People all over the world are exposed to cancer-causing chemicals present in air, water, food, consumer products, and even in soils and dusts. In their places of work some people come into contact with additional cancer-causing agents, generally at higher exposure levels than those experienced by the general population. Some people deliberately expose themselves, and incidentally expose others, to the large number of known and suspected carcinogens present in tobacco smoke. People are also exposed to various physical agents – ultraviolet radiation from the sun and sunlamps and other forms of natural and artificially produced radiation – that increase cancer risks. We are all being assaulted by chemical and physical carcinogens. Add to this the substantial viral and genetic contributions. No wonder the chances of developing some form of cancer over our lifetime is about one in three (for women) and one in two (for men).

But we are moving too quickly. Before we can begin to contemplate the contribution of all these environmental carcinogens to the total cancer problem we need to acquire a better understanding of what is meant by the terms “carcinogen” or “cancer-causing chemical” and of how certain substances get to carry these labels.

We shall begin with a little history, and then move to a discussion of cancer statistics and the causes of cancer, and then provide some background on cancer biology and the mechanisms of tumor development. Some of the general characteristics of chemical carcinogens will also be covered.