The US Navy's submarines are run with CO2 levels varying from 300-11,300ppm.[1] The military did plenty of studies in the 60s and 70s and failed to find significant cognitive effects in environments as high as 4% CO2.[2]
> Thus, CO2 at 40,000 ppm for 2 weeks did not affect performance on multiple tests of cognitive function in physically fit young airmen, a population probably not unlike submariners.
> A number of studies suggest that CO2 exposures in the range of 15,000-40,000 ppm do not impair neurobehavioral performance. Schaefer (1961) reported that 23 crewmen exposed to CO2 at 15,000 ppm for 42 days in a submarine showed no psychomotor testing effects but showed moderate increases in anxiety, apathy, uncooperativeness, desire to leave, and sexual desire.
> In a 5-day exposure of seven subjects at a CO2 concentration of 30,000 ppm, Glatte et al. (1967) reported no effects on hand steadiness, vigilance, auditory monitoring, memory, or arithmetic and problem solving performance.
> CO2 exposure did not affect performance on the tracking task or any of the six RPM subtests (Storm and Giannetta 1974).
There's also an argument from biology. When sitting around, people exhale 4-5% CO2. That's 40,000-50,000ppm. An extra 2,000ppm is far less than the variance across a typical breath, and it won't increase ppCO2 in the blood stream nearly as much as standing up and walking around. If CO2 hurt cognition as badly as these studies claim, then even minor physical exertion should turn people into drooling idiots.
It's for these reasons that I am extremely skeptical of the recent claims about CO2 impairing cognition.