ISO 16258-1:2015 specifies the analysis of respirable crystalline silica (RCS) in samples of air collected on 25 mm-filters by X-ray diffraction, when using an analytical approach where the dust on the air sample filter is directly analysed by the instrument. This part of ISO 16258 includes information on the instrumental parameters, sensitivity of different sampling apparatus, uses of different filters and correction for absorption effects. In this part of ISO 16258, the expression RCS includes the most common polymorphs quartz and cristobalite. The less common polymorphs of crystalline silica, such as tridymite, are not included within the scope of this part of ISO 16258 because a standard reference material is not available. Under certain circumstances (i.e. low filter dust loads, low silica content), the analytical approach described in this method may not fulfil the expanded uncertainty requirements of EN 482. Guidance for calculation of uncertainty for measurements of RCS is given in ISO 24095.