The invention concerns a method for detection of the concentration of at least one gaseous target substance (10) in a gas or a gas mixture (20), especially air, wherein: a) a first measured value (11) of the concentration of the at least one gaseous target substance (10) is saved as the first measured value (11) in a predetermined way and then b) the at least one target substance (10) is kept away from the sensor (1) with a gas shielding device (2) for a predetermined period of time, so that the concentration of the at least one target substance (10) measured by the sensor changes, c) before the end of the shielding of the sensor (1), the magnitude of the measured concentration and/or the direction of the change in concentration of the gaseous target substance (10) as detected by the sensor (1) is saved as the second measured value (12), d) the sensor (1) is then exposed again to the gas (20) with the at least one gaseous target substance (10), while e) on the basis of the difference between the first measured value (11) and the second measured value (12), an actual concentration of the at least one gaseous target substance (10) in the gas (20) is determined by means of a calculator (30). The invention also concerns a gas monitoring device and a use of such a device.