A carcinogen is any substance that is an agent directly involved in causing cancer. Carcinogens are not toxic right away. Their effect can be insidious. There are many natural carcinogens. Several radioactive substances are considered to be carcinogenics. Carcinogenics can arise in both natural and synthetic substances. As dangerous as carcinogens are, most of them are common in our everyday life. Most carcinogens require long term use to cause cancer to show up in the body. Some examples of carcinogens are alcohol, tobacco and cigarette smoke. Tobacco smoke, in 1992, was classified as a Group A carcinogen. Group A being the most dangerous class. Smokeless tobacco contains nicotine along with 28 other carcinogens. All of them are absorbed into the blood throughout the mucous membranes and the digestive tract. Most carcinogens are very addictive, and once you start you can not stop. Carcinogens cause cancer by producing changes in the genetic materials, or DNA, of a cell. Once the cells are filled with this, they result in uncontrolled cell decision. The DNA can be changed directly or it can react with other chemicals throughout the body to make substances that cause gene mutation. It is pretty common for cancer to appear 20 to 25 years after you start using a carcinogen.