Apparatus for determining concentration of a targeted gas in environmental air, the apparatus includes a non-dispersive infrared (NDIR) sensor, a pressure sensor coupled in fluid communication with an interior of the NDIR sensor; and a processor. The processor is configured to receive pressure data from the pressure sensor based on gas pressure within an interior of the NDIR sensor, receive a target-gas concentration signal from the NDIR sensor, and produce a pressure-compensated concentration signal based on the target-gas concentration signal, a predetermined reference pressure and the pressure data from the pressure sensor.