Around half of all vehicles randomly checked on the roads were polluters, with exhaust emissions exceeding legal limits, the Pollution Control Department said.
Officials ordered 234,176 commercial vehicles - lorries, trailer trucks, buses and pickups - to pull over for exhaust fume checks last year, and 114,888 of them, or 49%, were over the limit, director-general Atthapol Charoenchansa said in a statement issued on Monday.
Private cars, motorcycles and other vehicles registered for private use fared little better, with 48% of 123,640 vehicles inspected by the department found to be polluting the streets.
The figures were compiled from random checks across the country last year. The department sets up roadside checkpoints and randomly checks vehicles' exhaust fumes. The Land Transport Department and the Bangkok Metropolitan Administration also take part, and run their own checks.
Violators face fines and the vehicles can be ordered off the road until the engine is brought up to standard.
Mr Atthapol said more checkpoints were planned on main and secondary roads in the capital and neighbouring provinces this year, with Greater Bangkok plagued by harmful particulate levels.