Tukuk-OSOutdoor air in 2026 carries roughly 420–430 ppm CO₂. A room fills with the same number plus exhaled breath: a person breathes out air at about 40,000 ppm, and every occupant adds about half a litre of it per breath. Close enough doors in a populated room and levels climb through 1,000 — the conventional “adequate ventilation” mark — to 2,000 and beyond by late morning in schoolrooms worldwide; measured classroom peaks exceed 3,000 ppm routinely when windows stay shut. The occupants notice nothing at 800 and feel the room at 2,000.
Study after study of offices and schools finds measurable costs in that stale band: slower response times, reduced concentrated arithmetic, morning drowsiness attributed to the lesson rather than the air. A few parts per million of a gas aren’t toxic at these levels — the effect is subtle physiology we do not fully understand, which is exactly why cheap prevention makes sense. The same sensors that track outdoor AQI track this for under the price of a meal.
The fixes are old and new: open a cross-breeze (air exchange is the whole game), exhaust at the source (kitchen and bathroom fans actually used), and in polluted-climate buildings, filtered mechanical ventilation with heat recovery. Plants, contrary to every office poster, clean a room’s CO₂ negligibly — the photosynthetic sink is tiny compared with one sleeping human’s output; ventilation beats chlorophyll for this specific job every time.