buy a purifier before the season starts, when stock and filters are easier to find.
Size by volume, not floor area
Purifier boxes usually say something like "suitable for up to 40 m²". That figure assumes a particular ceiling height and cleaning rate that you can't see. Calculating from the room's volume is more reliable.
Target CADR (m³/h) = room volume (m³) × air changes per hour
The calculator starts with an adjustable target of 4.8 air changes per hour and rounds the resulting CADR up to the nearest 10. It does not guarantee a measured clean-air change rate in your room.
Example, a condo bedroom 4 × 5 m with a 2.5 m ceiling:
Volume = 4 × 5 × 2.5 = 50 m³
Target CADR = 50 × 4.8 = 240 m³/h
In cfm: 240 ÷ 1.699 ≈ 141 cfm
| Room (2.5 m ceiling) | Volume | Target CADR (m³/h) | Approx. cfm |
|---|---|---|---|
| 3 × 4 m (12 m²) | 30 m³ | 150 | 88 |
| 4 × 5 m (20 m²) | 50 m³ | 240 | 141 |
| 5 × 6 m (30 m²) | 75 m³ | 360 | 212 |
| 6 × 8 m (48 m²) | 120 m³ | 580 | 341 |
Reading the label
Use the smoke CADR if the label lists it. Smoke particles are closest in size to PM2.5.
Check the units: US labels give CADR in cfm, others in m³/h. 1 cfm ≈ 1.699 m³/h.
Check the fan speed: CADR is measured at the highest speed. If you'll run the purifier on a quiet setting at night, buy more CADR than the target.
Two units add together: two 150 m³/h purifiers give 300 m³/h in the same room.
Running and filter costs
The purchase price is only the start. Before buying, check:
Replacement filters: the price of genuine filters, and whether they're sold locally. In peak season, filters for some brands sell out.
How often filters need changing. Heavy use in smoke season can shorten their life.
Power use at the speed you'll actually run it.
No ozone: avoid devices that produce ozone. It's a lung irritant.
Use the purifier ownership cost calculator (/en/calculators/purifier-ownership-cost/) to compare total cost over several years.
Get more out of any purifier
Keep doors and windows closed while it runs. A purifier can't keep up with an open window.
Use the aircon's recirculate mode rather than bringing in outside air.
Concentrate on the bedroom, where you spend the longest stretch of time.
Check your aircon's filter. Some units have optional finer filters.
FAQ
Is a smaller purifier better than none? It helps, but it will clean the room more slowly. Run it on a higher speed, or use it in a smaller room.
Do I need one in every room? Prioritise bedrooms and the room you work in. Use the calculator for each room separately.
Size a purifier for a 4 × 5 m room