Cold weather has meant that rather than rising into the sky, the smoke has been pushed down into low-lying areas.
Thick smoke and clearing during the dry season have sullied Chiang Mai, one of Thailand's most popular tourist destinations.
Poor air quality
Boonlert Buranupakorn, Chiang Mai Mayor, said: "According to our indicator, it has shown that haze is now higher than our acceptable standard to the point that it can affect our health.
"Comparing to last year, our air quality has worsened by ten per cent."
"If we cannot put out the fires, we may declare a state of emergency, but we have not reached that stage yet"
Kasem Sanitwong Na Ayutthaya, environment minister
The unusually large number of fires has meant the level of airborne dust particles is almost triple the level considered safe.
One resident, Laduean Kaewjinda, said: "In the morning I always have tears in my eyes, and cannot breath comfortably and some of my classmates experienced asthma symptoms and needed to consult doctors."
The cancellation of flights due to the smoke has resulted in damage to the tourism economy, a lifeline for many in the north.
In response, the Royal Thai Air Force 'rainmakers' flew through clouds over northern Thailand for a third day on Tuesday, hoping to coax rains to clear away the thick smoke.
The planes have been seeding clouds with a cocktail of chemicals since Sunday.
Deputy Prime Minister Paiboon Wattanasiritham said: "We hoped the artificial rain-making would help wash out the haze, but the attempts have failed."
|'Rainmakers' hope to coax rains to clear |
away the thick smoke
The far north has not seen rain since November in what has been an unusually long and dry cool season in Thailand.
Chiang Mai's main hospital is treating 100 people with serious respiratory problems, three times the normal caseload for this time of year.
Pregnant women, children and the elderly have been urged to stay inside or wear face masks; most outdoor events have been cancelled.
In Bangkok, Kasem Sanitwong Na Ayutthaya, the environment minister said various levels of government were pooling resources to fight the fires.
"If we cannot put out the fires, we may declare a state of emergency, but we have not reached that stage yet."