Evidence for variations in the pathogenic effects of the different forms of commercially used asbestos. A review of the literature

The main commercially used types of asbestos are chrysotile, amosite and crocidolite although relatively small amounts of anthophyllite and tremolite are also used by industry. Since the use of asbestos is subject to strict controls in most countries it is most important to know if there are differences in the potential harmfulness between the different types. Unfortunately, the available evidence is fragmentary and to some extent contradictory.Evidence from human epidemiological studies indicates that exposure to crocidolite asbestos results in a much greater likelihood of developing mesotheliomas than exposure to other asbestos types. However, it appears certain that both chrysotile and amosite can produce these tumours in humans although anthophyllite may not be able to do so. There is also some evidence to suggest that crocidolite exposure may be more likely to produce bronchial carcinomas than chrysotile.Extraction of asbestos dust from human lung tissue indicates that levels of amphibole asbestos are higher and chrysotile lower than would be expected from exposure data. However, two papers from France suggest that chrysotile predominates in the pleural tissues and in mesotheliomas.In vivo experimental studies mostly using rats have used the techniques of intrapleural or intraperitoneal injection of dust or inhalation to administer different varieties of asbestos. Almost all the injection studies showed that chrysotile was at least as carcinogenic as the amphibole dusts and usually more so.In one publication only, UICC crocidolite had produced more mesotheliomas than UICC chrysotile but a “”superfine”” sample of chrysotile had produced even more. All inhalation studies where tumours developed have found chrysotile more effective in producing bronchial carcinomas than amphibole dusts and chrysotile also produced at least as many mesotheliomas.With one exception all in vitro studies using phagocytic cells have found chrysotile more cytotoxic than amphibole samples although one group of workers using non-phagocytic cells did find that amphibole samples killed a higher proportion of cells than chrysotile. Without exception a series of studies examining the haemolytic effects of asbestos have reported that while chrysotile is highly haemolytic, the amphiboles show very little effect.Possible reasons for the inconsistencies between results from humans and animal experiments are discussed. For the purposes of protection of individuals exposed to asbestos, epidemiological evidence must take priority over that from animal in vitro experiments. However, it is important to reconcile the differences and further studies using all available techniques are required. “”