BB101 explained: the CO₂ numbers a school is judged on

BB101 isn't a test you pass once. It's written as a daily average and a twenty-minute limit — which means it's only true or false over time.

BB101 (2018) is the Department for Education guidance on ventilation, thermal comfort and indoor air quality in schools. It sets the CO₂ levels a classroom is judged against, and Ofsted and the DfE look at whether schools meet it.

A standard written over a day can't be proven by a measurement that lasts an afternoon.

The CO₂ numbers

CO₂ is the proxy for fresh air. The more people breathe in a sealed room, the higher it climbs, so the reading is really a stand-in for whether the room is being ventilated at all. BB101 puts numbers on it, and they depend on how the room is ventilated:

• Naturally ventilated rooms (windows and vents): a daily average below 1,500 ppm during occupied hours, and no more than 2,000 ppm for over twenty consecutive minutes.
• Mechanically ventilated rooms (fans and ducts): a daily average below 1,000 ppm, and no more than 1,500 ppm for over twenty consecutive minutes.

Why a spot check can't prove it

Look closely at those numbers and they're all about time — a daily average, a twenty-minute exceedance. A one-day survey, or a sensor dropped in for a week, captures one weather pattern, one timetable, one season. The standard asks about every occupied hour. A snapshot answers about one.

And classrooms swing. A room can read fine at nine in the morning and breach by eleven with thirty pupils and the windows shut. The breach lives in the shape of the day, not in the average a visitor happens to catch.

Continuous is how the standard is written

The cleanest way to prove a continuous standard is to measure continuously. A CO₂ sensor in each classroom, reading every few minutes, produces the daily average and the twenty-minute exceedances directly — the exact quantities BB101 names. No modelling, no sampling, no inference.

• You see the breach in the room and the hour it happened, not at year-end.
• The evidence is the standard's own metric, not a proxy for it.
• The same sensor covers thermal comfort — overheating against the adaptive limits — at the same time.

It's a learning question too

The CO₂ that fails BB101 is the same CO₂ that makes a room hard to think in. Concentration fades as the air goes stale, usually well before anyone thinks to open a window. So the number does two jobs at once: it's the compliance line, and it's a fair measure of whether the room is a good place to learn.

The point

BB101 is a continuous standard wearing the costume of a single number. Treat it as a one-day test and you're guessing. Measure it the way it's written, and the proof is simply there — every hour, ready for whoever asks.

A daily average is only as honest as the day you measured. So measure every day.