Commentary on “possible effects on occupational lung cancer from smoking related changes in the mucus content of the lung”

Abstract

THE HYPOTHESIS that smoking may protect workers from the adverse consequences of inhaling dusts, fumes or gases on the job is an interesting one. As Sterling points out, workers in dusty occupations do sometimes claim that they smoke because smoking helps them to get rid of the dust from their lungs. And doctors have on occasion been known to support this view. In the 1950s in Britain, it was found that elderly coal miners without radiographic evidence of dust in the lungs were sometimes more likely to be disabled than miners of comparable age with the most advanced stage of simple pneumoconiosis. The hypothesis was entertained that this might have been due to cigarette smoking causing bronchitis, which protected the lungs from pneumoconiosis but which resulted in obstructive airways disease and consequent disability. In the present paper, the focus is on lung cancer. Sterling advances the hypothesis that smoking by increasing the mucus content of the lung reduces the impact of respiratory carcinogens. The pathological concept is of a layer of mucus lining the respiratory passages, which either blocks the access of the carcinogens to the mucus membrane, or dilutes and clears them more effectively than normal. Three kinds of evidence are advanced to support this hypothesis. First, studies are cited that indicate that the lung dust burden is less in smokers than in non-smokers. Second, certain epidemiological studies have shown or been reported to show a higher risk of lung cancer in workers exposed to respiratory carcinogens who do not smoke compared to those who do. Third, an experimental study on Beagle dogs is quoted to support the contention that the incidence of cancers in the respiratory tract is greater in animals exposed to carcinogenic dusts alone than in animals exposed to carcinogenic dusts and cigarette smoke. I should like to comment on these studies. The study by Pinto et al. [l] on pensioners from a copper smelter, who had been exposed at work to arsenic was of particular interest to me, since, for the past few years, my colleagues and I have been trying to disentangle the relative importance of exposure to arsenic trioxide and smoking in the cause of lung cancer among smelter workers. Table 1 (Table 3 from Pinto’s paper), shows the appropriate comparison. It will be seen that the number of cancer cases was small-only 3 among the non-smokers and 3 more among the