The crystalline silica conundrum: the effect of impurities on
the respiratory toxicity of diatomaceous earth and synthetic
cristobalite

Abstract

Crystalline silica, in the form of quartz and cristobalite, can cause respiratory diseases, such as silicosis. However, the observed toxicity and pathogenicity of crystalline silica is highly variable. This has been attributed to a number of inherent and external factors, including the presence of impurities that can react with the silica surface. Substitutions of aluminium (Al) for silicon in volcanic cristobalite, and phases occluding its surface, have been hypothesised to decrease its reactivity.

Diatomaceous earth (DE), an industrial source of cristobalite exposure, is studied to determine if structural substitutions in cristobalite are universal and whether they influence toxicity. Crystalline silica is also synthesised with incremental amounts of Al and sodium (Na) dopants. The effect of impurities on cristobalite toxicity is investigated through the physicochemical characterisation of DE and synthetic crystalline silica, and toxicological studies assessing the biological reactivity of these particles.

The physicochemical properties of DE varied by deposit source and processing technique. In processed DE, substantial quantities of cristobalite were observed. However, crystalline silica content could not be correlated with the potential toxicity of DE and this was attributed, in part, to impurities in the cristobalite structure and low cristobalite abundance at the particle surface.

Al-only doped or co-doped (Al+Na) synthetic cristobalite contained structural substitutions (1-4 oxide wt.% Al+Na). Co-doped samples also contained Al/Na-rich phases, such as albite. Doping reduced toxicity compared to non-doped cristobalite. Al-only doping was more effective at decreasing cristobalite reactivity than Al+Na doping. The reduction in cristobalite toxicity was attributed to structural impurities.

This thesis concludes that impurities can reduce the toxic potency of cristobalite and may explain the low reactivity of cristobalite-rich volcanic ash and DE. Whilst further work is required, to determine if these effects are sustained long-term in the lung, the research has substantial implications for the regulation of crystalline silica exposures.