Before the industrial revolution the CO2 levels outside were around 250ppm or at least less than 300.

Current levels outside as measured by my device just now are about 450ppm.

Here is a nice historical chart: https://www.climate.gov/sites/default/files/CO2_emissions_vs...

My measurement is also about 450ppm and it's fine for me. How do you discovered lower level is better for you?

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.