What CO₂ levels mean in a classroom
Outdoor CO₂ sits around 420 ppm. A 30-student classroom with the door and windows shut typically passes 1,500 ppm within 20 minutes of a lesson starting, and can exceed 2,500 ppm by the end of a 50-minute period.
UK Department for Education guidance recommends classroom CO₂ stays below 1,500 ppm on average across an occupied period, with peaks below 2,000 ppm. Research links sustained CO₂ above 1,500 ppm to measurable reductions in cognitive performance, reading speed and exam scores.
UK Department for Education guidance recommends classroom CO₂ stays below 1,500 ppm on average across an occupied period, with peaks below 2,000 ppm. Research links sustained CO₂ above 1,500 ppm to measurable reductions in cognitive performance, reading speed and exam scores.
What to monitor, where, and how
One monitor per classroom, ideally mounted at student head-height on an internal wall (not next to a window or door, which gives a misleading reading). Sample at 1-minute resolution at minimum — slower sampling misses the rapid CO₂ rises around lesson changes.
For a 10-classroom pilot, place monitors in: 4 ground-floor rooms, 4 first-floor rooms, 1 small group room, 1 hall or large space. After 30 days you will have a clear picture of which rooms are ventilation-limited.
For a 10-classroom pilot, place monitors in: 4 ground-floor rooms, 4 first-floor rooms, 1 small group room, 1 hall or large space. After 30 days you will have a clear picture of which rooms are ventilation-limited.
What to do with the data
The first month surprises most schools. Common findings: rooms with mechanical ventilation can still exceed 2,000 ppm during peak periods if the system has not been commissioned for occupancy; rooms that look 'fine' on inspection routinely sit at 1,800 ppm because the trickle vents have been painted shut; first-floor rooms with cross-window ventilation are usually the best in the school.
Use the data to: set a window-opening policy (with science-based thresholds, not 'if it feels stuffy'); identify rooms that need MVHR retrofit; produce evidence for the SBMT or trustees on which capital projects matter most.
Use the data to: set a window-opening policy (with science-based thresholds, not 'if it feels stuffy'); identify rooms that need MVHR retrofit; produce evidence for the SBMT or trustees on which capital projects matter most.