Carcinogen

The idea that chemicals could cause cancer was first promoted in 1775 by Percivall Pott, a London physician. Dr. Pott noted that young chimney sweeps had a high incidence of scrotal cancer. Because most sweeps began their careers very early in life and seldom washed or changed clothes, the sweeps were exposed to soot repeatedly and for long periods of time, leading to scrotal cancer in young adulthood. Not until 150 years later were the actual carcinogenic substances in soot identified, but Dr. Pott made his case for more humane treatment and better working conditions for the chimney sweeps by noting the connections between cancer and this profession.

With the advent of industrial development in the nineteenth century, other connections between certain cancers and chemicals were noticed. Shale oil and coal and tar workers had a high incidence of skin cancer. Dyestuff workers developed bladder cancer. And a chemical called vinyl chloride, used in the manufacture of leather goods, caused a rare liver tumor.

In response to growing concern about cancer in industrial workers, the International Agency for Research on Cancer and the National Toxicology Program formulated a system to classify chemicals according to their cancer-causing risk. A chemical could be classified either as a probable carcinogen, or as a non-carcinogen.

A problem with this and other classification systems is that much of the research is based on experiments on animals. If cancer can be induced in experimental animals with high levels of a chemical, it is sometimes assumed that humans could also be at risk even with lower exposure levels over a long period of time. The science of risk assessment investigates the possibility of developing cancer from very low levels of exposure to chemicals that cause cancer at very high levels.

Dioxin is a case in point. In the early 1980s, dioxin was sprayed on the roads of Times Beach, Missouri, to seal the pavement. Dioxin had been classified by the Environmental Protection Agency (EPA) as a probable carcinogen. When the townspeople discovered that dioxin had been sprayed on their roads, the town was abandoned and lawsuits against the road contractor were initiated which were challenged by the defendants.