The present invention relates to a device for indicating toxic substances in air that is supplied to accommodation used for personnel, preferably the cab of a motor vehicle, this having error compensation for humidity and temperature in one or a plurality of semiconductor gas sensors that are used, in which connection the sensor heating voltage is controlled to provide constant heating output and the sensor temperature pick up as well as the necessary amplifier (control stage) used to generate the temperature-varied heating voltage are arranged in a common housing, and that in the area of the sensors this housing is perforated. In order to permit compensation of the toxic substance, temperature, and humidity correlated output signals that are picked up, the present invention proposes that the characteristic curves of the sensor are reproduced in a voltage divider which, outside the sensor and jointly with the sensor forms a voltage divider for a common supply voltage, such that the influence factors compensate each other in equal proportional changes in value of both the sensor and of the outside wiring.