Before the Second World War, case reports had indicated that working with certain chemicals might lead to the development of cancer. After the war, these occupational risks were investigated more thoroughly. Arsenic had long been suspected to be carcinogenic, and in 1945 the British Factory Department set up a committee to investigate the possible relationship between arsenic exposure and lung cancer. A Bradford Hill and E Lewis Faning investigated mortality among workers in a factory producing arsenic powder. Their report published in the British Journal of Industrial Medicine in 19481 is the first description, I have come across, of how to undertake a proper historical cohort study of an occupational group. According to the authors, information required for the study included: “(a) a list of the male employees, divided according to their occupation, for each year from 1943 … back to 1900; (b) the approximate dates of birth of these men so that the age constitution of the population at risk could be computed at different points of time between 1900 and 1943; (c) information regarding all deaths occurring in that period of time”. Therefore, “[g]iven these data the recorded numbers of deaths from different causes could then … [be] compared with the deaths which …