I don't live in Chiang Mai, but I do visit there frequently. Most recently I was there and the haze that is some of the world's worst air pollution was very evident. This is caused by agricultural burning in the surrounding area and for some reason that I don't understand, the topography of Chiang Mai is perfect for containing the smog in one particular area.
The "burning season" as they call it is where farmers set controlled fires to clear old crops and others are burning the undergrowth in forests searching for mushrooms. It appears as though the fires are not so controlled at the moment.
They don't know who set them or why, or how it got out of control but now the entire mountainside is burning and there are currently 12 firefighting teams working on it. The teams state they need more help as they are not making much progress in containing the blaze but are largely ignored by a government that is mostly focused on the Corona-virus.
There are a number of theories about why these fires are started in the first place, but the main one is that the farmers who start these fires have been doing it for centuries and don't like the government stepping in and telling them that they have to change their ways.
Some have even speculated that the farmers are intentionally burning government forestry land in protest of being told that they have to change their ways or else. I have no idea if this is true but things are getting pretty nasty up there. Civil unrest was always going to happen when the government tries to curb old traditions. I can't imagine being so stubborn that I would be willing to destroy all the nature around me rather than letting someone tell me that I can't use it the way i want to. Shameful.