FACTORS AFFECTING THE RESPIRATORY TOXICITY OF DIATOMACEOUS EARTH: AN IN VITRO TOXICOLOGY AND PHYSICOCHEMICAL STUDY

Diatomaceous earth (DE), a deposit of silica-rich diatom skeletons, is mined globally due to its unique physicochemical properties, which make it ideal for use in the filler and filter industries. Epidemiology and toxicological studies have shown the toxic potential of DE, and that occupational exposure to DE can lead to diseases such as fibrosis and cancer. The high crystalline silica content in processed DE is thought to be the cause of the observed diseases. Crystalline silica has been shown to have variable toxicity, attributed to surface properties of the particles, which could be affected in DE. Here, we investigate whether impurities in DE samples can affect their potential pathogenicity.

Physicochemical characterisation and in vitro toxicology assays were performed on DE powders sourced globally, including processed (calcined; heated, and flux-calcined; heated with sodium carbonate) and unprocessed (raw) samples. The samples contained up to 71% <10 µm material, making some of them highly respirable in the workplace. Cytotoxicity and haemolytic potential did not correlate with crystalline silica content. Flux-calcined samples (bulk and respirable fractions) were not haemolytic or cytotoxic at concentrations tested, despite these samples having the highest crystalline silica content. In contrast, raw and calcined DE elicited some haemolytic response, were cytotoxic toward mouse macrophages, and caused a pro-inflammatory response measured by the release of TNF from mouse macrophages. Variations in surface area could not fully explain the observed differences between particle toxicity (flux-calcined material has a low surface area due to agglomeration of particles during processing). We hypothesise that the presence of CaO in raw DE contributed to increased haemolysis, but has no influence on the toxicology of flux-calcined samples. The addition of a fluxing agent could aid the uptake of impurities into the crystalline silica lattice, affecting the surface properties and potentially reducing its toxicity. This will be further investigated by high-resolution compositional analyses by electron microprobe. Early results suggest that both site-specific properties (level of impurities and particle morphology), and processing play a key role in determining the potential pathogenicity of DE.