I don't know for sure, but there are certain things I notice which I think may have been improved when I started keeping track of CO2 and keeping my windows open. (I don't spend most of my time outside and I certainly don't sleep outside; outside levels might be 450 but inside levels even with my mitigations are usually around 700)

Like so many things I think it is a small effect; it's not like I can go anywhere for a week to experience 300ppm air.

I haven't gone to the levels of designing experiments and data tracking on myself. The fact that I've lived with allergies and a broken nose most of my life probably hasn't helped my personal respiration characteristics.