The thick haze yesterday spread across seven northern provinces and poor visibility interfered with a Bangkok Airways flight to Lampang.
The 70-seat aircraft from Bangkok had to stop over at Sukhothai Airport in the morning and wait for the weather to clear before proceeding to Lampang.
The aircraft finally arrived at Lampang Airport at about 11.35am, more than an hour behind schedule.
The Lampang weather bureau had issued an alert for a sharp drop in visibility.
"We have warned motorists to be extra careful when driving through valleys too," said Tiwa Phanmaisee, the bureau's chief.
The Pollution Control Department reported that the smog in the seven northern provinces had soared beyond safe limits.
While safety standards call for small dust particles of up to PM10 microns in size to stay within 120 micrograms per cubic metre of air, the latest readings revealed worrying findings.
Phayao was up to 237.46, while Phrae hovered around 233.17. Lampang ranged between 210.21 and 173.21 depending on where the air-quality measurements were taken.
Nan, Chiang Rai and Lamphun were averaging 151.71, 151.67 and 180.42, while Chiang Mai was still about 176.54.
Worapoj Khunanurak, a weatherman at the Northern Meteorological Centre, blamed smoke from fires in the North and neighbouring countries for the worsening atmospheric conditions.
Chiang Mai Governor MR Panadda Diskul said he was trying to ask the Agriculture Ministry to carry out rainmaking operations now to reduce the air pollution.
"We had planned to wait a few more weeks to let garlic farmers harvest their crops first, but now the situation suggests we may have to do it immediately," he said.
Dr Chamnan Hansuttivejkul, public health chief for Chiang Rai, said smog could harm people's health. Common symptoms were asthma, allergic reactions, respiratory disorders and eye/skin inflammation.
Local administrative bodies should strictly bar locals from lighting fires to burn garbage or clear farmland, he said.
"Issue warnings first but if they repeat the offence, fine them under the Public Health Act," he said.
The law imposes a fine of Bt2,000 (S$82.6) on people who start fires that cause a public nuisance, he said.
The Meteorological Department issued a summer thunderstorm warning, citing the moderately high pressure ridge that has reached upper Vietnam and Laos.
"Soon, it will come to Thailand's Northeast," it said.
The North will likely see rainstorms, strong winds and a temperature drop of a few degrees Celsius, it added.