The Action of Different Forms of Pure Silica on the Lungs of Rats

Abstract

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 r-and 1-quartz (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. Cristobalite.* -Cristobalite was prepared by heating Loch Aline sand for one hour at 16200 C. without using any flux.Tridymite.*-Tridymite was prepared by dissolving the glassy impurities from a silica cement, which had had long service at approximately 13800 C. in a gas retort house. Fused Silica.-Fused silica was obtained from the Thermal Syndicate. It had been made from pure quartz crystals from Madagascar.The Belgian glass sand contained 99-2% silica and the other materials 99-5% or more by chemical analysis.Fractions below 2 microns were prepared from the four samples by repeated sedimentation in water. Before this was done the cristobalite, tridymite, and the fused silica were ground in a mechanically driven agate mortar.Chemical analyses, solubilities, and size distributions of the samples are given in Table 1. Fig. 1A shows x-ray diffraction diagrams and B, C, D, and E show electron micrographs of the samples. The diffraction diagrams agree with those given in the literature, and there is no indication of contamination by other silica modifications, except for the tridymite, which shows the strongest quartz line at 3.340 A as a very faint line, indicating 0-5 to 3% of quartz. Preparation of Dust SuspensionsAmounts of 2 g. of the individual dusts (fused silica, quartz, cristobalite, and tridymite) were weighed into 10...

Silica exposure and silicosis among ontario hardrock miners: III. Analysis and risk estimates

“…First, it seems likely that some forms of crystalline silica such as cristobalite and tridymite have a greater fi brogenic potential than silica itself. This was demonstrated in animal experiments by Gardner [I938] and King et al [1953a]. For this reason, the epidemiological surveys of Fulton et al [I939] in the silica brick industry and by Cooper and Cralley [1958] in workers exposed to calcined diatomite cannot be compared to the present data since the airborne dust in their studies contained very high proportions of both cristobalite and tridymite.…”

Surface reactivity in the pathogenic response to particulates.

“…In some pioneering works the biological effects of different forms of the same mineral (mostly silica) or of chemically modified samples were compared and related to the physicochemical properties (1)(2)(3)(4). Hemolysis of red blood cells was one of the first effects caused by mineral dusts to be extensively investigated over a large variety of specimens (5,6).…”

Minerals, Fibrosis, and the Lung

“…The most striking determinant of silicotic fibrogenesis to be identified experimentally was the orientation ofthe Si-O tetrahedra on which physical form depends. With intratracheal dose and particle size standardized as accurately as possible, the severity ofthe fibrotic reaction was least for amorphous silica, but much greater for the crystalline forms, increasing through quartz, then cristobalite to a maximum for tridymite, yet all varieties again had similiar solubilities (9). Still greater divergencies were revealed by the high-pressure, high-temperature forms of silica administered by injection.…”

“…Of the earlier hypotheses, silica solubility achieved prominence, originating with the observations ofGye and Purdy (12) on the toxic properties of colloidal silica. However, this concept fell into disrepute when disparities emerged between solubility and fibrogenicity, in respect ofamorphous and different crystalline modifications ofsilica and ofdifferently sized particles (5,9), as well as when silica particles of various forms were etched with hydrofluoric acid or sodium hydroxide (13). An extended solubility theory envisaged that precursors of collagen adsorbed silicic acid, which induced them to polymerize into mature forms (14), but this idea too did not overcome the objections.…”