Before this June, diesel engine exhaust was listed as a 'possible carcinogen.' Now the World Health Organization has classified the fumes as cancer causing. As semis roll down the road, there's a new threat level when it comes to their emissions…Health experts say diesel engine exhaust is now in the same carcinogenic category as asbestos, benzene and tobacco smoke. "The studies seem to suggest that there's an increased risk of lung cancer, and possibly bladder cancer," said Stephen Campbell, M.D. of Mayo Clinic Health System.