Bradford-hill

7

Section ofOccupational Medicine

295

Meeting January 141965

President's Address
The Environment and Disease: Association or Causation?
by Sir Austin Bradford Hill CBE DSC FRCP(hOn) FRS (Professor Emeritus ofMedical Statistics, University ofLondon) Amongst the objects of this newly-founded Section of Occupational Medicine are firstly 'to provide a means, not readily affordedelsewhere, whereby physicians and surgeons with a special knowledge of the relationship between sickness and injury and conditions of work may discuss their problems, not only with each other, but also with colleagues in other fields, by holding joint meetings with other Sections of the Society'; and, secondly, 'to make available information about the physical, chemical and psychological hazards ofoccupation, and in particular about those that are rare or not easily recognized'.
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 wedetermine what are physical, chemical and psychological hazards of occupation, and in particular those that are rare and not easily recognized? observed association to a verdict of causation? 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 beremote and indirect underlying the observed association. But with the aims of occupational, and alnost 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 may 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 toattribute 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?

There are, of course, instances in which we 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 knownto be toxic to man and therefore suspect on the factory floor. Sometimes, alternatively, we may be able to consider what might a par-ticular 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 eventsfor 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 specific example, some form of respiratory illness is associated with a dust in the environment. In what circumstances can we pass from this
can

(1) Strength. First upon my list I would put the strength of the association. To take a very old example, bycomparing 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. 'Even as late as the second decade of the twentieth century', writes Richard Doll (1964), 'the mortality of chimney sweeps from scrotal cancer was some 200...