The Transport Ministry insisted Friday that most of the Bangkok Mass Transit Authority's (BMTA) buses had been converted to run on natural gas which means they no longer produce toxic black smoke.
Dirty diesel vehicles were banned from city streets on Thursday in a campaign to improve air quality in Bangkok.
The statement came one day after the Supreme Administrative Court ruled that the BMTA must not allow any buses under its care to emit toxic fumes at a level exceeding the safety standards.
The court judgement came as a victory for an advocacy group bringing its fight against the BMTA's contribution to air pollution in the capital to the court about 12 years ago.
The court's ruling also requires the BMTA to submit reports of tests on the levels of toxic exhaust fumes emitted from all the buses belonging to it and its concessionaires every three months for a full year to prove that it has abided by the ruling.
The court also ordered the Pollution Control Department (PCD) to rigidly enforce the law to ensure full supervision of the BMTA and report the results of emissions tests every three months to the court for a year. In 2002, the Foundation for Anti-Air Pollution and Environmental Protection and a group of Bangkok citizens filed four lawsuits with the Central Administrative Court against the BMTA and the PCD for failing to tackle foul black smoke emitted by city buses.
The court ruled in 2006 that the BMTA was guilty.
Caretaker Transport Minister Chadchart Sittipunt said the engine conversions to allow the BMTA buses to switch from running on diesel to natural gas for vehicles (NGV) has dramatically cut the exhaust emissions problem.
A number of BMTA buses that still run on diesel are regularly tested to determine if they all strictly meet the standards of acceptable levels of black-smoke emission, Mr Chadchart said.
"We're going to heed the court's ruling and strictly follow it," he said.
"We now test the buses for high black-smoke levels twice a year.
"I have to admit that some buses emitting far too high levels of black smoke might have gone undetected for some reason."
But from now on, he said, the ministry will try its best to better control the exhaust problems with the BMTA's buses, Mr Chadchart said.
BMTA acting director Nares Boonpiam said the authority would not have any problems implementing the court's requirement for testing.
He said the BMTA already carries out emissions tests twice a month and it is also required to have its buses tested by the Land Transport Department twice a year.
Mr Nares said the BMTA now has 2,670 buses. Only 450 still run on diesel while the rest have been converted to run on NGV.
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Writer: Amornrat Mahitthirook and Mongkol Bangprapa