The respirable quartz component of dusts is being surface-analyzed for sub-micrometer thick alumino-silicate occlusion or coating. This respirable component is being surface-analyzed for dusts with some observed anomaly in disease risk, that is, for silica and silicate dusts whose activity for fibrotic lung disease is not fully predicted by the respirable quartz component. This is to determine whether biological availability of the surfaces of silica particles is a parameter in defining the risk of fibrosis from silica dust exposures. Scanning electron microscopy (SEM)-energy dispersive X-ray analysis is modified by collecting Si and AI X-ray spectra from a particle at high and at low electron beam energies, e.g., 20 keV and 5 keV. Both an analytical mathematical model, using a Beer's law expression for electron beam and X-ray attenuation in the particle, and a more general model using Monte Carlo numerical solution techniques, provide a basis for comparison of the two spectra. This is to infer whether the particle aluminum content is structured as an alumino-silicate coating on a silica core particle or whether the aluminum is distributed throughout the particle. Fields of particles from a sample are subjected to automated analysis at each of the two electron beam voltages, and the ratio of measured Si and AI X-ray line intensities determined for each particle to infer its surface structure. Respirable dust samples have been analyzed from a clay mine and preparation facility, from low temperature ashed samples of MSHA-sampled respirable coal mine dusts, and from a group of metal mines and pottery works in China. MSHA samples were from mines of differing coal rank from US regions exhibiting the anomalous "coal rank effect" in coal workers pneumoconiosis prevalence. The Chinese dust samples were from metal mines and pottery works in China for which a recent epidemiological study found differences in silicosis risk when normalized for total dust exposure and to calculated respirable silica exposure. Fractions of respirable silica particles evidenced surface occlusion by aluminosilicate submicrometer coatings. These were observed by SEM not to be particle aggregates. The fraction of particles with such occlusion generally decreased with coal mine dusts of increasing coal rank, and generally increased for the Chinese pottery dusts versus metal mine dusts. Results suggest that a decreased fraction of alumino-silicate surface-occluded silica particles is associated with increased fibrotic lung disease risk. The surface analysis method employed did not detect trace contamination of particle surfaces such as can be seen by Auger spectroscopy or X-ray photo-electron spectroscopy. The occlusion observed was on the order of 0.01 to 0.1 micrometer thickness. This may represent surface coatings which would persist for extended times for respired particles in vivo, suppressing or significantly delaying biological contact with the underlying quartz surface and the consequent onset of fibrogenic processes. -
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