The effect of joint exposure to alcohol and tobacco on risk of cancer of the mouth and pharynx.

INTRODUCTION IT IS known that alcohol consumption and smoking are risk factors for cancer of the oral cavity [l-3]. It therefore seems useful to determine how the joint action of the two factors is related to oral cancer risk. The basic question addressed in this paper is the following: do these two exposures confer their risks independently or in some ‘interactive’ fashion? It is necessary to define precisely the concept of independent action, as a basis for defining interaction. Measures of disease risk can be interpreted as probability measures. Persons not exposed to a certain risk factor have a specific probability for developing a disease, and persons exposed have a higher probability. The risk factor increases the probability by an amount ‘x’ -the difference between the two probabilities. If another risk factor increases the baseline probability by an amount ‘JY, then those persons with exposure to both should experience an excess probability of x+v, since the probabilities for rare independent events can be added together to obtain the probability that either event will occur. If one were dealing with a very common disease it might be necessary to modify this expression to take account of the problem of competing risks. For most diseases, however, the simple additive expression is sufficiently accurate. This criterion for independence of risk factors in disease is consistent with the concepts presented by MacMahon and Pugh [4] and by Cole and MacMahon [SJ. The key concept is the additivify of the excess risk associated with different exposures. If the excess risk for persons exposed to both factors is greater or less than x + y, an interaction effect, either synergy or antagonism, may be said to be present. The question of interaction between alcohol and tobacco in the causation of oral cancer was examined by Wynder et al [2]. However, their analysis was based on the