[Mortality of silicosis patients among migrant workers]

Migrant workers known in Japanese as "dekasegi" refer to workers who migrate seasonally from their town of residence to areas where work is readily available. The eastern part of Toyama Prefecture is well known as a source of migrant workers who engage in jobs associated with dust exposure such as tunnel projects. Most of these workers suffered from silicosis. A total of 695 migrant workers suffering from silicosis who had underwent health screening between 1977 and 1982 were followed until the end of 1983. For cases of death, the cause and date of death were individually confirmed on the basis of death certificates. Based on these data, the person-years of risk and cause-specific mortality rates were calculated. The mean person-years of risk per person was 4.7. During this period of observation, there were 75 deaths among these silicosis patients, giving a mortality rate of 23.0 per 1,000 person-years of risk. When classified by the Japanese roentgenographic category of pneumoconiosis, the mortality rate was 10.5 for category 1, 21.3 for category 2, 38.6 for category 3 and 49.3 for category 4. The mortality rates of categories 3 and 4 were significantly higher than those of categories 1 and 2. The highest cause-specific mortality rate among silicosis patients per 1,000 person-years was 5.2 for malignant neoplasms followed by 3.7 for pulmonary tuberculosis, 3.1 for both cardiovascular diseases and pneumoconiosis, and 2.8 for pneumonia and bronchitis. High mortality rates in the 50-69 age group were found among silicosis patients belonging to categories 3 and 4. By cause of death, the mortality rates of all malignant neoplasms (especially lung cancer), pulmonary tuberculosis, and cerebrovascular diseases were relatively high in this age group. In the 70-89 age group, the mortality rate of those belonging to categories 2, 3 and 4 was high and by cause of death the mortality rates of pneumoconiosis, pulmonary tuberculosis, all malignant neoplasms (especially lung cancer), cardiovascular diseases and pneumonia and bronchitis were high. The mortality rates of silicosis patients with abnormal findings in %VC, FEV1% and AaDO2 by pulmonary function tests tended to be higher than those of silicosis patients without such abnormalities.