The action of different forms of pure silica on the lungs of rats.

Pure silica (SiO, ) exists at room temperature in the four different modifications: quartz, tridymite, cristobalite, and fused silica. The first three are crystalline, but fused silica is a non-crystalline solid, a glass. Tridymite, cristobalite, and fused silica are metastable, but at room temperature they do not invert within measurable time to the only thermodynamically stable modification, quartz. It can be regarded as proven that quartz can produce tissue changes in the lungs of animals similar to those found in human silicosis. It was also thought up to quite recently that silica was unique in producing such changes. Experiments with rand 1quartz (King, Rogers, Gilchrist, and Nagelschmidt, 1945) had shown that there was no difference in the action of either of the optically active forms from the effect of the racemic type. It was thought desirable next to test the effect of the four different modifications of silica under comparable conditions. The only previous information in this field came from Gardner (1938) who had injected rabbits by the ear vein with 1 g. lots and had found progressive nodular fibrosis of the liver with quartz and fused silica, but a more severe diffuse fibrosis with cristobalite and a still more rapid reaction with tridymite which resulted in the death of all his animals within three months or less. Gardner stated that his samples were analysed and were all below 3 micron diameter, but he did not give any results of analyses or counts. We decided to make comparable sized fractions of the four silica modifications and to use them for intratracheal injection into rats in order to study the effect of the crystal structure on the type and amount of tissue reaction.