I live in Chiang Mai most of the year and it really depends upon how sensitive you are to air pollution. If you are very sensitive, I would avoid Chiang Mai from mid-February until the end of April. (March and April are insufferably hot also.)

If you aren't so sensitive, then you're probably good to go until the end of February/beginning of March. It really varies by person (and by year) as to when things become unpleasant.

Personally, mid-March to the end of April is when I find it worst. Others would disagree.