The fate of siliceous dusts in the body; a comparison of the in vivo solubilities of cement, carborundum, quartz, and moulding sand.

The pulmonary lesions found in silicosis were formerly thought to be due to the abrasive nature or spiked shape of the inhaled dust particles, but this theory was questioned by Gardner (1923), who demonstrated that carborundum dust is harmless. The later work of Kettle (1932) and of Denny and others (1937) was interpreted as showing that the tissue damage is due to silicic acid formed by the solvent action of the plasma on dust. Solubility determinations by many workers, including Briscoe and others (1937), Emmons and Wilcox (1937), King and McGeorge (1938) and Whitehouse (1938) appeared to show that, in general, the dusts from which more silica dissolves are those most likely to produce fibrosis; but a detailed study of the solubility in vitro of cement and flint dusts (Holt and Briscoe, 1949) emphasized the difficulty of deducing the fate of mineral particles which enter the body from solubility data obtained under fixed and arbitrary conditions. Cement dust is of particular interest in that, although the atmospheric dust concentration is usually far higher in works *where it is manufactured or handled than in places where materials give rise to dusts producing silicosis, if cement produces pulmonary lesions they are comparatively benign. Experiments previously described (Holt and Briscoe, 1949) have shown. that cement has a very low silica solubility when extracted with water but that the solubility is greatly increased by traces of carbon dioxide. In the present work the solubilities of dusts in vivo have been studied by determining the silica excreted in the urine after the administration of dust samples. This assumption that dissolved silica is completely excreted is substantially but perhaps not entirely correct (King and others, 1933). The administration of dusts by inhalation is impracticable for the present purpose both because it is impossible to transfer a dust sample quantitatively to the lung and also because inhaled dust is partly removed by the cilia and eventually reaches the alimentary tract. Whitehouse (1937) in a study of the effect on silica excretion of the oral administration of dusts in man found that, although finely powdered quartz and flint gave no significant increase in silica excretion, Scotch whinstone increased the output by about 30 mg. and flint of