A disposable respirator, the 3M 9910, was evaluated for its ability to reduce dust exposure, and a practical method for determining the effectiveness of a respirator in a workplace was described. A measure of the effectiveness of a properly used and functioning respirator was called the workplace protection factor (WPF) and was defined as the particle concentration outside the mask divided by the concentration inside the mask. Over a 3 day testing period, 22 pairs of samples were collected from seven workers at a workshop which mixed and packaged a concrete patching compound. The total particulate concentrations inside and outside the respirator were simultaneously measured while the volunteer was engaged in normal work activities. To determine particle size distribution for three locations of the work area, Marple cascade impactors operating at a flow rate of 2 liters per minute were used to collect eight area samples. These three locations were: at a distant corner of the work area (location 1); beside the small mechanical mixer (location 2); and on the packaging table (location 3). Locations 1, 2, and 3 were found to have mean concentrations of 3.7, 3.7, and 16 milligrams per cubic meter, respectively, and mean aerodynamic mass median diameters 8.0, 8.7, and 20 microns, respectively. The 3M 9910 respirators reduced the dust exposure of workers to levels below the American Conference of Governmental Industrial Hygienists threshold limit value of 10 milligrams per milliliter. The WPFs were adequately described by a log normal probability distribution. The authors recommend that simultaneous inside mask and outside mask dust concentrations be routinely measured to ensure the effectiveness of respirators and their use in the workplace.