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IT is known that the incidence of lung cancer amongst heavy cigarette smokers is higher in urban than in rural districts. While this might be attributed solely to exposure to the carcinogenic substances present in the urban atmosphere (e.g. particularly smoke from coal fires, also diesel fumes and car exhaust fumes (Kotin, 1956)), there is also the possibility of a co-carcinogenic action between cigarette smoke and city smoke. The following series of experiments was designed to test this hypothesis. Since the experiments were begun three years ago, Gellhorn (1958), Roe, Salaman and Cohen (1959) and Wynder and Hoffman (1960) have confirmed the presence of incomplete carcinogens in cigarette smoke as foreshadowed by Hamer and Woodhouse (1956) and Gwynn and Salaman (1956).

Tumour Promotion by the Neutral Fraction of Cigarette Smoke