Experimental Evaluation of the Threshold Limit? of Cristobalite—Calcined Diatomaceous Earth

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Abstract
Dogs, guinea pigs, and rats were exposed daily, 6 hours per day, 5 days per week for periods up to 2.5 years to flux-calcined diatomaceous earth of 61% cristobalite content and 0.7 μ mass medium particle size. Dogs were given an additional 10-month period of observation to study possible progression of tissue reaction. Dust exposures were at levels of 2 and 5 mppcf of air as well as high-level, intermittent exposures at 50 mppcf both alone and superimposed on the 5 mppcf level. Detailed histologic evaluation of tissues of serially sacrificed animals showed that no fibrosis of the pulmonary parenchyma resulted in any of the species under any condition of exposure. However, there was no dust level that did not exhibit a histocytic and giant cell infiltration in the pulmonary parenchyma, and moderate numbers of hyalinized fibrotic nodules developed in hilar lymph nodes in dogs at the 5 mppcf level, and to a lesser degree at the 2 mppcf level. Data indicates little if any safety factor in the present limit particularly against diatomite exposures of higher cristobalite content.

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