FROM about 1930 it became apparent that there was an increase in the incidence of lung cancer that was out of proportion to the increase in cancer as a whole, and that the causative agent must be something comparatively new, probably something that had made its appearance during the 1930’s. What was it?
To elucidate such problems there are well-established methods of scientific investigation: evidence is collected, hypotheses suggested, further facts sought, hypotheses modified or if they are not in accord with the evidence abandoned, perhaps new hypotheses put forward, and so on – and always, when a fact and a hypothesis contradict one another, it is the fact that must be retained.
There are plenty of facts available about the increase in lung cancer, and by about 1940 three main hypotheses were being considered: the action of urban smoke, cigarette and tobacco smoke, and diesel smoke. We need to consider which, if any, of these is in accord with the available facts.