Are glass fibres carcinogenic to man? A critical appraisal.

In an editorial entitled 'Fiber carcinogenesis: is asbestos the only hazard?', Stanton (1974) drew attention to the possibility that inhaled fibres other than asbestos, and in particular glass fibres, could be carcinogenic to humans. The hypothesis was based on experiments carried out on rats. Specially-bred experimental animals were subjected to intrapleural application of large numbers of specially-milled glass fibres with dimensions of the order of 20 ,tm or greater in length and less than 3 ,tm in diameter. Similar experiments were carried out using other substances including three types of asbestos. Later, when these animals had been killed, Stanton observed the development of a high percentage of pleural tumours (mesotheliomata). Such tumours have been found to occur in humans occupationally exposed to asbestos, particularly crocidolite or blue asbestos. Stanton infers that fibreglass may therefore similarly represent an occupational hazard as a human carcinogen. He relates the alleged carcino-genicity of glass fibre to its fibrous nature (Stanton and Wrench, 1972). The hypothesis that fibreglass may be an inhaled human carcinogen because of its fibrous nature can be questioned for several reasons: 1. To compare the results of experiments carried out on one animal species with those of another can lead to fallacious conclusions; for example, meso-thelioma of the human pleura, when it does occur occupationally in asbestos workers, is closely connected with exposure to crocidolite. Other forms of asbestos such as chrysotile and amosite have been found to induce occupational tumours very rarely. Nevertheless these minerals are fibrous, and past occupational exposures have been heavy. The fibrous state therefore seems to be unrelated to 47 human mesothelioma caused by asbestos, and Stanton's findings with rats do not discriminate between the three sorts of asbestos. 2. The route of application of the specially-milled glass fibres and their site of lodgement in the experimental animal (that is, by opening the chest wall and applying them over a large area of the pleura) is artificial. It cannot be duplicated in vivo and it ihas no relationship with human exposure or animal inhalation. It has been shown by Gross et al. (1970) that in rats and hamsters subjected to inhalation of exceedingly high concentrations (100 mg/m3) of glass fibres, with mean diameter 0 5 ,um and length ranging from 5 ,um to 20 ,m, such fibres were cleared almost completely from the parenchyma with no interstitial fibrosis. There was no evidence that the glass fibres …