Diesel fumes cause cancer, the World Health Organisation's cancer agency has declared - a ruling it said could make exhaust as important a public health threat as secondhand smoke.
The risk of getting cancer from diesel fumes is small, but since so many people breathe in the fumes in some way, the science panel said raising the status of diesel exhaust to carcinogen from "probable carcinogen" was an important shift.
"It's on the same order of magnitude as passive smoking," said Kurt Straif, director of the IARC department that evaluates cancer risks. "This could be another big push for countries to clean up exhaust from diesel engines."
Since so many people are exposed to exhaust, Mr Straif said there could be many cases of lung cancer connected to the contaminant.
He said the fumes affected groups including pedestrians on the street, ship passengers and crew, railway workers, truck drivers, mechanics, miners and people operating heavy machinery.
The new classification followed a week-long discussion in Lyon, France, by an expert panel organised by the International Agency for Research on Cancer. The panel's decision stands as the ruling for the IARC, the cancer arm of the WHO.
The last time the agency considered the status of diesel exhaust was in 1989, when it was labelled a "probable" carcinogen. Reclassifying diesel exhaust as carcinogenic puts it into the same category as other known hazards such as asbestos, alcohol and ultraviolet radiation.
The US government, however, still classifies diesel exhaust as a likely carcinogen. Experts said new diesel engines emit fewer fumes but further studies are needed to assess any potential dangers.
"We don't have enough evidence to say these new engines are zero risk, but they are certainly lower risk than before," said Vincent Cogliano of the US Environmental Protection Agency. He added that the agency had not received any requests to re-evaluate whether diesel definitely causes cancer but said their assessments tend to be in line with those made by IARC.
Experts in Lyon had analysed published studies, evidence from animals and limited research in humans. One of the biggest studies was published in March by the US National Cancer Institute. That paper analysed 12,300 miners for several decades starting in 1947. Researchers found that miners heavily exposed to diesel exhaust had a higher risk of dying from lung cancer.