PROTEIN DIGESTION AND ABSORPTION

non-smokerswould be of the order of 1 in 300. The Council's report points out the particular importance of the observationon the effect of giving up smoking, since ."it indicates that men who ceaseto smoke, even in their early forties, may reduce their likelihood of developing the diseaseby at least one-half". The comment is added that the evidenceis further strengthened"by the observation from several sourcesthat the extent of the relationship with smoking differs for different .types of lung tumour which can be distinguishedonly by microscopical examination"-areferenceto the finding that the relationship appearsto hold only for epidermoid and anaplastic cancers (including oat-cell cancer) and to apply to a less degree (if at all) to adenocarcinoma. The laboratory evidence shows that five substanceshave been found in tobacco smoke which can, in certain circumstances,cause cancer in animals,and that painting concentratedextracts of tobacco tar on the skin of animals has produced tumours. There is no certaintythat the low concentrations of known carcinogensin tobacco smoke could be harmful to human beings, but "the finding of carcinogenicagents in tobaccosmoke is an important step forward, in that it provides a rational basis for the hypothesisof causation". The question of the possible role of atmosphericpollutior. is consideredin the report, and the following conclusion is reached: "On balance, it seemslikely that atmospheric pollution plays some part in causing the disease,but a relatively minor one in comparison with cigarette smoking." Further reference to this last-mentioned question, leading to similar conclusions,will be found In a paperby E. L. Wynder,' which deals constructivelywith the whole problem of causation of lung cancer. The Medical ResearchCouncil report goes on to state that many factors other than tobaccosmoking are undoubtedly capableof producing lung cancer; but so far no adequate explanatiou for the large increasein tlie incidence of the diseasehas been advancedsave that cigarettesmoking is indeed the principal factor in its causation.