I'm a bit confused by this, since indoor CO2's higher (double?) so wouldn't we already be experiencing it?
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/
Hypercapnia
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.
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".