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Thirty-two years after injection of thorium dioxide (Thorotrast) for diagnostic x-ray studies in a female patient deposits were found by light microscopy in the liver macrophages (Kupffer cells). They were shown by electron microscopy to be located inside secondary lysosomes, and by autoradiography and x-ray microanalysis they were identified as thorium.

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Scanning electron microscopy of lung tissue, ashed at low temperature, and obtained from an insulation worker who had died of pleural mesothelioma, showed the presence of numerous inorganic particles and fibres. A regional variation in fibre concentration in different tissue samples was found, and the size distribution of naked fibres and asbestos bodies was determined. By energy dispersive x-ray microanalysis the fibres were identified mainly as amphibole asbestos. This method also showed the presence of particles containing titanium and of fragments of diatom shells. Despite a mean concentration of 33 x 106 fibres per gram of dry tissue no significant lung fibrosis was found.The causal relation between asbestos exposure and pleural and peritoneal mesothelioma was established in 1960.1 Since then the increased incidence of mesothelioma in individuals exposed to asbestos has been widely documented.2 The relation to asbestos exposure may, however, be obscured by a long period between initial exposure and the appearance of the tumour. The dose required for the initiation of a mesothelioma is small, and may be acquired by non-occupational exposure to asbestos.3A relation between the lung asbestos burden and asbestos-related diseases has been suggested. [4][5][6] Identification and quantification of mineral particles in the lungs requires the use of microanalytical techniques.7 For in situ identification of inorganic substances in lung tissue-for instance, in the study of pneumoconiosis-scanning electron microscopy coupled with energy dispersive x-ray microanalysis has proved particularly useful.8 This instrumentation has also been used for quantitative analysis of mineral fibres in lung residues obtained by tissue degradation methods. Gylseth et a19 reported that low temperature plasma ashing is the most convenient and reliable technique for this purpose.In the present study we have used these methods