A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. Several radioactive substances are considered carcinogens, but their carcinogenic activity is attributed to the radiation.This maybe because of the ability to damage the genome or to the disruption of cellular metabolic processes. Carcinogens may increase the chance of cancer by damanging DNA directly in cells.
After carcinogen enters the human body it goes through a process called biotransformation to try and eliminate it.

Some sources that cause this:
*Lifestyle factors.
*Medical Treatments [radiation and medicines including chemotherapy, hormone drugs and etc.]
* Workplace and House exposures.