There is described a mobile breathing-air compressor unit arranged for use in an explosive environment and including an air compression unit, an air inlet provided with a gas detector which is arranged to stop the air compression unit, means of keeping the operating temperature of the air compression unit below a limit value, a) by the air compression unit being provided with several compressors which are arranged to be subjected to a maximum load which lies considerably below the maximum capacity of the compressor, or b) by the air compression unit is provided with means arranged to carry an energy-carrying fluid across a heat -exchanging surface on each of the compressors, or c) by each of the compressors is provided with a temperature controller effecting operation at intervals of each of the compressors, or a combination thereof; and the breathing-air compressor unit being arranged in a transport container. There is also described a method of producing compressed breathing air by the use of a mobile breathing-air compressor unit.