Performance of powered air purifying respirators was evaluated in field and laboratory tests. In laboratory tests, filter leakage was evaluated using dioctyl-phthalate aerosol at a flow rate of 42.5 liters per minute for approximately 10 seconds. Airflow was measured before and after exposure to silica dust, concentration 50 to 60 milligrams per cubic meter (mg/m3). In field tests, the respirators were worn by workers while bagging silica. Personal samples were taken simultaneously inside and outside the facepiece for sampling times of 84 to 320 minutes. Workplace protection factors, defined as the ratio of dust concentrations outside to those inside the facepiece, were calculated. In the laboratory test, penetration of dioctyl-phthalate on new and used filters ranged from 0.01 to 0.001 percent. The United States Bureau of Mines penetration standard is 0.03 percent. Minimum air output of the respirators after 4 hours of silica dust exposure was above 4 cubic feet per minute. The amount of silica dust collected inside the facepiece was less than the limit of detection, 0.3 milligrams. In the field tests, dust concentrations inside and outside the facepiece were 0.03 to 0.56mg/m3 and 1.98 to 36.76mg/m3, respectively. Workplace protection factors ranged from 16 to 215. The authors note that the protection factors are significantly less than the assigned factor value of 1000. The respirators under field use do not provide the degree of protection expected for this kind of equipment. The poor performance could be due to respirator leakage, user behavior, or inconsistencies in sampling and analysis.