It might just be slow enough that we don't notice. We know that increased CO₂ indoors leads to poorer performance from studies. A widely discussed concern is that outdoor CO₂ is increasing and that this could raise indoor baseline levels, meaning buildings might reach cognitively relevant concentrations more easily unless ventilation improves. The issue is not that today’s outdoor CO₂ levels are directly toxic; rather, higher outdoor concentrations reduce the “headroom” between outdoor air and indoor levels generated by occupants.