Surface composition of respirable silica particles in a set of U.S. anthracite and bituminous coal mine dusts

Respirable particles of high-percentage silica content from anthracite and bituminous coal mine dust samples were analyzed for aluminosilicate clay surface coating, by measuring silicon and aluminum X-ray spectra using scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDS). Silicon and aluminum elemental ratios were determined with incident electron energies of 20 and 5 keV to reveal whether surface occlusion was present. Some 20 respirable-sized, non-agglomerated particles with silicon fraction of signal of 75% or more (for elements above sodium) were analyzed for each of 12 coal mine dust samples. Mine dust samples were from U.S. anthracite and bituminous coal mining regions involved in epidemiological studies of the U.S. National Study of Coal Workersʹ Pneumoconiosis. Some particles of high-percentage silica content exhibited a decrease in the ratio of silicon-to-aluminum K-alpha line intensities with decreasing electron beam accelerating voltage, consistent with aluminosilicate clay surface contamination or occlusion of a silica particle. Significantly lower frequencies of particles manifesting occluded behavior were found for anthracite dusts versus all bituminous dusts. It is suggested that such occlusion alters the biological availability of the surface of those particles. This may be a factor in the results of attempts to correlate disease prevalence with conventionally measured dust composition, as in the classical failure of coal workersʹ pneumoconiosis disease prevalence to correlate with silica exposure while being correlated with cumulative total respirable dust exposure and with coal rank.