A gas mask is provided for normal-pressure and overpressure operation with at least one breathing port, an expiration valve with a valve spring, an adjusting element affecting the prestress of the valve spring and a pushing element having a first switching position and a second switching position. The apparatus asynchronously samples an analog read signal generating from the optical storage system and subtracts an estimated DC offset from the asynchronous sample values to generate a sequence of asynchronous DC-removed sample values. A cooling air is diverted from the compressor and passed through the bearing cooling passages for cooling the bearing, and then the cooling air is directed into the combustor. In several embodiments, the fastener and corresponding mating fastener are two mating zipper portions of standard length that can be combined to form a zipper irrespective of the garment size. Further disclosed are certain novel fluorescent whitening agents suitable for preparation of the whitening pigments and a process for their preparation.