The departments Inspectorate carried out the tests at the citys two inter-provincial bus stations, Mien Dong and Mien Tay, and three other stations from July 21 to 29.
While nearly 100 per cent of inter-provincial coaches with 50 seats and above met emission standards, most vans and city buses did not.
Under Vietnamese emission standards, petrol vehicles should not emit more than 4.5 per cent carbon monoxide.
In case of particulate matter, the emission should not be more than 1,200 parts per million (ppm) in case of four-stroke engines, 7,800 ppm in case of two-stroke, and 3,300 in case of special engines.
Diesel vehicles are allowed a maximum of 72 HSU (Hartridge Smoke Unit, which measures the opacity of smoke).
Le Hong Viet, deputy head of the Inspectorate, blamed the high emission rates on transport operators failure to pay attention to the maintenance of vehicles many of which are old.
Most inter-provincial buses met emission standards because operators constantly replenish their fleets to attract passengers, Viet said.
Many of the vehicles that failed to meet emission standards too had recently been inspected and certified by transport agencies.
Viet explained that to pass the inspection, many owners service their vehicles and make them more fuel-efficient before taking them for inspection. Later they readjust the engines to make their vehicles run faster, which also makes them less fuel-efficient.
But the inspection was meant more to apprise vehicle owners of the need to meet emission standards and they were not fined, he said.
On August 15 the Inspectorate will carry out another test and penalise offenders this time, especially if they have been warned already, he said.
Besides fines of VND200,000-300,000 (US$10-15.7) on offending vehicles, inspectors will also temporarily seize their papers and suspend their operations until they meet emission standards and obtain certificates from official agencies, he said.
Vietnam News