All of the vehicles were still within their permitted licence periods. And while some of the buses had only been on the road for two to three years, they still were major polluters.

An official with the Environment Police Department, Nguyen Quoc Trung, said the results meant that in future "We will be strict with these violations. Bus services can’t avoid the regulations just because they are public."

Under regulations, fines for violation are between VND150,000-200,000 (US$8.3-11). The department is working with bus-service operators and vehicle registration centres to clarify the responsibilities of each party.

For instance, registration centres must do exhaust checks before issuing licences, which means they could share some of the responsibility if a vehicle is found to be creating too much pollution.

"No (public authority) one has ever touched public buses before," Trung said. There are many reports about the heavy pollution in big cities such as Ha Noi and HCM City. We will be serious in dealing with violations, this time.

"In the near future, we will widen vehicle checks to cars and motorbikes as well," said Trung.

Since 2007, 11,000 cars out of more than 75,800 checked have failed to meet pollution standards.