CO2 monitors in classrooms: reading them and acting on them
The Department for Education put CO2 monitors into state-funded classrooms so schools could see, in real time, when a room needs more fresh air. But a monitor only helps if staff know what the numbers mean and what to do when they climb. This guide turns the readings into a simple classroom routine — and explains where air filtration fits alongside them.
Why CO2 is used as a proxy
People exhale carbon dioxide constantly, so in an occupied room CO2 rises as the air gets "used up" and falls when fresh air comes in. It doesn't measure viruses or particles directly, but it's an excellent, cheap indicator of how well a room is ventilated — and poorly ventilated rooms are exactly the ones where airborne pathogens and fine particles accumulate.
What the readings mean
- Below ~800 ppm: good ventilation. Fresh air is keeping up with the room.
- 800–1,500 ppm: acceptable but watch it — increase ventilation if it's trending upward.
- Above ~1,500 ppm: ventilation is inadequate; take action — open windows and doors, reduce occupancy if practical, and use filtration to keep the air clean while you do.
Key point: a CO2 monitor tells you when a room is under-ventilated. It does not clean the air. Bringing readings down is a ventilation job; keeping particles and aerosols low the rest of the time is a filtration job.
A simple classroom routine
Site the monitor at head height, away from windows, doors and draughts, and away from where pupils breathe directly on it. Glance at it at the start and middle of each lesson. If it drifts above 1,500 ppm, open windows on opposite sides for a few minutes to flush the room (a short, wide-open purge loses less heat than leaving a window cracked all day). In cold weather, purge between lessons rather than during them.
Where air filtration fits
Ventilation and CO2 monitoring solve the fresh-air problem. They don't, on their own, remove the fine particles — PM2.5, pollen, smoke and virus-carrying aerosols — already circulating in the room, and in a British winter you can't leave every window open all day. A MERV-13 air purifier runs continuously and quietly in the background, holding the room's particle load down whatever the CO2 reading. Used together, the monitor tells you when to ventilate and the purifier keeps the air clean the rest of the time — the Lancet COVID-19 Commission found that reaching six air changes per hour can cut airborne risk by up to 80%.
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Get a school quote or buy online →Frequently asked questions
What CO2 level should trigger action in a classroom?
Sustained readings above roughly 1,500 ppm indicate inadequate ventilation and should prompt action — increase fresh air, and use filtration to keep particle levels down while you ventilate.
Do air purifiers lower CO2?
No. Air purifiers remove particles and aerosols, not CO2. Only fresh air lowers CO2. That's why monitoring and filtration are complementary, not alternatives.
Where should we place the CO2 monitor?
At roughly head height, away from open windows, doors, draughts and direct breath, and not next to a heat source. You want it sampling the air pupils are actually breathing.