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could explain an increaxd frequency of pleural plaques in exsmokers because damaged clearance mechanisms unaided bv continued smoking may lead to higher "effec tive ' doses of asbestos in the tissues. The clinical significance of a relationship between the prevalence of pleural plaques and smoking depends on the clinical significance of plaques. The immediate impor tance of these lesions is negligible but there is some sus picion that plaques may be indicators of an enhanced risk of lung cancer'" " and mesothelioma " With respect to lung cancer risk the evidence is equivo cal Kiviluoto et al' found 700 cases of calcified plaque in 6.000 adults during a mass chest x-ray survey in Tuusniemi commune, Finland, in 1962. The lung cancer incidence between 1962 and 1977 was almost identical to the incidence in controls without plaques in Maaninka commune. 13 and 14. respectively. It may be important to note that this study was limited to calcified plaques. In a case-control study of 69 lung cancer cases there was an elevated risk of lung cancer of 2.8 only when there was pulmonary fibrosis in addition to calcified plaques. Infor mation on smoking habits was not provided. Edge" has reported an increased incidence of lung cancer in 429 older British shipyard workers with pleural plaques unassociated with pulmonary fibrosis found be tween 1964 and 1971 who were compared with a matched control group of 429 men in another city where there was neither known asbestos exposure nor plaques. After excluding six men with lung cancer who had clinical evidence of tumor at the time observation was begun in the men with plaques, the relative risk was 3.25 (13 cases in the men with plaques compared to 4 in the controls, chi square with Yates' correction = 3.84, p = 005). How ever. this comparison is probably invalid because only 231 of the 429 men with plaques were discovered by routine chest films and the remaining 198 were found by clinic or hospital films whereas all 429 controls had routine films. Thus, there was undoubtedly some selec tion bias operating to increase the incidence of lung can cer in the men with plaques. Comparing only the groups that had routine films, six (2.6%) of 231 men with plaques and four (09%) of 429 controls developed lung cancer, a difference which is not statistically significant at the 0.05 level Furthermore, half the lung cancer cases in the men with plaques were diagnosed in the first two years of follow-up and therefore these individuals presumably had their tumors at the time observation began. After ex clusion of the early cases, the relative risk, as compared to the incidence of lung cancer in the general population. remained at 1.0 until after six years of follow-up it rose to 2.1 (6 cases compared to 2.6 expected), which also u not statistically significant at the 0.05 level Most importantly, no smoking habit information was available for adjust ment of the relative risk. A similar problem indicating selection bias in the men with plaques operated with respect to the incidence of mesothelioma: most of the cases occurred early in the follow-up period and only two developed alter six yr.ir-, of follow-up. However, mesothelioma is so rare m tlv general population that even two cases in the mm wnii plaques is suspicious, especially since mi-soihrlKiiii.i is not related to smoking. Thus, at present the prognostic significance of pleural plaques with regard to the risk of lung cancer is doubtiol In the absence of smoking-habit data, no conclusions are warranted. If plaques are related to smoking as well as to asbestos exposure and if lung cancer is similarly related, then any association between plaques and lung cancer is likely to be spurious. References 1 Weisi W and Theodos PA Pleuropulmonary disease among asbeitos workers in relation to smoking and type of exposure / Occvp Med 20 341-34$, 1976 2 Harries PC. Rossiter CE. and Coles RM: Royal Naval Dockyards Asbestosis Research Project. Report No 1. December 1975 3. Rossiter CE and Harries PC: U K Naval Dockyards Asbestosis Study. Survey of the sample population aged S0-S9 years Sr I ind Med 36 281-291.1979. 4 Rossiter CE. Heath IR. and Harriet PC Royal Naval Dockyards Asbestosis Research Protect Nine-year <ollow-up study of men e>posed to asbestos in Devonport Dockyard / Roy Soc Med7} 3)7144 1980 $ Hillerdal C Pleural plaques in a health survey material Scant/ / Resprr Du S9 257-26). 1978 6 Hedenberg l, Hermansson l, Liden MA. and Thunnger C Directed health survey of workers exposed to asbestos Ltk.irudmr.gen 78 4151-4152, 1976 (Abstract in International Cancer Research Data Bank Cancergram on Environmental and Occupational Carcmo genesis. Series CK02, No 4, April 1979. p 6. National Cancer Institute! 7 Smoking and Health. A Report of the Surgeon General U 5 Department of Health. Education, and Welfare. DHfM Puhluatinn No (PHSl 79-50066.1979. Section 6. pp 6-32 and 6-33 6 Cohen D. Arai SF. and Brain ID. Smoking impairs long-term ilust clearance from the lung Science 204 514-517. 1979 9 Albert RE. Peterson HT. Bohning DE. and lippmann M Slwi term effects of cigarette smoking on bronchial clearance in humanArch Environ Health 31 361-3b7.1975 10 Kiviluoto R. Meurmin LO. and Hakama M Pleural plaaurs and neoplasia in Finland Ann NY Acad id 330 31-3). 1979 11 Edge IR Incidence of bronchial carcinoma in shipyard workers with pleural plaques Ann NY Acad Sci 330 289 294. 1979 430 Pleural Plaques and Cigarette Smoking/Weiss, Levin, and Goodman