Historical total and respirable silica dust exposure levels in mines and pottery factories in China.

Historical exposure estimates of total dust and respirable silica were made in a recent nested case-referent study of lung cancer among mine and pottery workers in China. Exposure to total dust and respirable silica was assessed in 20 mines and 9 pottery factories. The average total dust concentration was 7.26 mg center dot m-3, with a range from 17.68 mg center dot m-3 in the 1950s to 3.85 mg center dot m-3 in the 1980s, while the average respirable silica dust was 1.22 mg center dot m-3, with a range from 3.89 mg center dot m-3 in the 1950s to 0.43 mg center dot m-3 in the 1980s. The highest respirable silica dust occurred in the underground mining operations (1.43 mg center dot m-3), particularly for manual drillers (9.03 mg center dot m-3). Among all facility types, tungsten mines had the highest respirable silica dust exposure (1.75 mg center dot m-3), while the lowest exposure occurred in copper-iron mines (0.32 mg center dot m-3).

A nested case-control study of lung cancer among silica exposed workers in China.

In an attempt to assess whether silica induces lung cancer, a nested case-control study of 316 male lung cancer cases and 1352 controls was carried out among pottery workers and tungsten, copper-iron, and tin miners from five provinces in south central China. Exposure to dust and silica for each study subject was evaluated quantitatively by cumulative exposure measures based on historical industrial hygiene records. Measurements on confounders such as inorganic arsenic, polycyclic aromatic hydrocarbons (PAHs), and radon were also collected from the worksites. Information on cigarette smoking was obtained by interviews of the subjects or their next of kin. A significant trend of increasing risk of lung cancer with exposure to silica was found for tin miners, but not for miners working in tungsten or copper-iron mines. Concomitant and highly correlated exposures to arsenic and PAHs among tin miners were also found. Risk of lung cancer among pottery workers was related to exposure to silica, although the dose-response gradient was not significant. Risks of lung cancer were significantly increased among silicotic subjects in iron-copper and tin mines, but not in pottery factories or tungsten mines. The results of this study provide only limited support for an aetiological association between silica and lung cancer.

Indirect validation of a retrospective method of exposure assessment used in a nested case-control study of lung cancer and silica exposure.

Validations of retrospective methods of assessment used in occupational epidemiological studies have rarely been published. This study is an indirect validation of a quantitative retrospective assessment of exposure to silica used in a nested case-control study of lung cancer among workers at 29 metal mines and pottery factories in China. Indices of cumulative total dust and cumulative respirable dust were calculated by merging work histories with the historical exposure profile for each subject. To validate indirectly the methods of exposure assessment used in the study of lung cancer, trends for exposure response relation between the two indices of exposure to silica and risk of silicosis were evaluated with 376 patients with silicosis from the study population as the cases, and 1262 controls without silicosis for comparison. Age adjusted odds ratios (ORs) as a measure of risk of silicosis showed striking trends with both indices of exposure to silica. For cumulative respirable dust, the OR (95% confidence interval) rose from 7.6 (5.1-11.4) for low exposure to 20.0 (13.2-30.6) for medium exposure, and to 51.7 (31.0-86.8) for high exposure. The strength of the association between exposure to silica and risk of silicosis suggests that the retrospective assessment of exposure used in the case-control study of lung cancer would accurately reflect an exposure response relation between silica and lung cancer, if it existed.

Estimating historical exposure to silica among mine and pottery workers in the People's Republic of China.

A quantitative retrospective exposure assessment method was developed for use in a nested case-control study of lung cancer among mine and pottery workers exposed to silica dust in the People's Republic of China. Exposure assessment was carried out in 20 mines (10 tungsten, 6 iron/copper, and 4 tin) and nine pottery factories. A job title dictionary was developed and used in both the collection of historical exposure information and work histories of 1,668 (316 cases and 1,352 controls) study subjects. Several data abstraction forms were developed to collect historical and current exposure information and employees' work histories, starting in 1950. A retrospective exposure matrix was developed on the basis of facility/job title/calendar year combinations using available historical exposure information and current exposure profiles. Information on the amount of respirable, thoracic, and free silica content in total dust was used in estimating exposure to silica. Starting in 1950, 6,805 historical estimates had been carried out for 14 calendar-year periods. We estimated the average total dust concentration to be 9 mg/M3, with a range from 28 mg/M3 in earlier years to 3 mg/M3 in recent years. Several exposure indices [such as cumulative dust, average dust, cumulative respirable (< 5 mu in particle size) and thoracic (< 10 mu in particle size) silica dust, average respirable and thoracic silica dust, exposure-weighted duration, and the highest/longest exposure] were calculated for individuals by merging work history and historical exposure matrix for each study subject. We developed these various measures of exposure to allow investigators to compare and contrast different indices of historical exposure to silica.

Mortality among dust-exposed Chinese mine and pottery workers.

A cohort study of approximately 68,000 persons employed during 1972 to 1974 at metal mines and pottery factories in south central China was conducted to evaluate mortality from cancer and other diseases among workers exposed to different levels of silica and other dusts. A follow-up of subjects through December 31, 1989 revealed 6,192 deaths, a number close to that expected based on Chinese national mortality rates. There was, however, a nearly 6-fold increase in deaths from pulmonary heart disease (standard mortality ratio, 581; 95% confidence interval 538 to 626), and a 48% excess of mortality from nonmalignant respiratory diseases (standard mortality ratio, 148; 95% confidence interval, 139 to 158), primarily because of a more than 30-fold excess of pneumoconiosis. Pulmonary heart disease and noncancerous respiratory disease rates rose in proportion to dust exposure. Cancer mortality overall was not increased among the miners or pottery workers. There was no increased risk of lung cancer, except among tin miners, and trends in risk of this cancer with increasing level of dust exposure were not significant. Risks of lung cancer were 22% higher among workers with than without silicosis. The findings indicate that respiratory disease continues to be an occupational hazard among Chinese miners and pottery workers, but that cancer risks are not as yet strongly associated with work in these dusty trades.