Carbon Dioxide

Outdoor carbon dioxide sits at roughly 420 ppm and rises slowly year over year, which sets the floor for any indoor reading. In a normally occupied bedroom without combustion sources, most of the rise above that floor comes from the people in the room, and outdoor air is what removes it. A closed bedroom with two people will typically climb through the night and level off wherever the room's leakage and the occupants' output balance.

That relationship is what makes the measurement useful. At the concentrations found in homes this is not primarily a toxicity question. It is the cheapest available indicator that a room is not getting enough outdoor air. Elevated occupant-generated carbon dioxide often indicates that other occupant-generated pollutants and moisture are also being removed slowly, since ventilation is what clears all of them — though each has its own sources, sinks and deposition behavior, so their concentrations do not track one another exactly.

The evidence on direct cognitive effects at moderate indoor concentrations is genuinely mixed. Some controlled studies have found decision-making decrements in the low thousands of ppm; others have failed to replicate them, and separating the effect of carbon dioxide itself from everything else that accumulates in underventilated air is methodologically hard. Treating the number as a ventilation signal is well supported. Treating it as a measured dose of a cognitive toxin is not.