I mean, I can understand CO, maybe Radon, but CO2 really seems pointless and only driven by the modern green trends.
In fact there's no floor - even small increases in CO2 have small but measurable effects.
[1] https://www.hsph.harvard.edu/healthybuildings/2021/09/09/imp...
Unless you mean headache-inducing levels of CO2 which, per my actual sensor, is upwards of 3000 because at that level I still didn't notice anything. The air feels warmer, but that's attributable to me breathing in a small and enclosed space
Genuinely curious if there's something in our bodies that has been shown to serve as CO2 detector, or if you just mean "I can smell stale air and why don't you just do the same" (which I'm pretty certain relies on proxy factors like dust, mould smells like in a moist cellar, temperature, a combination, and/or perhaps other stuff that didn't come to mind)