I am a long term thai resident and from experience the smog is unbearable for a couple of months a year. That's mostly from end January till mid April. Some people escape to the South during these months. Others can't. Although I like Chiang Mai I would not want to live there and shorten my life by a couple of years.
According to data:
Chiang Mai has experienced varying numbers of days with PM2.5 air pollution exceeding safety standards over the past three years:
2023
101 days with PM2.5 levels surpassing the safety threshold. This period was marked by approximately 13,000 hotspots and 1,760 square kilometres of burned areas, leading to around 35,000 cases of respiratory illness.
2024
80 days of PM2.5 pollution above safe levels, indicating a reduction from the previous year. This improvement is attributed to enhanced preventive measures and increased public awareness.
2025 (as of March 31)
37 days with PM2.5 levels exceeding safety standards since January 1. Notably, the onset of the haze season was delayed to mid-March, and overall air quality has shown significant improvement compared to previous years.