Abstract: A method for determining a concentration of a target gas in blood of a human or animal animate being. The method includes the steps of: generating electromagnetic excitation radiation having a carrier frequency, wherein the carrier frequency is selected such that the target gas and an absorbing gas absorb the excitation radiation, modulating an amplitude or the carrier frequency of the excitation radiation with a modulation frequency, illuminating an absorption path superficial to a skin of the animate being with the excitation radiation, generating a sound wave by an absorption of the excitation radiation in the absorption gas, and detecting at least one of an amplitude and a phase of the sound wave as a measure of a concentration of the target gas on the absorption path. Also provided is a photoacoustic sensor for determining a concentration of a target gas in blood of a human or animal animate being.