The smell of the classroom

Carbon dioxide is the gas we exhale, and in a closed room full of people it accumulates. Outdoor air contains roughly four hundred parts per million. A classroom of thirty students with the door and windows shut can exceed two thousand within an hour.

At those concentrations the gas is not dangerous in any conventional sense. What several studies find is a measurable effect on decision-making and concentration, with performance on tasks requiring sustained attention falling as the level rises. The effect is not subtle at the top of the range and is noticeable well before anyone feels unwell.

What makes this difficult is that nobody can detect carbon dioxide directly. It has no smell. The stuffiness people describe is other compounds accumulating alongside it, which correlate with the gas without being it, and the human nose adapts within minutes so that a room entered later smells worse to a visitor than it does to the people who have been sitting in it for an hour.

This is a case where a cheap instrument beats judgement entirely. A monitor costing less than a textbook shows the number, and the number tells somebody to open a window several minutes before anybody would have thought of it.

The pattern is worth noticing in general. Where the human sense adapts and the harm accumulates, a number on a wall will outperform a roomful of attentive people indefinitely.