Comparisons of the pathogenicity of long and short fibres of chrysotile asbestos in rats

( i) Long-term inhalation and intraperitoneal injection studies were undertaken with laboratory rats using short and long fibre samples of Canadian chrysotile asbestos.( ii) The short fibre sample was prepared by the Institute for research and development of asbestos using the sedimentation techniques described by JOLICOEUR et al., (1981) and a respirable dust cloud was generated from the sample supplied using a fluidised bed aerosol generator. The long fibre dust cloud was generated using a Timbrel1 dust generator and chrysotile from the same batch as used to produce the short fibre dust. In this case the raw commercial chrysotile (grade 4T.30) was loaded directly into the generator without any preliminary treatment.( ill) As routinely monitored by Phase Contrast Optical Microscopy, the long fibre dust cloud contained approximately 5500 fibres/ml >5pm in length while the short fibre cloud contained 1100 fibres/ml >5�m in length. These figures indicate that the bulk 1.5Kg sample of short fibre chrysotile supplied, contained a higher proportion of relatively long fibres than the previously reported preparation which was produced only in milligramme quantities. However, the difference in fibre numbers in the longer size ranges increased progressively with length. For fibres >30�m in length the long fibre dust cloud contained 320 fibres/ml while the short fibre cloud contained only M fibres/ml.( iv) Following a one year inhalation period at a respirable dose level of 10mg/m3, there was no difference in the overall survival of the group of rats treated with either long or short chrysotile. Untreated control animals did survive significantly longer than the two dusted groups.( v) Rats treated with long fibre chrysotile developed six times more advanced interstitial fibrosis (asbestosis) than animals treated with short fibre chrysotile.( vi ) Rats treated with long fibre chrysotile developed three times more pulmonary tumours and mesotheliomas than rats treated with short fibre chrysotile. There were no differences between these two groupsof rats or controls in the occurrence of tumours in other sites.( vii) At the end of the 12 month dusting period, roughly three times more short fibre chrysotile than long had been retained in the rat lung tissues. During the following six months, however, the short fibre chrysotile was removed from the lungs much more rapidly than the long. The percentage clearance was 89% for short fibre chrysotile and 5^% for long.(viii) Following intraperitoneal injection at a mass dose of 25mg of dust, both long and short fibre chrysotile produced mesotheliomas in more than 90% of rats. At a dose level of 2.5mg of dust the short fibre chrysotile produced mesotheliomas in only one third as many rats as the long fibre dust which still produced mesotheliomas in more than 90% of the animals injected. At a dose level of 0.25mg of dust the short fibre chrysotile produced no mesotheliomas while the long fibre chrysotile still produced these tumours in 66% of rats. In the two highest doses, where the short fibre chrysotile produced mesotheliomas, the mean tumour induction period was significantly longer than for tumours produced by long chrysotile. “”