A device is provided for increasing humidity for respirating patients, especially for a respiration humidifier or an anesthesia reflector. The device includes a gas channel for passing through breathing air, a control and/or regulating device, a temperature sensor and a dew sensor for measuring the humidity present in the breathing air by electromagnetic radiation with a generating device and with a detection device for electromagnetic radiation. The humidity of the breathing air is determined at a low cost and in a reliable manner and condensation is avoided in the respiration system without any additional effort for sterilization. Humidity in the breathing air is determined by the a dew sensor by the change in the reflection of the electromagnetic radiation as a function of dew formation at a condensation-fogged and condensation-free boundary surface between breathing air and a medium having an optical density higher than that of breathing air.