Effects of carbon dioxide on mental performance.

Although elevated levels of inspired CO2 have been recorded in work situations, little is known about effects of concentrations greater than 6% on mental performance. We have measured the effects of inhaling 0, 4.5, 5.5, 6.5, or 7.5% CO2 for 20 min on normal young adults unhabituated to CO2. Performance of reasoning tasks, such as AB logic problems, was significantly slowed at the higher levels of CO2, with a threshold at end-tidal PCO2 close to 51 Torr. Accuracy of reasoning and short-term memory were not significantly affected. More prolonged studies with inhalation of 6.5% CO2 showed that after the first 10 min ventilation tended to increase, end-tidal PCO2 to fall, and slowing of reasoning to recover, but substantial decrement in performance continued for 80 min, by which time performance virtually stabilized. On return to air breathing, preexposure performance was resumed within 10 min. Also caused by 6.5% CO2 was a significant rise in subjectively assessed irritability and discomfort, with no significant change in alertness or in either registration or recall of long-term memory.