The most threatening conditions were in Mae Hong Song, where the air quality index reached 219, officials said.
Health officials said an index that high warrants an "emergency condition," the Bangkok Post reported.
An air quality reading over 100 can cause health problems, Thailand's Pollution Control Department said.
When air pollution gets bad, officials warn children, the elderly and people with asthma to stay indoors but the Post report said "the entire population is more likely to be affected" with a reading as high as that in Mae Hong.
The Lampang Meteorlogical Station reported an AQI of 159, Phrae had 134, Mai City 106 and Saraburi 104.
Satellite photos and charts from Nasa showed many wildfires set at this time of year to prepare fields for a new crop when spring begins, the Post reported.
Residents said the haze this year is especially bad.
"It is the worst I have seen in years," a Lampang resident told the Post.