Chemicals as tumor-inducing agents.

Extract: There is overwhelming evidence that the majority of cancer deaths in Western countries are caused by exogenous factors such as tobacco, alcohol, diet, infections, and occupational exposures. Besides physical and biological factors, such as UV light or oncogenic viruses, chemicals were first considered as carcinogenic agents in the 18th century when two English physicians observed the occurrence of nose polyps or skin cancer due to exposure to snuff and soot. Today it has been established that a great range of chemicals and their mixtures present in our environment are carcinogenic in humans. Examples are metals such as nickel, cadmium or lead, aromatic amines, halogenated olefines (e.g., vinyl chloride) or paraffines, N-nitrosamines, aromatic compounds (e.g., benzo[a]pyrene; 2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD), estrogen-receptor agonists or antagonists with residual agonistic effects (e.g., tamoxifen), natural compounds (e.g., aflatoxins), and so forth.