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An in vivo intratracheal study in rats demonstrated a clear difference between reference quartz and quartz with occluded surfaces extarcted from bentonite. The clear difference in toxicological proerties  is further supported by two in vitro studies investigating the cytotoxic and genotoxic potential of reference quartz and various bentonite samples with different chemical modifications.  

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Creutzenberg (2008)

A 90 day toxicity study in male rats was performed with bentonite (60% quartz) and a reference quartz sample (87% quartz). The animals were dosed with an equivalent dose of quartz: a total of 15.2 mg/kg bentonite or 10.5 and 4.7 mg/kg bodyweight of reference quartz and titanium dioxide, respectively were administered by intratracheal instillation over 4 consecutive days. At the end of the study period, rats were subject to bronchoalveolar lavage (BAL) and histopathology. The morphology of test material in lungs was also assessed. There were no deaths, changes in bodyweight or other treatment-related effects observed. An inflammatory response as assessed by BAL total and differential cell counts was observed for both bentonite and the quartz reference after 3 days. There was no progression of these effects at 28 and 90 days after administration of bentonite. The reaction was strongest for the reference quartz and after 28 and 90 days, only the quartz reference was significantly different from control. This finding was confirmed histopathologically. Additional histopathological observations included very slight fibrosis of the lung after 28 and 90 day in most rats exposed to bentonite. Again, the reaction was significantly stronger with the reference quartz.

Characterization of particles revealed differences in both chemical composition and crystal organization and size.Pre-instillation particles: chemical composition was different primarily due to significant levels of montmorillonite in the bentonite sample. The crystal structure was also different: reference quartz consisted of highly angular fragments of quartz crystals while the bentonite consisted of more round agglomerates of quartz with a monmorillonite coating.

Recovered particles:the surface morphology or chemistry of either test substance was not changed following recovery from lung after 90 days, however there was an increase in the number of fine particles for the reference quartz sample.

Geh 2006a

The cellular uptake and cytoxic-, apoptotic- and haemolytic- potential of bentonite with different quartz concentrations (0.5-6%) were determined. The effect of activation (acidic, alkaline, and organic) was also assessed. Cellular uptake was determined by transmission electron microscopy of monolayer cultures of the human lung fibroblast cell line IMR90 incubated in the presence of bentonite particles. Cytoxicity was assessed by determining viability of the same cell line. Hemolytic activity was determined by mixing bentonite with red blood cells and assessing the release of haemoglobin spectroscopically. DNA fragmentation (electrophoretic separation) and propidium iodide and immunologic Annexin V staining followed by fluorescence activated cell sorting was used to determine apoptotic activity. Cellular uptake (1.3 x10-3to 4.6x 10-3particles per um2when exposed to 10 ug/cm2for 24 hours) was higher for activated samples; however uptake was primarily limited to the monmorrillonite crystals and they were only found in the cytoplasm. Higher quartz content and activation was associated with higher toxicity. At low concentrations, cell proliferation was enhanced. Hemolytic activity was dose-dependent and significant (1.25-5 mg/ml, higher than for pure quartz) for all types of bentonite. Finally, bentonite induced moderate apoptosis and necrosis. Quartz content and activation did not influence this endpoint.

Geh 2006b

The genotoxic potential of bentonite with different quartz concentrations and activations was assessed by exposing human lung fibroblasts (IMR90) cells to 1-15µg/cm2of bentonite for 36, 48, or 72 hours. Genotoxicity was assessed by micronucleus- and kinetochore assays, and formation of hydroxyl radicals. There were no effects of untreated bentonite in the micronucleus assay while only a very weak response was seen with treated (acid or alkaline) bentonite with high quartz content. There was an increase in formation of hydroxyl radicals with treated and untreated bentonite. The increase was generally dependent on the transition metals present in the sample. In conclusion native (untreated) and organic activated bentonite particles did not show genotoxic effects in most of the experiments. Also, bentonite particles with a quartz content <1% were negative in the micronucleus assay.