CO₂ in classrooms
and student concentration

Of all the rooms in a school, the classroom is the one designed with the least air per person. Forty students in a room built for thirty, doors closed against corridor noise, windows closed against dust and heat and traffic. It is also the room where the cost of degraded performance is highest, and the one nobody measures.

What a classroom actually does over a day

The pattern is consistent enough to describe from the curve alone. The room starts near outdoor air — 420 to 500 ppm — before assembly. Within fifteen minutes of the first period it is at 800. Somewhere between forty and sixty minutes it crosses 1,500. If the door stays shut it will continue climbing towards 2,500 ppm and beyond, and each break resets it only partially, because a five-minute changeover does not exchange a room's air.

By the afternoon session, in a room that has been occupied all day with no deliberate ventilation, 2,000 to 2,500 ppm is normal. The teacher describes it as the class getting restless after lunch. That description is accurate; the attribution usually is not.

The research

Schools are an unusually good setting for this research, because the outcome measure — standardised test performance — is already being collected for other reasons, at scale, on a fixed schedule. Three findings are worth knowing in detail.

Ventilation rate and test scores

Haverinen-Shaughnessy, Moschandreas and Shaughnessy (2011) measured ventilation rates in 100 elementary-school classrooms across the south-western United States and matched them against standardised maths and reading results. They found that each additional litre per second per person of outdoor air supply was associated with a rise in the proportion of students passing the tests — roughly in the range of half a percentage point to three percentage points per unit, depending on the subject and model.

It is an observational study, so it demonstrates association rather than proof of cause, and the authors are appropriately careful about that. But the effect held after adjusting for the obvious confounders, and it points the same way as the controlled work below.

Ventilation rate and speed of work

Wargocki and Wyon (2007) ran controlled experiments in real classrooms, varying the outdoor air supply rate while children performed standard schoolwork tasks. Higher ventilation improved the speed at which the children worked — by roughly 8 to 15% on several tasks — without any increase in error rate. That is the signature of an alertness effect rather than an accuracy one: the same children, doing the same work, simply moving through it faster.

Ventilation rate and absence

Mendell et al. (2013) examined illness absence across 162 classrooms in California and found lower absence associated with higher ventilation rates. That makes intuitive sense — ventilation dilutes airborne respiratory particles along with everything else — and CO₂ is the practical way to know whether the dilution is happening.

Taken together: better-ventilated classrooms show faster work, better test outcomes and less absence. None of the individual effects is enormous. All of them are free, in the sense that the fan is usually already installed.

Why Indian classrooms start further behind

The studies above were conducted in schools with lower occupant density and, in many cases, mechanical ventilation systems that at least attempt a minimum outdoor-air rate. An Indian classroom generally has neither.

• Higher density. Forty to fifty students in a room sized for thirty is common. CO₂ scales directly with the number of people.
• No mechanical fresh air. Ventilation is a window, and a window is closed for perfectly good reasons — road noise, dust, heat, monsoon rain.
• Air conditioning that recirculates. In air-conditioned schools the incentive to keep the room sealed is stronger still, and the AC contributes no outdoor air.
• Long occupied blocks. Longer school days with short changeovers mean less chance for the room to recover.

What a school can actually do

The good news is that the equipment is almost always already there. Nearly every Indian classroom has an exhaust fan or openable windows. What is missing is a trigger.

1. Measure a few rooms first. Not every classroom — three or four representative ones. A top-floor room, a ground-floor room near the road, a computer lab, the staff room. A week of logged data will tell you which rooms have a problem and how bad it is.
2. Set a threshold and automate it. Fan ON at 1,500 ppm, OFF at 900 ppm is a sensible starting point for a classroom. This has to be automatic, because a teacher mid-lesson has forty other things to think about, and a policy that depends on somebody remembering is a policy that lasts a fortnight.
3. Use the data. Once you have a week of curves, the argument for where to spend the ventilation budget makes itself. It also tells you which rooms are fine and need nothing — usually a surprise, and it saves money.
4. Expand on evidence. Extend to the rooms the data says are worst, not the rooms that are most convenient.

A note on what this is not

Ventilation is not a substitute for teaching, and no CO₂ controller ever raised a board result on its own. The effects in the literature are real but modest — single-digit percentages on task speed, a few percentage points on pass rates.

What makes them worth acting on is the asymmetry. The intervention is cheap, it uses equipment the school already owns, it requires nothing from teachers once configured, and the same air exchange also reduces illness absence and makes the room more pleasant to be in. Very few things available to a school have that profile.