You can get a decent CO2 meter from stores like IKEA. You’ll come to the conclusion that the cozy feeling you get sometimes is directly related to the ppm being well over 1000. It affects your thinking, but it creates “atmosphere”.
The first 440 ppm you’ll get for free. This used to be lower and has risen steeply in the last 100 years and continues to do so.
For homeowners, I highly recommend making sure you have some way to bring it fresh air on demand, and then automate it in conjunction with a CO2 monitor. I added a duct fan and ERV before my HVAC system's fresh air intake, and then tied the fan to an air quality monitor.
My personal experience is that when the CO2 levels hit over 1200 in a room, it feels more cozy like a bar or crowded place. With all the up and downsides. Would hate to live in a world where this fog is a constant, as it impedes thinking.
The human body can tolerate a short sauna session. It cannot tolerate living in a sauna 24/7.
That said, the authors note that many people regularly do indeed experience long periods of higher CO2 levels, and that this may in part explain the HCO3− blood level trend they're seeing and has to be taken into consideration.
They also note "more recent studies of humans and animals have shown harmful effects of CO2 exposure at [1000 to 3000 ppm CO2], such as changes in heart rate, kidney calcification, oxidative stress, neural damage and inflammation".
The first 440 ppm you’ll get for free. This used to be lower and has risen steeply in the last 100 years and continues to do so.
See the historical graph at https://www.co2levels.org/