Photo by twicepix on Flickr.

In a report released yesterday, the World Health Organization (WHO) declared exhaust from diesel engines to be a carcinogen [PDF] — the same status as secondhand smoke. In 1989, the fumes were deemed a “probable carcinogen.” The suspected culprit? Particulate matter expelled during diesel fuel combustion. Gasoline exhaust, with a different chemical makeup, remains a possible carcinogen.

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As reported by CBS News, the WHO study looked at a population of 12,000 miners over the course of the past 60 years. Those regularly exposed to diesel exhaust had three times the rate of lung cancer deaths as their peers.

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This doesn’t mean that people who live in urban areas need to move to farms. The threat is akin to secondhand smoke: If you live in a closed room with a constant smoker, you’re far more at risk than if you simply walk by someone on the street. As the WHO put it:

Large populations are exposed to diesel exhaust in everyday life, whether through their occupation or through the ambient air. People are exposed not only to motor vehicle exhausts but also to exhausts from other diesel engines, including from other modes of transport (e.g. diesel trains and ships) and from power generators.

Diesel exhaust is most dangerous in places where you can’t escape the fumes: in the mines, for example, or in areas where housing is located near a facility that emits a lot diesel fumes. (That could mean an increased risk for people living in communities like West Harlem — see, for example, this Grist profile from back in the day.)

The diesel industry responded exactly as you might predict: by downplaying the findings.

As recently as April 12, 2012, findings of this landmark study sponsored by U.S. Environmental Protection Agency (EPA), the California Air Resources Board (ARB), industry and HEI suggest ‘few biologic effects to diesel exhaust exposure.’

“Few” is not the same as “none.” We’re going to go ahead and take the WHO’s word on this one, thanks.