Document pE4r4r7qENGB0NGraxX7GN7k

`7 . Section of 0ccuparlo;lal Medicine 295 / Meeting January 14 1965 President's Address `I`tieEnvironment and Disease : ,issociation or Causa#ion? by Sir Austin Bradford Hill CEIEDSCFRCP(hon) FRS professor Emeritus of Medical Siarisrics, U,tiwrsit.v of Lorrdon) Amongst the objects of this newly-founded Section of occupational Medicine are firstly `to provide a means, not readily afforded elsewhere, whereby physicians and surgeons with a special knowledge of the relationship between sickness and injury and conditions of work may discuss their prob- lems, not only with each, other, but also with colleagues in other fields, by holding joint meet- ings with other Sections of the Society'; and, secondly, `to make available information about the physical, chemical and psychological hazards of occupation, and in particular about those that are rare or not easily recognized'. I At this first meeting of the Section and before, with however laudable intentions, we set about instructing our colleagues in other fields, it will be proper to consider a problem fundamental to our own. How in the first place do we detect these relationships between sickness, injury and conditions of work? How do we determine what are physical, chemical and psychological hazards of occupation, and in particular those that are rare and not easily recognized? There are, of course, instances in which we can reasonably answer these questions from the general body of medical knowledge. A particular, and perhaps extreme, physical environment cannot fail to be harmful; a particular chemical is known to be toxic to man and therefore suspect on the factory floor. Sometimes, alternatively, we may be able to consider what might a particular environment do to man, and then see whether such consequences are indeed to be found. But more often than not we have no such guidance, no such means of proceeding; more often than not we are dependent upon our observation and enumeration of defined events for which we then seek antecedents. In other words we see that the event B is associated with the environmental feature A, that, to take a jpecific example, some form of respiratory illness is associated with a dust in the environment. In what circumstances can we pass from this observed association to a verdict of ccnt.~sotio't?i Upon what basis should we proceed to do so? I have no wish, nor the skill, to embark upon a philosophical discussion of the meaning of `causation'. The `cause' of illness may be immediate and direct, it may be remote and indirect underlying the observed association. But with the aims of occupational, and almost synonymously preventive, medicine in mind the decisive question is whether the frequency of the undesirable event B will be influenced by a change in the environmental feature A. How such a change exerts that influence m a y call for a great deal of research. However, before deducing `causation' and taking action we shall not invariably have to sit around awaiting the results of that research. The whole chain may have to be unravelled or a few links may suffice. It will depend upon circumstances. Disregarding then any such problem in semantics we have this situation. Our observations reveal an association between two variables, perfectly clear-cut and beyond what we would care to attribute to the play of chance. What aspects of that association should we especially consider before deciding that the most likely interpretation of it is causation? (I) Strength. First upon my list I would put the strength of the association. To take a very old example, by comparing the occupations of patients with scrotal cancer with the occupations of patients presenting with other diseases, Percival Pott could reach a correct conclusion because of the enormous increase of scrotal cancer in the chimney sweeps. `E\en as late as the second decade of the twentieth century', writes Richard Doll (l964), `the mortality of chimney sweeps from scrotal cancer was some 200 times that of workers who were not specially exposed to tar or mineral oils and in the eighteenth century the relative difference is likely to have been much greater.' To take a more modern and more general example upon which I have now reflected for over fifteen years, prospecti\ e inquiries into smoking have shown that the death rate from cancer of the lung in cigarette smokers is nine to ten times the rate in non-smoken and the rate in heavy cigarette smokers is twenty to thirty times