The distribution of length and diameter and the aspect ratio of crocidolite asbestos, a mineral substitute for asbestos (wollastonite), a manmade mineral fibre (lass wool), and synthetic fibres (pobpopylene and polyacrylonitrite) were determined by light microscopy with phase contrast and, for crocidolite, also with trasission electron microscopy. The synthetic organic fibres and manmade mineral fibre used were of a size exceeding that considered respirable. Respirable materials were given to rats by the itratracheal method and after exposure for a standard time interval the main indices of fibrogenic effects-the total hydroxyproline content, the wet weight of the lung, and the total lipid content in the lung-were estimated. We decided, therefore, to determine its fibrogenicity in vivo. Materials and methods MORPHOMETRIC ANALYSISThis study examined the morphometric variables of asbestos (crocidolite), of a manmade mineral fibre (glass wool), of synthetic organic fibres (polypropylene, polyacrylonitrile), and of two sources of wollastonite, a recent mineral substitute for asbestos. ' The length, diameter, and aspect ratio of fibres were determined by light microscopy with phase contrast as previously described'2; crocidolite was also examined by transmission electron microscopy.Microphotographic recordings of the individual variables of fibre size and distribution were analysed. For all variables, the average, geometric mean, and SD were calculated. ANIMALS AND CONDITIONS OF EXPOSUREThe samples of fibres that could be given to the experimental animals (specific pathogen free male Wistar rats, about 300 g body weight) were obtained by mechanical treatment (crushing, sedimentation, etc).In the fibrogenicity test'3 the samples of fibrous dusts were given intratracheally during mild ether anaesthesia. For comparison highly fibrogenic standard quartz and low fibrogenic mineral dust of magnesite of mine origin were given. Particles of these two samples were of respirable size (diameter < 5pm).In our experiment wollastonite fibres were given and their effects were compared with the action of crocidolite, quartz, mineral mine dust, and controls.As the synthetic organic fibres were not respirable they were not expected to be fibrogenic.
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