Not only is tobacco addictive but the smoke from burning tobacco is toxic.In 1992 the environmental protection agency classified environmental tobacco smoke or secondhand smoke as a group a carcinogen .This is the most dangerous class of carcinogens a carcinogen is a cancer causing substance. Common examples of non-radioactive carcinogens are inhaled asbestos, certain dioxins, and tobacco smoke. Carcinogens may increase the risk of cancer by altering cellular metabolism or damaging DNA directly in cells, which interferes with biological processes, and induces the uncontrolled, malignant division, ultimately leading to the formation of tumors.After the carcinogen enters the body, the body makes an attempt to eliminate it through a process called biotransformation. The purpose of these reactions is to make the carcinogen more water-soluble so that it can be removed from the body. However, in some cases, these reactions can also convert a less toxic carcinogen into a more toxic carcinogen.