We placed an Envora One in the back corner of a Year 6 classroom for thirty days during the spring revision window. Class size 28. Two openable windows. A single trickle vent.
The data tells a story.
Mornings start at 600 ppm. The overnight baseline. Within fifteen minutes of registration, the room is at 900. By the end of the first 45-minute lesson, 1,300. The breaktime window-opening drops it to 700, briefly, then the climb resumes.
Revision sessions held the line at 1,800. During pre-SATs revision in the final two weeks, with windows kept closed to reduce noise from the playground, the steady-state climbed to 1,800 ppm and held there for the full session. On three separate afternoons, the peak crossed 2,200.
The performance literature is consistent. The COGfx data and a meta-review by Wargocki (2017) point to roughly 5% performance drops per 1,000 ppm above background, on tasks that mirror exam content — reading comprehension, mental arithmetic, working memory recall.
A class doing exam-prep revision in 1,800 ppm air is doing it with a measurable cognitive handicap.
What it costs to fix, in this room.
- A second trickle vent on the opposite wall, retrofit cost ~£180. Steady-state CO₂ during revision dropped to 1,200 ppm — still high, but in COGfx terms a 35% performance recovery.
- Pre-venting the room before the lesson. Open both windows for ten minutes before children enter; close them as the bell rings. The starting CO₂ is 500 instead of 900, and the room never reaches 1,800.
- A clip-on CO₂ display visible to the teacher. Knowing the number changes the behaviour of the adult in the room far faster than any policy.
The thing about classroom air is that nobody owns it. The teacher owns the lesson plan. The estate manager owns the building. The headteacher owns the policy. The air owns nobody — and the children own the consequences.