Cancer is a series of diseases characterized by abnormal cell growth and spread of cancerous cells through the body. A carcinogen is a substance that can cause cancer in humans or animals. Sometimes substances promote cancer or aggravate cancers but do not cause them. These substances also may be called carcinogens. Carcinogens include both naturally occurring substances (radon gas) and substances created as a result of production (the components of tobacco). Lists of known carcinogens are maintained by national and world agencies, such as the National Cancer Institute, the International Agency for Research on Cancer, the National Toxicology Program, and the World Health Organization.

Geographers have been involved in the study of cancer because there are obvious geographical differences between nations, within states and provinces in ...