The Pollution Control Department (PCD) on Fridy revealed five southern provinces with the best air quality in the country.
They are Satun, Yala, Phuket, Surat Thani and Songkhla, said Siwat Pongpienchan, spokesperson of PCD's Centre for Improving Air Quality, a special unit assigned to monitor and solve air pollution.
Those five provinces had low daily average PM2.5 levels from June-September last year, the centre's data revealed. They also had lower major air pollution particles.
Their level of carbon monoxide, measured in eight-hour segments, was less than nine parts per million (ppm) and their ozone gas was under 70 ppm over the same period of time.
Nitrogen oxide levels in average one-hour spells were less than 170 parts per billion (ppb), sulphur dioxide were less than 300 ppb and PM10 -- fine dust particles -- less than 120 microgrammes per cubic metre (µg/m³) in an average 24 hours.
However, Mr Siwat said it was too soon to conclude that the whole southern region was free from air pollution as some areas had suffered from seasonal air pollution caused by haze from forest fires in peat and palm oil plantations in neighbouring countries as far away as Indonesia.
Meanwhile, 17 northern provinces were hit the hardest by forest fires.
The Third Army Region closed down forest areas in 17 northern provinces to the end of April to control bushfires as PM2.5 dust levels have risen since last month. For example, Chiang Mai has been rated on March 9 as the world's most polluted city, according to the popular IQ AirVisual air monitoring website.
Authorities said the situation had caused 30,000 people to seek hospital treatment for respiratory illnesses over the past three months.
Third Army Region spokesman Col Rungkhun Mahapanya said the army had closed many northern forests during March and April to stop people entering and lighting fires.
Laws will be strictly enforced in Mae Hong Son, Chiang Mai, Lampang and Tak, which have been hit by the surge of the dust, he said.
Government and army aircraft and helicopters were used to douse a significant number of fires in elevated areas that were inaccessible by foot, he added.