A CO2 monitoring system provides oxygen (O2) to a subject's mask with known characteristics. The CO2 monitoring system receives, from the mask, gas samples including exhaled CO2 diluted by O2. The CO2 monitoring system uses an adaptive noise canceller to cancel the diluting O2 by using the known characteristics of the O2 provided to the mask, the result of which process is restoration of the original concentration level of the CO2 as exhaled by the subject into the mask.
