STATISTICAL-ANALYSIS OF RESULTS OF CARCINOGENICITY STUDIES OF SYNTHETIC VITREOUS FIBERS AT RESEARCH AND CONSULTING COMPANY, GENEVA

Five inhalation studies of synthetic vitreous fibres have recently inv estigated experimental tumorigenic responses to four different refract ory ceramic fibres (RCF), two fibre glasses, one stone (rock) wool and one slag wool. Except for one RCF, the source materials were typical commercial products. Three studies included positive control groups ex posed to chrysotile or crocidolite asbestos. The studies were conducte d using state-of-the-art technologies for fibre size separation, fibre lofting and nose-only inhalation exposure. The target average fibre s ize was 20 mu m long by 1 mu m diameter. Hamsters exposed to a kaolin RCF yielded a mesothelioma rate of 38%, but no lung cancers. There wer e no tumours among the chrysotile-exposed hamsters. At the highest dos e of 30 mg m(-3) in rat studies, the commercial RCF all produced signi ficant numbers of lung tumours, and some mesotheliomas. The fourth RCF , which had been heat-treated to simulate an after-service fibre, did not produce a significant excess of lung cancers, but did produce one mesothelioma. A rat multi-dose experiment with three lower doses of th e kaolin RCF yielded one mesothelioma among 379 rats, but no excess of lung tumours. The overall dose-response relation for lung cancer did not appear to be linear, consistent with the possibility of a threshol d close to the Maximum Tolerated Dose. No insulation wool (glass, ston e or slag) exposure group had a lung tumour rate that differed statist ically significantly from the tumour rate for the respective concurren t control groups, sham-exposed to filtered air. There was no significa nt difference in the total tumour rates between the four insulation wo ol groups and the control animals, and no significant dose-response re lation above the respective sham-exposed control tumour rates. The tot al lung tumour rates for rats in both chrysotile and crocidolite expos ure groups were significantly raised. One animal in each asbestos-expo sed group developed a mesothelioma, whereas no air control or insulati on wool-exposed animal did so.