Experts at the World Health Organisation (WHO) say diesel engine exhaust fumes can cause cancer in humans.

They say they belong in the same potentially deadly category as asbestos, arsenic and mustard gas.

After a week-long meeting, the International Agency for Research on Cancer reclassified diesel exhausts from its group of probable carcinogens, to its group of substances that have definite links to cancer.

It says diesel emissions cause lung cancer and increase the risk of bladder cancer.

They say their decision was unanimous and based on "compelling" scientific evidence.

The director of New York's Clean Fuels and Vehicles Project, Rich Kassel, has told CNN the WHO has confirmed what has been suspected for some time.

"Anybody who lives in Beijing, Mexico, New York or any congested city has probably felt the feeling of holding their breath when the bus pulls away from the curb leaving you in a ... puff of black smoke," he said.

"This study basically confirms that we're right to hold our breath when the bus pulls away."

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The pollution that we care about from diesel - buses, trucks and other diesel engines - is technically called particulate matter. We all know it is soot. It's fine, fine particles that are small enough to get past our throat, past our lungs into the deepest part, the deepest of our lungs, where they trigger asthma attacks, bronchitis, emphysema, heart disease and now of course we've learned cancer."

The WHO has acknowledged tougher fuel regulation has led to improved diesel quality and trucks do not billow big clouds of soot so often anymore.

But it says it is not yet clear whether these changes have reduced the risks.

The Cancer Council's chief executive, Professor Ian Olver, says the WHO also has not confirmed what levels of exposure cause cancer.

"Most of the data in the world relates to occupational exposure, such as diesel equipment in mines, or transport, particularly railway workers, exposed to diesel," he said.

"So the first group that we ought to be looking at are those that [are exposed] to the heavy diesel output machinery."

Professor Olver says there is no data available for the levels of exposure in cities.

"The difficulty is that all the pollutants, whether it is a petrol engine or a diesel engine, are all mixed together and that is why the data upon which this was based had to be the more specialised sort of high-level exposure of various occupations," he said.

Andrew Bourne has been in the diesel fuel injection industry for more than 30 years, and runs a diesel business in Toowoomba in Queensland.

"We're certainly dealing with emissions every day," he said.

"The machines that we see coming through our business have health problems with either the engine or the fuel system and as a result of that, often their emissions are one of the main telltales.

But he says although his workplace may be more exposed to exhaust than most - his workers' general health is fine.

"It's not as if we actively breathe in exhaust fumes. When we are testing vehicles, we tend to try and avoid it [and have] an open-air area to do that," he said.

Mr Bourne says the WHO's announcement is a good reminder.

"Perhaps we might be more aware. We already do take measures to protect ourselves from those fumes," he said.

"We direct exhaust fumes outside of our building through piping from the exhaust. Perhaps with these findings we might be a little more careful with making sure that we evacuate that gas more actively."