The air quality in Chiang Mai is shit today, and it's probably going to get worse.
From http://aqmthai.com/
That last indicator (PM10) shows fine particles in the air - basically soot particles upto 10 micrometres in diameter.
Thailand says the safe level is 120.
In the UK, the scale is:
low 0 - 49
moderate 50-74
high 75-99
very high >99