PhotoFor the last 24 years the World Health Organization (WHO) has believed diesel exhaust to be a "probable" source of cancer. Now the agency has removed the world "probable" from the statement.

After input from international health experts, the WHO's International Agency for Research on Cancer has classified diesel engine exhaust as carcinogenic to humans, based on sufficient evidence that exposure is associated with an increased risk for lung cancer.

It also found a "positive association" between diesel exhaust and development of bladder cancer.

Diesel exhaust's cancer-causing potential has been the source of intense study over the last two decades, especially because of findings in epidemiological studies of workers exposed in various settings. Earlier this year health researchers published the results of a large US National Cancer Institute/National Institute for Occupational Safety and Health study of occupational exposure to such emissions in underground miners, which showed an increased risk of death from lung cancer in exposed workers.

Compelling evidence

“The scientific evidence was compelling and the Working Group’s conclusion was unanimous: diesel engine exhaust causes lung cancer in humans.” said Dr Christopher Portier, Chairman of the IARC working group. “Given the additional health impacts from diesel particulates, exposure to this mixture of chemicals should be reduced worldwide."

Because of growing environmental concerns over the past two decades, governments in North America and Europe have already taken some action to tighten emission controls on both diesel and gasoline engines. Among the changes have been reductions in the sulfur content of diesel fuel, as well as changes in engine design to promote efficiency.

The researchers concede that recent changes in diesel fuel and engine technology may have mitigated the risk somewhat, but said more research is needed to determine just how much. Meanwhile, they say that gasoline exhaust remains a suspected carcinogen, but current research has not offered conclusive proof that it is.