Tuesday’s announcement comes after international experts spent a week reviewing new research, including an influential long-term study of more than 12,000 miners who were heavily exposed to diesel exhausts.

The rating upgrades one from 1988, when the WHO’s International Agency for Research on Cancer stated that diesel exhaust was “probably carcinogenic” to humans. This prompted many countries to tighten diesel emission standards. Businesses responded with innovations such as lower-sulphur fuel, new engine designs and better exhaust controls, especially in North America and Europe.

Diesel industry lobbyists criticize WHO’s reliance on the mining study because researchers lacked exact data on exposure levels during the study’s early years. Meanwhile, diesel engine makers point to their new design that produce 95 percent less nitrogen oxide, particulate, and sulfur emissions than older truck and bus engines.

The US Environmental Protection Agency continues to classify diesel more cautiously than the WHO, calling it a “likely” carcinogen.

But Ken Donaldson, a professor of respiratory toxicology at the University of Edinburgh, told CBS News, “It’s pretty well known that if you get enough exposure to diesel, it’s a carcinogen.” However, he adds, “For the man on the street, nothing has changed… It’s a known risk but a low one for the average person, so people should go about their business as normal… you could wear a mask if you want to, but who wants to walk around all the time with a mask on?”