A carcinogen is defined as any substance that causes cancer.  Such substances affect the metabolic process in DNA.  Many elements in the environment act as carcinogens.

One of the first links observed by researchers between the environment and cancer was radioactivity, a link made evident by the bombing of Hiroshima, Japan, during World War II.  Following this event, the cancer rate in surrounding areas of Japan skyrocketed.  Subsequently, many other environmental agents have been found to be carcinogenic as well.  These include synthetic chemicals, sunlight, air pollutants, heavy metals, chemical pesticides, and cigarette smoke.  Heavily industrialized areas often exhibit a higher cancer rate.

Cigarettes are the leading cause of lung cancer, a dominant type of cancer in the United States.  Not only smoking, but even secondhand inhalation of smoke has been found to be carcinogenic.  Air pollution is a cause of cancer as well.  Inhabitants of urban areas have a 33 percent higher risk of developing lung cancer.

Occupational exposure to various chemicals is also a cause of cancer.  Before the Toxic Substances Control Act of 1976, potentially hazardous chemicals were often not tested before being put on the market, and carcinogenic substances, such as asbestos, led to many workers' deaths.  Vinyl chloride, used in plastic production, and anesthetic gases from operating rooms, have been known to cause cancer among workers exposed to them.  In addition, overexposure to ultra-violet radiation, as a result of ozone layer depletion, is a leading cause of skin cancer.

The EPA on Carcinogens