You need to be a bit careful about drawing too many conclusions from the results of optical fga’s.
To work they first need the CO to be set at “zero” in fresh air. Thereafter in use they will measure oxygen and relative CO compared to the zero setting. CO2 is not measured, it is calculated using an algorithm against the combustion gas that the analyser has been set to measure.
In essence, breathing on the exposed probe will not accurately measure the CO2 from your lungs. Any readings it gives will be calculated from measured CO levels and the combustion gas(s) the analyser has been configured and set for.
They are a simple cost effective tool for measuring products of combustion in a specific situation. They are not a true gas chromatograph, which identifies and measures the presence of all gases from their reflected wavelength.