Asbestiform Talc and Lung Cancer
Exposure levels prior to 1945 were sufficiently high, in both mines and mills, to result in the pneumoconioses cases described above with quartz levels in air as great as 10 fold higher than today's permissible exposure limit for respirable quartz of 100 µg/m3. Respirable quartz is a known human lung carcinogen, with elevated risks particularly when exposures are sufficient to result in silicosis. That respirable quartz exposures were a concern has been confirmed by autopsy studies performed by Dr. Jerrold Abraham of 8 GTD workers. Two of the 5 workers with a history of more than 20 years of talc mining had silicosis.
The second study that has been used to implicate a risk between exposure to asbestiform talc and lung cancer is the NIOSH 1979 study of Grouvenor Talc Company workers. GTC went into operation in the late 1940's using a wet drilling method that would have suppressed exposure to respirable quartz dust as noted in the above table. The NIOSH study has been criticized because of a number of short comings. It would be important to highlight these short comings since they have been addressed in later epidemiological studies of these workers. Specific concerns with this study included its small size; inclusion of all workers, including those that had only worked days; lack of assessment of the contribution of prior exposures; no study of exposure-lung cancer relationships; and no adjustment for smoking effects (Brown, et al, 1983). Any prior mine work among GTC employees would have likely involved high level exposures to quartz dust. Stille and Tabershaw (1982) were able to nearly double the size of the cohort. They found that the SMR for lung cancer among workers who had only worked at GTC was less than expected (76) and that tuberculosis, a disease associated with silicosis, was a significant finding (SMR 680). This study did not correct for smoking history, exposure or identify non-GTC exposures that many have been a concern.
Lamm, et al. (1988) presented a re-analysis of the Stille and Tabershaw (1982) data set in which the occupational histories of workers dying of lung cancer were presented. 8 of 11 workers who died of lung cancer had worked in mines other than talc mines or in quarries elsewhere than at GTC. The SMR for lung cancer in mill workers was 72 for those workers who had worked at least one year at GTC. For those for workers who worked less than one year and had first worked to GTC 20-24 years prior to their death, the SMR for lung cancer was 1111. The latter group would have included workers with prior exposures to mine dust prior to the putting in place of dust control technologies.
Gamble (1993) performed a nested case control study on NIOSH's second evaluation of 710 GTC workers (NIOSH, 1990) to address concerns of confounding. They found that when using fellow GTC workers as controls, all of the excess lung cancer risk could be ascribed to smoking. When looking at past exposures they found that essentially all talc exposure could be ascribed to work at GTC. They were able to give more complete exposure histories for the lung cancer cases: 8 of the 22 cases had worked as drillers at mines or quarries other than GTC and 17 had worked in metal mines prior to working at GTC. Work in mines would have been expected to be associated with exposure to either quartz dust (exposures would have likely been even higher in metal mines than in talc mines because of quartz content of base rock) or radon daughters, a known cause of excess lung cancer risk in metal miners. That drillers may be at particular risk of quartz exposure has been noted by Rubino, et al. (1976) who found that dust generated from drilling operations my contain up to 18% quartz, even though talc itself is relatively free from quartz. In metal mines, drilling dust can contain up to 39% quartz (McDonald, et al., 1978).
Dezell, et al. (1995) further expanded the cohort to 818 workers and increased the latency time to an average of 21 years for GTC workers. They were able to address the concern that prior studies did not address incorporate an exposure-response analysis by estimating respirable dust exposures. When compared to past dust measurements, there was an excellent correlation between the two with a correlation coefficient of 0.78. They found no relationship between dust exposure at GTC and lung cancer. Increases in lung cancer were limited to workers hired prior to 1955 with deaths from non-malignant respiratory disease concentrated in this group as well. When adjusting for exposure they found an inverse relationship between lung cancer and exposure to all subjects, to those workers who were first employed prior to 1955 and to those workers who had worked at GTC for more than one year. The Gamble and Dezell, et al. studies discount the finding of an exposure-related risk of lung cancer for GTC workers with smoking and/or prior exposures to cancer-causing quartz dust or radon being likely contributors to the risk.
Asbestos and Cancer: An Overview of Current Trends in Europe
Maria Albin,1 Corrado Magnani,2 Srmena Krstev,3 Elisabetta Rapiti,4 and Ivetta Shefer1*
1Department of Occupational and Environmental Medicine, Lund University Hospital, Lund, Sweden; 2Cancer Epidemiology Unit - CPO Piemonte, S. Giovanni B. Hospital and University of Torino, Torino Italy; 3Institute of Occupational and Radiological Health, Belgrade, Yugoslavia; 4Osservatorio Epidemiologico Regione Lazio, Rome, Italy