The government declared Chiang Mai and Mae Hong Son disaster zones yesterday after air quality worsened over the weekend.

A pall of thick smoke has blanketed the region since March 1.

Deputy Prime Minister and Social Development and Human Security Minister Paiboon Wattanasiritham said the decision was made after the Pollution Control Department reported the deterioration in air quality. Paiboon is in charge of the crisis.

Meanwhile, the National Park, Wildlife and Plant Conservation Department yesterday deployed 300 forest-fire fighters from the Northeast to Chiang Mai, Mae Hong Son, Phrae and other provinces to help battle burning fires.

Daily fine-particle dust recordings in Mae Hong Son rose yesterday from 278 micrograms per cubic metre (mpcm) to 284 mpcm. The acceptable standard is 120 mpcm. Recordings in Chiang Mai rose to 196 mpcm, Paiboon reported.

"The number of people treated for smoke-related health problems in Mae Hong Son has risen to more than 3,500 from about 400 on March 17. In Chiang Mai the number rose to more than 4,500 from almost 1,400," he said.

In Chiang Rai fine-particle dust levels increased from 159 mpcm on Sunday to 201 mpcm yesterday. The number of residents seeking help for smoke-related conditions rose from almost 1,800 to 11,150. The situation in Lampang eased slightly with particle levels falling 3 mpcm to 131 mpcm.

Declaring a disaster allows provincial governors to allocate funds to fight the crisis and impose outdoor-burning bans.

The ministry is asking the Army to use helicopters search for fires.

Following the disaster declaration, Mae Hong Son Governor Direk Konkleep introduced seven measures, including the prosecution of landowners responsible for fires.

Chiang Mai officials were discussing solutions but are yet to introduce measures.

Public Health Ministry permanent secretary Dr Prat Boonyawongvirot said pollution-related cases in the region had reached almost 34,800. Most were eye irritations but there had been 159 heart attacks attributed to the smoke.

He recommended wearing face masks outdoors. The ministry has sent almost a quarter of a million masks to the area.

Prat expressed concern for Yupparat School in downtown Chiang Mai where smoke has caused traffic congestion and sent dust levels to more than 200 mpcm.

Chiang Mai Deputy Governor Vilas Rujiwatanapong said aerial spraying had stabilised dust levels but the results were poorer than hoped.

He is especially concerned about Saraphi district, which has the country's highest incidence of lung cancer. The district has no fires.

The Council for National Security sent 10,000 mask to Vilas.

Chiang Mai traffic police have been sent for medical checks as they spend between four and five hours a day exposed to smoke.