Relationship between surface properties and cellular responses to crystalline silica: Studies with heat-treated cristobalite

A fibrogenic sample of cristobalite dust, CRIS (crystalline silica of miner al origin), was heated to 1300 degrees C (CRIS-1300) to relate induced phys icochemical modifications to cytotoxicity. Heating did not affect dust micr omorphology and crystallinity, except for limited sintering and decreased s urface area of CRIS-1300. Thermal treatments deeply affected surface proper ties. Electron paramagnetic resonance showed surface radicals progressively annealed by heating, mostly disappearing at greater than or equal to 800 d egrees C. Surface hydrophilicity or hydrophobicity, evaluated with water va por adsorption, still showed some hydrophilic patches in CRIS-800, but CRIS -1300 was fully hydrophobic. Heating modified the biological activity of cr istobalite. Cytotoxicity, tested on proliferating cells of the mouse monocy te macrophage cell line J774, showed that CRIS was cytotoxic and CRIS-800 w as still cytotoxic, but CRIS-1300 was substantially inert. Cytotoxicity of CRIS to the rat lung alveolar epithelial cell line, AE6, as measured by col ony forming efficiency, was greatly reduced for CRIS-800 and eliminated for CRIS-1300. The rate of lactate dehydrogenase release by rat alveolar macro phages was lowered for CRIS-800, and release was completely inactivated for CRIS-1300. The absence of surface radicals and the onset of hydrophobicity may both account for the loss of cytotoxicity upon heating. Differences ob served between CRIS-800 and CRIS-1300, both fully deprived of surface radic als, indicate that hydrophobicity is at least one of the surface properties determining the cytotoxic potential of a dust.