Although no one ever believed that inhaling black, grit-filled smoke would do you any favors, the World Heath Organization notes that doing so is much worse than merely unkind: the WHO has officially decreed that inhaling diesel fumes can cause lung cancer and has added the fumes to its list of Group 1 Carcinogens, those known to cause cancer in humans. That puts diesel plumes in the same homicidal company as arsenic, strontium-90, neutron radiation and being a painter, and makes it worse than second-hand smoke.
However, according to The New York Times, scientists say you only need be concerned if your job or environment places you in direct, long-term contact with diesel exhaust. Workers like miners and toll collectors were mentioned as prime examples, with one study showing that miners subected to diesel fumes for years had a lung cancer risk that was seven times greater than nonsmokers. Just walking by an accelerating school bus or owning a diesel car won't be an issue, though. Said one scientist, "I don't think it's bad to have a diesel car. I don't think it's good to breathe its exhaust."
The American Cancer Society is expected to follow the WHO's move shortly. If your job or corner of the world makes this a troubling bit of news, you might find some temporary solace by noting one of the other hugely popular substances lumped with diesel exhaust in the Group 1 Carcinogens: alcoholic beverages.