Study underestimated difference in risk

EDITOR—In their study of lung cancer and other diseases in male pipe and cigar smokers, Wald and Watt seem to have underestimated the difference in the risk of lung cancer between male current smokers and lifelong non-smokers (a 16-fold difference in their study).1 Non-smokers accounted for seven of 102 lung cancers in the study (figures for most ex-smokers were not supplied). This is very different from the experience of Capewell et al, who studied 3070 Scottish patients with lung cancer.2 Only 0.7% of men with lung cancer were lifelong non-smokers. My experience in Salford (three (0.8%) non-smokers and 217 current smokers among 380 men with lung cancer) is almost identical with that of Capewell et al. On the basis of local data on the prevalence of smoking, I calculate a 62-fold increased risk of lung cancer for Salford smokers compared with non-smokers. Capewell et al's figures would also imply a risk of cancer of at least 50-fold for current smokers.

There are several possible explanations for the high proportion of non-smokers with lung cancer in Wald and Watt's study. Firstly, this was an elite group of subjects (business and professional men), of whom only a fifth smoked; one would therefore expect proportionally more cancers among non-smokers. In a previous study of professional British male subjects (the British doctors study), however, only seven (1.6%) of 441 deaths from lung cancer occurred in non-smokers.3 Secondly, the number of lung cancers in Wald and Watt's study was small (102 cases), so the finding of seven cancers in non-smokers (compared with an expected finding of one or two cancers based on the above studies) may have occurred by chance. Thirdly, some long term ex-smokers in Wald and Watt's study may have described themselves as non-smokers (Capewell et al found that at …