A powered respirator comprises a motor-driven fan unit (11) drawing air through an upstream filter (16) for delivery to a facepiece (1). Operation of the fan motor (13) is controlled in response to a differential pressure sensor (17) tapped in across the fan unit (11) so as to reduce the fan throughput in response to an increase in the pressure differential and to increase the fan throughput in response to a decrease in the pressure differential, and thereby match the delivery of air to the breathing demand of the wearer. The diaphragm (20) or other pressure-responsive member of the sensor (17) is isolated from unfiltered air and can therefore provide no leakage path for contaminants to the facepiece (1).; In an alternative embodiment (Fig. 4, not shown) the positions of the fan (12) and filter (16) are reversed so that the fan (12) is upstream of the filter (16), and similarly controlled in response to the differential pressure across the fan (12). In the latter arrangement any contaminant which could pass through the pressure sensor is removed by the downstream filter (16) before delivery to the facepiece (1).