The modern cognitive-effects-of-CO2 literature begins with Satish 2012 at LBNL and the Allen 2016 COGfx study at Harvard. Both used environmental chambers with otherwise-identical conditions and varied only the CO2 level. Both found measurable, dose-dependent declines in cognitive performance across the 600-1,400 ppm range. This is the range that most homes, offices, schools, and bedrooms sit in for substantial parts of the day, which is why the finding matters at all.
Allen 2016 tested 24 office workers at 550, 945, and 1,400 ppm in random order across multiple days. The composite cognitive-function score dropped 15% at 945 ppm and 50% at 1,400 ppm relative to the 550 ppm baseline, across nine cognitive domains. The hardest-hit domains were strategic thinking, information usage, and crisis response: roughly, the executive-function side of cognition rather than reaction time or simple recall. Satish 2012 ran the same shape of experiment four years earlier with a different population and found the same dose-response pattern.
The school-classroom corollary is independent confirmation. Multiple studies (the Mendell classroom CO2 work is the canonical citation) document associations between classroom CO2 and standardized test scores, attendance, and teacher-rated attention. Classrooms routinely run 1,500-3,000 ppm by mid-afternoon under typical US ventilation practice. The dose-response in field studies is consistent with the chamber studies: it appears to be monotonic above ~700 ppm, with no clean threshold below which CO2 is "fine".
The mechanism is not settled, which is why this literature still has skeptics. The leading hypothesis is CO2-mediated cerebral blood flow changes plus mild systemic acidosis affecting neuronal function; this is consistent with the executive-function pattern (prefrontal cortex is metabolically demanding) and with the chamber-study findings, but the human mechanistic studies are still in flight. For practical purposes the policy conclusion is unchanged: keeping occupied rooms below ~1,000 ppm CO2 appears to preserve cognitive performance that is otherwise quietly degrading. See CO2 rebreathing detection for the diagnostic pattern, reducing CO2 for the playbook, and office IAQ and classrooms for the high-stakes applications.
