石綿セメント管(水道管)の修繕作業にともなう石綿曝露

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Asbestos cement pipes (ACPs) containing 15 to 20% chrysotile or crocidolite have been used for underground conduits. Even today 16.2% of all conduits in Japan are ACPs, though the production of ACPs was suspended in 1985. When such a conduit is accidentally damaged the workers belonging to the Waterworks Bureau of a local government cut off the damaged conduit using a high-speed disk cutter and replace it with a new conduit. This operation develops a cloud of dust and the workers involved run the risk of asbestos exposure. It was the aim of the present study to estimate asbestos exposure levels among these workers. First, in the experiment, we established the typical working conditions and requested an experienced worker to cut an ACP using a high-speed disc cutter in a hole dug in the ground as he routinely does. The experiment was repeated three times. During a bout of each experiment, dust was sampled at several points both inside and outside the hole. Second, a self-administered questionnaire survey was conducted to obtain information from the workers regarding their working conditions in cutting ACPs. The subjects of the survey were 1,048 men belonging to conduit repair sections of the Waterworks Bureau of 119 local governments. The results obtained can be summarized as follows. 1) Each bout of cutting ACPs requnired about five minutes. The concentration of asbestos fibers longer than 5μm with 3: 1 aspect ratio ranged from 48 to 170 fibers/ml (92 fibers/ml on an average) inside and 1.7 to 15 fibers/ml outside the hole. The concentration inside the hole exceeded the ceiling limit (10 fibers/ml) recommended for asbestos by the Japanese Association of Industrial Health. A concentration of 92 fibers/ml is equivalent to 0.96 fibers/ml as 8-h time-weighted average. 2) The number of subjects with experience of cutting ACPs was 849 (81.0%). The average length of service in conduit repair section was 14.2yr. Based on the information obtained from each subject regarding the average working days per yr for each decade from 1946, the cumulative days to date expended in cutting ACPs was estimated to average 235d, that is, 17d per yr. Only 18.1% of the subjects used a protective respiratory device. 3) On the assumption that these workers worked 17d per yr in cutting ACPs for 14yr and were exposed to 1 fiber/ml of asbestos per d, the life-time excess mortality rate of lung cancer would be 30 per 100,000 and that of mesothelioma 30 per 100,000 by OSHA's model, and that of lung cancer would be 15 and that of mesothelioma 28 by Hughes' model. These results suggested that workers belonging to the Waterworks Bureau have a possibility of being exposed to relatively high levels of asbestos when cutting ACPs and have a risk of developing asbestos-related diseases.
社団法人日本産業衛生学会の論文
1993-05-20

石綿セメント管(水道管)の修繕作業にともなう石綿曝露

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