If you're in Thailand between roughly late February and April, you'll hear locals and expats alike talk about "burning season." It's the period when farmers across northern Thailand and neighboring countries burn crop stubble to clear fields, and when seasonal forest fires are most common. Combined with Chiang Mai's mountain-basin geography, which traps smoke instead of letting it disperse, the result is some of the worst air quality readings in the world for a few weeks each year.
2026 was a clear example. On March 30, Chiang Mai briefly ranked as the most polluted major city on Earth, with an AQI of 233 and PM2.5 readings around 188 micrograms per cubic metre â well above what's considered hazardous. Hospitals in Chiang Mai and Chiang Rai reported a rise in patients with breathing problems during the same stretch, according to IQAir's local reporting.
The severity varies year to year depending on wind patterns and how much burning happens regionally, but the general window is consistent: worst in late February through March, sometimes extending into April, and clearing up once the rains arrive around May.
If you live in or are planning to visit northern Thailand (Chiang Mai, Chiang Rai, Mae Hong Son, and nearby provinces) during this window, a few practical steps help: check a real-time AQI app like Air4Thai (the Thai government's own network) or IQAir before heading out, keep a fitted N95 or KN95 mask on hand for high-AQI days, run an air purifier indoors if you have one, and consider that southern Thailand and the islands are largely unaffected â many people simply shift travel plans south for those weeks. If you or someone you're traveling with has asthma or another respiratory condition, it's worth being extra cautious and checking with a doctor before spending extended time outdoors during peak burning season.