In terms of cognitive performance: Harvard research links adequate ventilation to a 26% improvement in cognitive function, with carbon dioxide concentration being the key factor. In enclosed spaces with insufficient ventilation, carbon dioxide builds up steadily while the area is occupied. Levels of around 1,000 ppm — typically reached within 30 to 45 minutes in a standard single-person enclosed space — lead to measurable declines in decision-making quality, reaction speed and sustained attention.