How much CO₂ is too much in a classroom?

Standards put the line for classrooms at 1000 ppm to 1500 ppm as a daily average. Measurement campaigns find that real classrooms spend most of the school day above it: in one school the recommended 1000 ppm limit was exceeded 89.3 % of the time (F-0038), and across the European SINPHONIE campaigns schoolchildren were exposed to a mean of 1433 ppm, with individual measurements reaching 4957 ppm (F-0039).

Where the carbon dioxide in a classroom comes from

It comes from the pupils. Carbon dioxide indoors is a continuous and constant source in the technical sense: people exhale it steadily for as long as they are in the room, so the concentration is set by how many people are in the room and how fast the air is exchanged, not by any material or product.

That is why the number is used as a proxy rather than as a hazard. At the concentrations reached in classrooms carbon dioxide is not itself the thing to worry about; it is the visible part of a problem that also carries everything else people and rooms release — odours, volatile compounds, and the respiratory aerosols by which infections spread. Ventilation removes them together, and carbon dioxide is the only one of them that a school can afford to measure continuously.

Exposure limits by jurisdiction

None of these is a toxicity limit. They are ventilation targets: the value at which the body issuing them considers the air exchange adequate.

Body	Threshold	Applies to	Source
UK Department for Education, BB101 (2018)	1000 ppm daily average (mechanical ventilation); 1500 ppm daily average (natural ventilation); not above 2000 ppm for more than 20 consecutive minutes	classrooms during occupied hours	BB101
German Environment Agency, UBA (2008)	below 1000 ppm hygienically harmless; 1000–2000 ppm elevated, ventilate; above 2000 ppm unacceptable	all indoor spaces	UBA
ANSI/ASHRAE Standard 62.1	about 700 ppm above the outdoor concentration at the standard’s ventilation rate	design reference	ASHRAE 62.1

Slovakia, the United Arab Emirates and Hong Kong are being compiled; the comparison across jurisdictions is kept in the indoor air quality limits table.

What classrooms actually measure

Three independent campaigns give the picture.

What was measured	Value	Where	Finding
Share of school-day time above the recommended 1000 ppm	89.3 %	one school	F-0038
Mean exposure of schoolchildren, range 269–4957 ppm	1433 ppm	SINPHONIE, European campaigns	F-0039
Average school-day concentration (standard deviation 405 ppm)	841 ppm	monitored classrooms	F-0041
99.9th percentile of daily maxima	5080 ppm	the same classrooms	F-0042

The two averages look contradictory and are not. A mean of 841 ppm across whole school days (F-0041) sits alongside daily peaks past 5000 ppm (F-0042) in the same set of classrooms: the average is pulled down by empty rooms, breaks and open windows, while the peak is what a full room reaches before anyone opens one. This is why a single spot measurement says very little, and why the standards are written as daily averages plus a short-term ceiling.

What lowers it

Mechanical ventilation. Two of the campaigns compared classrooms that had it with classrooms that did not, and both found lower concentrations where it was present: in SINPHONIE a mean of 1087 ppm against 1510 ppm (F-0040), and in the second campaign lower concentrations and less variability in schools with central mechanical ventilation (F-0043). Taken together these support a claim of moderate strength: in classrooms, mechanical ventilation may lower indoor carbon dioxide (C-0018).

Where mechanical ventilation does not exist, the lever is the air exchange rate achieved by opening windows — and the evidence we hold does not tell us how much any particular window-opening routine achieves. That is a gap, not a recommendation.

How strong is the evidence

Grade C. All four studies behind the numbers above are measurement campaigns rather than experiments: they measure what classrooms are like, and where mechanical ventilation is present they measure it alongside everything else that tends to come with a newer, better-equipped building. The direction is consistent across two independent campaigns in different countries, which is why the claim is graded C and not D — but consistency between observational campaigns is not the same as a controlled comparison, and no study here randomised anything.

The limits in the table come from a different kind of document again: guidance and standards, not measurements. They tell you what a body decided was acceptable, not what happens at that value.

What this article does not show is any effect on pupils. The evidence layer behind it contains no finding linking classroom carbon dioxide to attention, test results or absence — so no such sentence appears here, however often it is repeated elsewhere.

What would settle it

Two things. First, a controlled comparison: the same classrooms, the same pupils, ventilation changed deliberately, with the outcome measured — not a survey of schools that already differ in a dozen ways. Second, our own measurements. A logged school day in a specific classroom, against the thresholds in the table above, answers the question that a parent or a head teacher actually has, which is not “what do classrooms look like on average” but “what does this room do at 11 a.m. in February”.