This standard describes the procedure and equipment for the determination of the respirable fraction of crystalline free silica at the workplaces. Samples are collected on a PVC filter using a respirable sampler and analysed by infrared spectrophotometry. The field of application of the analysis ranges from 10 micrograms to 500 micrograms of crystalline free silica in the collected sample. Then the procedure allows to determine concentrations of crystalline free silica (respirable fraction) from 0,02 mg/m3 to 1,0 mg/m3 for an air sample volume of 500 l. The procedure of measurement described can be used to carry out measurements to determine the occupational exposure concentration, measurements for comparison with limit values (occupational) to evaluate the exposition via inhalation and periodic measurements. It may also be used to carry out screening measurements of time weighted average concentration (see UNE-EN 481, 4.2) and screening measurements of variation of concentration in time and/or space (see UNE-EN 481, 4.3).