You get used to it after a while. I've been living in an area with extreme levels of outside air pollution (think Mumbai or Beijing, but much worse) for many years, so my windows are almost always closed shut. I have a CO2 meter and it reports an average of about 2000 ppm. I feel fine, and don't feel any difference to the normal level of 440 ppm. I'm pretty certain it would show on objective cognitive tests, but personally I really don't feel any difference.