During the 1970's many cases of serious silicosis occurred among migrant workers doing tunnel construction.
We studied the prevalence of silicosis in the eastern part of Toyama prefecture. Questionnaires were sent to all male inhabitants aged 30 or over in the five selected areas. Eight hundred and eighty-five of respondents (41%) had worked as migrant workers. Of these, 580 men (66%) had worked on the job with exposure to dust such as tunnel construction. Of this number, 482 men were examined by chest roentgenography. Of the 482 migrant workers whose job exposed them to dust, 424 silicosis cases (88%) were found. These patients included 195 cases of category 1, 123 cases of category 2, 59 cases of category 3 and 47 cases of category 4 silicosis.
Most of the patients retired and returned to their home villages without having been given any diagnosis and medical care at their places of employment. The silicosis in 297 cases (70% of the total number of disease patients) was first detected in the course of our research.
Two thousand and seventy-seven of the respondents for questionnaire in 1977 and 1978 were followed-up until the end of 1983. For those who died, the causes and dates of death were confirmed by death certificates. The mean person-years of observation per a person was 5.9. The subjects were divided into three groups.
The mortality rate per 1, 000 person-years of migrant workers who had worked in jobs with exposure to dust was 22.3. The mortality rate was significantly higher than that of migrant workers who had not worked in jobs with exposure to dust (14.9) and those who were not migrant workers (9.1). Especially significance is the excess mortality rate of migrant workers whose jobs exposed them to dust in ages 40 to 69.
The mortality rates for pulmonary tuberculosis, pneumonia & bronchitis and pneumoconiosis among migrant workers whose jobs exposed them to dust were higher than those among non-migrant workers.