A method for operating the device of any of the claims 11-14, the method comprising: feeding ambient air to a gas sensor along a line path of a gas analysis device ; determining a first gas concentration in the ambient air with the gas sensor ; storing a first output signal of the gas sensor temporarily as an ambient air reference value; feeding calibrating gas to the gas sensor , the calibrating gas being fed to the gas sensor along the same common line path as the ambient air ; determining a second gas concentration in the calibrating gas with the gas sensor ; storing a second output signal of the gas sensor temporarily as a calibrating gas reference value; feeding exhaled air to the gas sensor along the same common line path; determining a third gas concentration in the exhaled air with the gas sensor ; storing a third output signal of the gas sensor temporarily as a measured value; determining a difference between the third gas concentration in the exhaled air and the first gas concentration in the ambient air using the ambient air reference value; feeding additional calibrating gas to the gas sensor along the same common line path; determining a fourth gas concentration in the additional calibrating gas; storing a fourth output signal of the gas sensor corresponding to the fourth gas concentration; comparing the fourth output signal to the calibrating gas reference value; storing a difference between the fourth output signal and the calibration gas reference value temporarily as a correction value; feeding additional exhaled air to the gas sensor along the same common line path; determining a fifth gas concentration in the additional exhaled air with the gas sensor ; storing a fifth output signal of the gas sensor temporarily as an additional measured value; and determining a second difference between the fifth gas concentration in the additional exhaled air and the first gas concentration in the ambient air using the ambient air reference value and the correction value. feeding ambient air to a gas sensor along a line path of a gas analysis device ; determining a first gas concentration in the ambient air with the gas sensor ; storing a first output signal of the gas sensor temporarily as an ambient air reference value; feeding calibrating gas to the gas sensor , the calibrating gas being fed to the gas sensor along the same common line path as the ambient air ; determining a second gas concentration in the calibrating gas with the gas sensor ; storing a second output signal of the gas sensor temporarily as a calibrating gas reference value; feeding exhaled air to the gas sensor along the same common line path; determining a third gas concentration in the exhaled air with the gas sensor ; storing a third output signal of the gas sensor temporarily as a measured value; determining a difference between the third gas concentration in the exhaled air and the first gas concentration in the ambient air using the ambient air reference value; feeding additional calibrating gas to the gas sensor along the same common line path; determining a fourth gas concentration in the additional calibrating gas; storing a fourth output signal of the gas sensor corresponding to the fourth gas concentration; comparing the fourth output signal to the calibrating gas reference value; storing a difference between the fourth output signal and the calibration gas reference value temporarily as a correction value; feeding additional exhaled air to the gas sensor along the same common line path; determining a fifth gas concentration in the additional exhaled air with the gas sensor ; storing a fifth output signal of the gas sensor temporarily as an additional measured value; and determining a second difference between the fifth gas concentration in the additional exhaled air and the first gas concentration in the ambient air using the ambient air reference value and the correction value. <EOS>