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Old Material, New Data: Asbestos Revisited

A recent study by McDonald et al. (1999) supports the hypothesis that amphibole fibers, including tremolite, are more carcinogenic than chrysotile fibers. However, the data were not sufficient to address the question "Are they fibrogenic to the same extent that they are carcinogenic?"

Reference: McDonald, J.C., A.D. McDonald and J.M Hughes 1999. Chrysotile, Tremolite and Fibrogenicity. Ann. Occup. Hyg. 43(7):439-42

Previous data from this group (McDonald and McDonald, 1997) suggest that the risks of lung cancer and mesothelioma in chyrsotile miners and millers were related to the estimated levels of fibrous tremolite in the mines in which they had worked. Their data suggest that tremolite fiber concentration was responsible for the differences in the incidence of carcinogenicity among miners. Would a similar difference apply to fibrogenicity? 

David Bernstein, Geneva, Switzerland has reported at the 7th International Symposium on Particle Toxicology, Maastricht, October 13, 1999, that pure chrysotile asbestos taken from the Cana Brava mine in Brazil which is without any tremolite or other amphibole contamination has been found to be cleared from the lung with a clearance half-time of 1.3 days for the long fibers (>20 µm) and 2.4 days for the shorter fibers.  An epidemiological study is underway in Brazil which so far suggests that there is no excess mortality associated with exposure to this chrysotile.  A fiber inhalation toxicity study using the new EC protocol is currently underway with this material.  An initial publication is in preparation and updated reports are expected.   Dr. Bernstein is presenting a poster on this study at the SOT meeting in Philadelphia this March. 

Reference: Personal Communication: David Bernstein, February 2000. davidb@iprolink.ch

Other news concerning tremolite:

The February 2000 newsletter of ACTS FACTS reported that vermiculite is back in the news. Why? Because the major source of vermiculite was mined from Libby, Montana and the dust from the vermiculite was contaminated with tremolite asbestos in amounts as high as 5%. Vermiculite has many uses, the most well known being as a potting soil. It is also used in packaging materials for shipping, as a ceramic and glass kiln insulation, and often being mixed with clay or plaster for texture. According to the report, the workers most at risk from asbestos-contaminated vermiculite were those mining it and those working in the 60 or more plants across the US, Canada, and Puerto Rico where it was processed and packaged. It has been reported that the vermiculite mine in Libby has killed at least 192 people around the country in the last 40 years; most had developed asbestosis, lung cancer or mesothelioma. The full extent of injury is still unknown. W.R. Grace closed the Libby mine in 1990 and vermiculite sold today is from tremolite-free mines.

Reference: ACTS FACTS Newsletter. February 2000. Monona Rossol (ed.). ACTSNY@cs.com

A recent article published in the American Journal of Epidemiology demonstrates elevated risk of mesothelioma and environmental exposure to tremolite. A case- control study on respiratory cancers was conducted in New Caledonia (South Pacific), where a high incidence of malignant pleural mesothelioma had been observed. The disease pattern could not be explained by occupational exposure to asbestos; instead an environmental exposure to asbestos or other mineral fibers was suggested. The first results showed that in some rural areas, tremolite asbestos as a friable rock derived from local outcroppings had been used as a whitewash for indoor and outdoor walls of houses. Samples of the rock were found to consist of virtually pure tremolite asbestos. This study population consisted of all cases diagnosed between 1993 and 1995 which included 15 pleural mesotheliomas, 228 lung cancers, and 23 laryngeal cancers and 305 controls. Information on past or present use of the whitewash, residential history, smoking, diet and occupation were collected. The authors reported that the risk of mesotheliomas was strongly associated with the use of the whitewash [OR= 40.9; (95% CI: 5.15, 3250]. All Melanesians (the indigenous people) had been exposed. In addition, among Melanesian women, exposure to the whitewash was associated with an increased risk of lung cancer [OR= 4.49; 95% CI:1.13, 21.2], and among smokers exposed to the whitewash had an approximately ninefold risk compared with women who never smoked and had never used the whitewash. In contrast, no association was noted by the authors between exposure to whitewash and lung cancer risk among indigenous men, presumably because of lower exposure levels.

Reference: Luce et al. 2000. Environmental Exposure to Tremolite and Respiratory Cancer in New Caledonia: A Case-Control Study. Am. J. Epidemiol. 151 (3):259-65.