Ganciclovir is considered a potential human carcinogen, teratogen, and mutagen. Formaldehyde is classified by the WHO as a known human carcinogen. The substance has low to moderate toxicity with a possible human carcinogen classification. Acetaldehyde is toxic when applied externally for prolonged periods, an irritant, and a probable carcinogen. Arsenic is classified as a Group-A carcinogen. Adding carbon black also prevents the formation of bromate, a suspected human carcinogen. Thallium is a suspected human carcinogen. "Aristolochia" has been shown to be both a potent carcinogen and kidney toxin. At high temperatures, a probable carcinogen called acrylamide can form. Tobacco is an environmental carcinogen and the major underlying cause of lung cancer. Safrole is still regarded by FDA to be a weak carcinogen in rats. Some say Thallium is a suspected human carcinogen. The United States Environmental Protection Agency lists parathion as a possible human carcinogen. Alcohol is an example of a chemical carcinogen. The FDA stated that hydroquinone cannot be ruled out as a potential carcinogen.