The interaction of asbestos and smoking in lung cancer: A modified measure of effect

Objectives: The ratio of the relative risk of lung cancer due to asbestos exposure in non-smokers to that in smokers has been termed the relative asbestos effect (RAE). In a review, Liddell [Liddell FDK (2001) Ann Occup Hyg; 45: 341-56] estimated that the RAE was -2. This measure is satisfactory when there is an appreciable relative risk due to asbestos but does not generalize to lower levels of exposure. A modified measure is proposed to overcome this difficulty. The modified measure, RAE(m), is defined as the ratio of the excess relative risk (RR - 1) in nonsmokers to that in smokers. Methods: The cohort studies combined in Liddell's 2001 analysis have been used to give a combined estimate of the modified measure. Results: The combined value of RAE(m) is 3.19 with 95% confidence interval 1.67-6.13. Conclusion: The excess relative risk of lung cancer from asbestos exposure is about three times higher in non-smokers than in smokers. The modified measure has been placed within a more versatile model of interaction. If interaction is present the relative risk from asbestos exposure changes only slightly between light and heavy smokers, but is higher in very light smokers and non-smokers. The relative risk estimated from epidemiological studies of a mixed population of non-smokers and smokers applies to smokers.