Until the twentieth century lung cancer was an uncommon disease of obscure causation. Then, as its incidence increased, questions arose about the factors responsible. Some blamed the introduction of tarred roads, others the fast-spreading habit of cigarette smoking. From the nineteen-eighties Richard Doll repeatedly expressed surprise that he and Bradford Hill had been able to establish the complicity of tobacco rather than road tar or vehicle exhaust fumes. Yet no convincing research ever linked road tar or motor exhaust with elevated lung cancer rates in humans. In contrast, many eminent scientists in Britain and overseas had proposed a connection between smoking and lung cancer.