Carcinogens are substances or agents that promote cancer. Benzene, kepone, EDB, asbestos, and the waste rock of oil-shale mining have all been classified as carcinogenic. As far back as the 1930s, Industrial and tobacco smoke were identified as sources of dozens of carcinogens, including benzopyrene, tobacco-specific nitrosamines such as nitrosonornicotine, and reactive aldehydes such as formaldehyde — which is also a hazard in embalming and making plastics. Vinyl chloride from PVC is a carcinogen. Certain viruses such as Hepatitis B and human papilloma viruses have been found to cause cancer in humans.