Carcinogens are substances, whether chemical or physical, that cause caner.  Caner is the uncontrolled division and growth of cells; these cells can spread and invade surrounding tissues, replacing normal cells.

Humans can be exposed to carcinogens when they are present in food, air, or water. In addition, some carcinogens are created within the body as products of metabolism or certain pathophysiologic states like inflammation or oxidative stress. Carcinogens from the environment likely contribute to a large percent, possibly the majority, of human caners when related to lifestyle, including diet and tobcao use.

Tumors are tissues that grow more rapidly than the surrounding tissue and form an enlarged mass of cells. Tumors can be either benign or malignant. Benign tumors are often encapsulated and grow slowly; they do not metastasize, and rarely exhibit anaplasia, the loss of cellular differentiation and function. Malignant tumors are not encapsulated, and they grow quickly and metastasize into the surrounding tissues. Metastasis is the ability of tumor cells to break free of the tumor and enter the bloodstream, where they travel to other parts of the body and begin to grow. They display anaplasia and are poorly differentiated.