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alpha-Quartz dust of respirable size--on inhalation--deposits in the lung, where it may remain for long duration and be a risk factor in the development of silicosis and (with increasing weight of evidence) lung cancer in humans. The risk assessment for these end points is fundamentally based on those properties of alpha-quartz that allow it to persist over time in the lung. For individual samples of respirable alpha-quartz it is likely that the specific toxicity of the alpha-quartz in the sample is related to this residence time, which, in turn, will be a function of its crystallinity because of the latter's potential influence on alpha-quartz's cytotoxicity and solubility. Current X-ray powder diffraction technology allows for the routine assessment of diffraction line broadening by measuring the full-width-at-half-maximum (FWHM) of a diffraction line. The FWHM for any accessible alpha-quartz diffraction line in a sample can then be used to directly assess Murata and Norman's Crystallinity Index for the alpha-quartz. This method greatly enhances the practicability of determining alpha-quartz crystallinity on individual respirable dust microsamples collected by personal sampling. It is likely that the Crystallinity Index will prove to be a useful measure of the specific toxicity of alpha-quartz and, hence, find incorporation into the risk assessment for alpha-quartz exposure.