Chemical factors

Ever since Sir Percival Pott made the astute observation that soot contributed to the prevalence of scrotal cancer in chimney sweeps, there has been a dramatic rise in the number of chemicals suspected to be carcinogenic. Their origins include industry, e.g. polycylic hydrocarbons, plants, simple organisms, the natural environment, and even the side-effects of medical chemotherapeutics. Some of these are related to occupational or environmental exposure and are dealt with under the respective section.

Early experiments in mice have produced a theory as to the interaction of chemical carcinogens with cells:

• a cell exposed to a suitable dose of chemical carcinogen is then more likely to give rise to a tumour, but this single event is inadequate to ensure tumour production
• permanent damage in DNA is believed to occur, hence, the elevated risk remains permanently once the initiator has been removed
• the effect of the chemical carcinogen is cumulative due to the irreversiblity of its actions and hence, whatever the interval between doses, as long as a threshold amount is exceeded, a tumour will evolve
• 'promoter' agents acting at a site other than DNA can enhance the likelihood of the initiating chemical carcinogen in causing a tumour; the action of the promoter is reversible and so diminishes with time

This theory is equally applicable to humans: bladder cancer has been linked with the cumulative effect of aromatic amine exposure and diethylstilboestrol is believed to be a promoter of postmenopausal endometrial carcinoma.