Three portable direct-reading dust monitors were tested in a recirculating dust tunnel and a calm air dust chamber against a range of industrial dusts with different size distributions to investigate sources of variation in their responses. Responses were found to be linear compared to reference gravimetric respirable samplers over a range of concentrations for a particular particle size distribution. Their calibration factors were dependant on particle size, particle composition and air velocity. If particle size and air velocity do not change significantly then the calibration factor can be applied to the monitor readings to give an accurate measure of dust concentration. The DataRam and HAM, factory calibrated against respirable dust concentration, were found to agree closely, whereas the Microdust gave higher readings, having been factory calibrated against total suspended particulate concentration. The calibration of the DataRam was significantly altered by either contamination of the optics with dust or by cleaning the optics. This was not observed with either the Microdust or HAM, since both monitors include a reference calibration element.