A CO2 monitoring system (100) provides oxygen (O2) (120, 230) to a subject's mask (140) with known characteristics. The CO2 monitoring system (100) receives, from the mask (140), gas samples (146) including exhaled CO2 (144) diluted by O2 (120, 230). The CO2 monitoring system (100) uses an adaptive noise canceller (166, 400) to cancel the diluting O2 (120, 230) by using the known characteristics of the O2 (120, 230) provided to the mask (140), the result of which process is restoration of the original concentration level of the CO2 (144) as exhaled by the subject into the mask (140).