Travel Advisory from the Royal Dutch Embassy in Bangkok
"In the North and North-East of Thailand (like Khon Kaen, Mae Hon Song, Chiang Mai, Chiang Rai, Nan, Lampang) the air-quality is very bad because of the seasonal burning of farm-land and smog. Some regional airports could be closed. For air-quality information see the website of World Air Quality Index."Personal add-on : This seasonal burning in March & April is not only a problem of Thailand but also in Myanmar, Laos and Cambodia. Although it is a recurring problem every year since I am living in Thailand now for almost 20 years, I think it has never been so bad and took so long as this year. And again although it is an recurring & predictable problem it seems to me that none of the involved countries have been very serious in tackling the long-term reasons that create this smog problem.
My personal advice would be that if you are sensitive to bad air quality you should check a website like Air Visual and make your own decision about traveling in North and North-East Thailand.