Crocidolite
Many studies support the conclusion that there is a causal association of exposures to crocidolite, a form of riebeckite, with mesothelioma. The relationship between crocidolite asbestos exposures and mesothelioma was demonstrated by applying the scientific method to epidemiology studies designed to formally examine the findings that Wagner and his colleagues published in 1960 (Wagner et al., 1960). A cohort study of a cigarette filter factory in Massachusetts consisted of 33 men exposed during the manufacturing process using crocidolite; 5 died of mesothelioma (Talcott et al., 1989). Two hundred thirty-one mesotheliomas were diagnosed among a group of 6908 persons (6493 men and 415 women) who had worked at a former crocidolite mine and mill in Wittenoom, Australia, at some time between 1943 and 1966. Nine percent of the known deaths in this group were attributed to mesothelioma (Berry et al., 2004). Among 3430 crocidolite miners in South Africa contributing about 49,000 person-years of follow-up, mesotheliomas were discovered in 20 men (Sluis-Cremer et al., 1992). These cohorts had average cumulative exposures of 17-120 f/ml-yr for crocidolite fibers (see Hodgson and Darnton, 2000). Gas mask production using crocidolite from Western Australia in plants was associated with 67 mesothelioma cases among 1172 workers in a plant in Nottingham, UK (Jones et al., 1976, 1996). Twenty-one men were heavily exposed to crocidolite during the construction of a saltpeter plant from 1928 to 1929, and two mesothelioma cases were reported by 1980 in the group with 0.21 cases expected (Hilt et al., 1981). In a cohort of 136 filter paper makers using crocidolite for gas masks and cigarettes working from 1943 to 1972 in Massachusetts (Plant A), 12 mesotheliomas were found, including another case in the wife of a worker (Gaensler and Goff, 1988). A study of 435 workers making gas masks in Leyland, UK, using primarily crocidolite, found 5 mesothelioma cases in the records; 3 pleural mesothelioma cases at Blackburn had amphiboles found in their lung tissue (Acheson et al., 1982).
Many cases of mesothelioma and other asbestos diseases for two cohorts and one subcohort are reported as associated with environmental and occupational exposures to crocidolite in a rural county in southwestern China. Not only were there exposures from ambient air and during the common application of crocidolite-containing clay as stucco, but also asbestos stoves and stove pipes were made in family-style production for selling locally and beyond the Da-yao area until this practice was officially banned in 1984. The annual mortality rate is 85-365 per million in that region of China. In comparison, the current rates in North America are about 15-20 cases per million in men and much lower in women (Sporn and Roggli, 2004). Also, the more highly exposed peasants in this Chinese study had a fivefold increased risk of mesothelioma relative to the counterparts with lower exposure (Luo et al., 2003).