Is CO₂ at classroom levels actually harmful?

Not directly, in the toxicological sense — classroom CO₂ peaks of 2,000–2,500 ppm don't cause acute medical harm. The reason it matters is that CO₂ is a proxy for ventilation rate. When CO₂ is high, fresh air supply is low, and everything else accumulates with it: PM2.5, formaldehyde, viruses, allergens. The cognitive effects show up at CO₂ levels well below any toxicity threshold — the Harvard COGfx study found a 50% drop in cognitive function scores at ~1,400 ppm versus a 550 ppm baseline.

Can't we just open the windows?

It works in May. It doesn't work in January at 5 °C with rain blowing in, which is precisely when classrooms are at their most occupied and exhalation rates are highest. The UK SAMHE study found daily-mean CO₂ exceedances on roughly 20% of days when outdoor temperatures dropped to ~5 °C, with most schools defaulting to closed windows for thermal comfort. The Department of Education's own guidance acknowledges this — 'open windows when CO₂ exceeds 800 ppm' is the floor, not the solution. The pragmatic answer is mechanical demand-controlled ventilation that delivers fresh air without losing the heat with it.

Will demand-controlled ventilation make the heating bill worse?

The opposite. This is one of the rare retrofits with no efficiency-versus-comfort trade-off. Demand-controlled ventilation supplies fresh air only when CO₂ rises above setpoint, which means the school is not heating fresh air it doesn't need. Vaisala's portfolio data records an average 38% HVAC energy saving across building types; the US Department of Energy's PARC programme found commercial buildings typically over-ventilate by up to six times the required minimum. Heat-recovery ventilation pairs with this: incoming fresh air is preheated by the outgoing stale air to the tune of ~97% of recovered heat in modern ERV units, so the net heating load barely shifts.

How does this fit with our existing solar PV or SEAI grant pathway?

It complements rather than competes. The Schools Photovoltaic Programme funds up to 6 kWp of rooftop PV plus a mandatory monitoring screen — most schools currently use that screen as a static poster. Adding continuous CO₂ and energy sensing turns the same dashboard into a live operational tool covering generation, consumption, ventilation status and air quality. On the funding side, eligibility is project-specific — we help schools navigate the SEAI Support Scheme for Energy Audits, the Non-Domestic Microgeneration Grant for solar expansion, and other routes where the school qualifies.

Seven of Eight Irish Classrooms Exceed CO₂ Guidelines. Here's What That Means for Learning.

In 2024, three Irish universities — Technological University of the Shannon, Trinity College Dublin and the University of Limerick — published a peer-reviewed measurement study of indoor air quality in eight naturally ventilated primary classrooms. The mean CO₂ readings ranged from 796 ppm to 2,469 ppm. Seven of the eight classrooms exceeded national guidelines. The follow-up nine-school study, published in Building & Environment in 2025, found the median classroom CO₂ across the wider sample was 1,319 ppm, and PM2.5 exceeded the WHO 24-hour 15 µg/m³ threshold on 48% of days.

If those numbers seem abstract, here’s the consequence in language teachers will recognise: at the CO₂ levels regularly observed in Irish classrooms, peer-reviewed cognitive-performance research records a 50% drop in cognitive function scores versus a fresh-air baseline. Not 5%. Not “a bit”. Half.

This isn’t a single eye-catching study. It’s a finding that’s now been reproduced across four countries and two decades of research. Here’s what the data actually says, why “open the windows” stopped being the answer, and what an Irish school can do about it without picking between heating bills and learning outcomes.

The Quick Version

The 2024 TUS / Trinity / University of Limerick study found 7 of 8 sampled Irish primary classrooms exceeded national CO₂ guidelines, with peaks at 2,469 ppm.
The 2025 multi-zone follow-up recorded median classroom CO₂ of 1,319 ppm and PM2.5 above the WHO threshold on 48% of measured days.
Harvard’s COGfx study (Allen et al., 2016) found cognitive function scores 50% lower at ~1,400 ppm versus a 550 ppm baseline.
Mendell et al. (2013) tracked 162 California classrooms over two years and recorded a 1–2% drop in absenteeism per 100 ppm fall in classroom CO₂.
The Department of Education’s COVID-era distribution of ~35,400 portable CO₂ monitors gave teachers eyes on the problem. None of those monitors act on it.
Demand-controlled ventilation typically saves 20–40% of HVAC energy while keeping classroom CO₂ inside guidelines. It is one of the rare retrofits with no comfort/efficiency trade-off.

The 2024 Irish Finding (And the 2025 Follow-up)

The 2024 paper — Indoor air quality in naturally ventilated primary school classrooms in Ireland, published in MDPI’s Environments journal — measured continuous CO₂, temperature, humidity and PM2.5 across eight primary classrooms in different parts of the country, over a full heating season. The headline numbers:

Mean classroom CO₂ ranged from 796 ppm to 2,469 ppm across the eight rooms.
Seven of the eight exceeded the 1,500 ppm action threshold the Department of Education’s Practical Steps for Deployment of Good Ventilation Practices in Schools identifies.
The single best-performing room was the only one fitted with mechanical ventilation; the rest relied on operable windows alone.

The 2025 follow-up — a multi-zone study across nine Irish schools published in Elsevier’s Building & Environment — broadened the picture. Median classroom CO₂ across the wider sample sat at 1,319 ppm. PM2.5 exceeded the WHO’s 24-hour 15 µg/m³ threshold on 48% of days. Median formaldehyde concentrations sat at 20 µg/m³.

These are peer-reviewed Irish numbers, measured in Irish classrooms, by Irish researchers — not extrapolated from UK, EU or US sources. Globally, the 2024 Garvey/Littlewood systematic review of 2,444 classrooms across 125 studies found a median occupied CO₂ of 1,487 ppm, with 81% of classrooms exceeding 1,000 ppm. The Irish picture is broadly consistent with the international one. It is not better than average.

Why CO₂ Is the Right Number to Watch

CO₂ at the levels typically found indoors — even at 2,500 ppm — is not directly harmful in the toxicological sense. You won’t faint at those concentrations. You won’t develop respiratory disease. So why is the Department of Education’s guidance, the REHVA threshold, BB101 in the UK, and EN 16798-1 across the EU, all built around CO₂?

Because CO₂ is a proxy for ventilation rate. Every occupant in the room is exhaling roughly 8 litres of CO₂ per hour, predictably. The CO₂ concentration in a closed room rises in direct proportion to occupancy and falls in direct proportion to fresh air supply. Measure it, and you have a reliable measurement of how much fresh air is actually reaching the people inside.

That matters because everything else accumulates with it:

Particulate matter (PM2.5 and PM10) from outdoor sources, kitchen activity, dust, and aging carpets. The 2025 Irish study recorded PM2.5 above WHO thresholds on nearly half the school days measured.
Formaldehyde and other volatile organic compounds from furniture, books, cleaning products, paint and adhesives — the 2025 Irish study found median formaldehyde at 20 µg/m³.
Bioaerosols including viruses (a topic the COVID years made real for everyone) and allergens.
Moisture that drives condensation and mould on cold surfaces — a separate but related problem in older Irish school buildings.

A high CO₂ reading doesn’t directly hurt a child. What it tells you is that ventilation is failing, and everything else in the air is therefore more concentrated than it should be. The cheapest single sensor in a classroom is a CO₂ sensor; it tells you about the whole air quality stack.

The Cognitive Evidence Is No Longer Thin

If there were one paper showing a 50% drop in cognitive function at 1,400 ppm, that would be interesting but easy to dismiss. There isn’t one paper. There’s a substantial multi-country, multi-decade evidence base, and the findings reproduce.

Harvard COGfx (Allen et al., 2016) is the most-cited reference. The Harvard team had office workers complete cognitive tasks under three controlled CO₂ conditions: 550 ppm, 945 ppm, and 1,400 ppm. Cognitive function scores dropped 15% at 945 ppm and 50% at 1,400 ppm versus the 550 ppm baseline. Crisis-response and strategic-thinking scores were the most affected; basic activity scores were largely unaffected. The takeaway isn’t that occupants forget how to type; it’s that the higher-order thinking that schools are explicitly trying to develop is the part that degrades fastest.

Bakó-Biró et al. (2012) worked specifically in eight UK primary schools. The team tested attention, concentration and memory at two ventilation rates — 1 l/s/p (a typical naturally ventilated classroom in winter) and 8 l/s/p (well-ventilated). Performance improved by up to 15% when ventilation was raised. The effect was visible within a single school day.

Petersen et al. (2016) ran a controlled study in Danish primary schools, doubling outdoor air supply from 1.7 to 6.6 l/s/p across regular school days. Test scores improved 3.2–7.4% across reading, grammatical reasoning and arithmetic — measurable improvements on the same tests, with the same children, by changing only the ventilation.

Mendell et al. (2013) is the most operationally important for school boards. The team tracked 162 California classrooms across 28 schools for two full years. They found a 1–2% relative drop in absenteeism for every 100 ppm fall in classroom CO₂. In a school of 200 pupils averaging 1,400 ppm, dropping average CO₂ to 800 ppm is a 6–12% reduction in sick days per pupil per year — quietly material to staffing, attainment and budget.

This is now a finding that has been replicated across four countries, in different climates, in different school estates, across two decades. It is no longer the kind of evidence a school board can responsibly dismiss as “a study said”.

What the Regulatory Floor Says (And Where It Falls Short)

Ireland has a regulatory framework, and it’s not nothing. It’s just not enough.

The Department of Education’s Practical Steps for Deployment of Good Ventilation Practices in Schools adopted the REHVA threshold in 2021: open windows when CO₂ exceeds 800 ppm, with an action point at 1,500 ppm. Building Regulations Part F sets the legal floor for ventilation in Irish buildings, including schools. SDG 03-TN01 covers MVHR specifically for schools.

Internationally, the UK’s Building Bulletin 101 (2018) sets a 1,000 ppm daily-average ceiling for mechanically ventilated classrooms and 1,500 ppm for naturally ventilated ones, and now explicitly favours hybrid demand-controlled ventilation as the design pattern. EN 16798-1:2019 is the EU framework, with category-based limits running roughly 950 / 1,200 / 1,750 ppm across four indoor-quality tiers.

The problem is what the regulations assume. They assume that 800 ppm or 1,500 ppm thresholds, plus a portable sensor, plus operable windows, are a working system. In May, in a building with windows the school is willing to leave open all day, that assumption holds. In January, at 5 °C, with rain blowing in, it doesn’t. The UK SAMHE study (Imperial-led, 1,300+ schools) reported daily-mean CO₂ exceedances on roughly 20% of days when outdoor temperatures dropped to ~5 °C. Schools default to closed windows because nobody wants to teach in a draught at 12 °C, and the CO₂ rises accordingly.

The regulatory floor identifies the problem. It does not solve it.

The 35,400 Monitors That Can’t Act

There’s one piece of school-sector instrumentation that needs naming, because it gets confused with monitoring every time the topic comes up.

During 2021 and 2022, in response to COVID, the Department of Education distributed roughly 35,400 portable CO₂ monitors to every primary, post-primary and special school in the country. Those units do an important job — they show classroom CO₂ to the teacher in real time, in the room. They’ve materially changed how Irish teachers think about ventilation.

But they don’t log historic data. They don’t send alerts. They don’t integrate with any building system. They don’t feed the school’s annual SEAI Monitoring & Reporting return. They are conscience devices, not data devices. They give the teacher eyes on the problem; they don’t give the school hands to act on it.

This is the most common confusion in school IAQ conversations: schools assume that because they have monitors, they have monitoring. They don’t. The State told schools that classroom CO₂ matters and gave them eyes on it. It didn’t give them a control system. That’s the gap continuous monitoring fills — and it’s the gap the Climate Action Summer Works Scheme is now starting to make load-bearing for grant eligibility.

Demand-Controlled Ventilation: The Honest Answer

Here is where the trade-off everyone fears doesn’t actually exist.

The naive way to fix classroom CO₂ is to run mechanical ventilation continuously at peak rate during occupied hours. That works for air quality. It also wastes huge amounts of heating energy, because the school is constantly bringing in cold outside air and heating it. If you’ve ever heard a principal say “we just can’t afford to run the ventilation that long”, this is the trade-off they’re navigating.

Demand-controlled ventilation (DCV) removes the trade-off. CO₂ sensors in each classroom modulate fresh air supply continuously — running full when 28 children are mid-lesson, ramping down at break times, almost off in empty rooms. The system delivers fresh air only when occupants need it and not before.

The numbers across the literature are extraordinary for a category that usually gets framed as “expensive ventilation”:

Vaisala’s commercial-portfolio data records average 38% HVAC energy saving across building types fitted with DCV, with the largest savings in cold climates and variable-occupancy uses — schools sit at the top of that list.
NIST’s state-of-the-art review reports DCV savings of 15–80% depending on baseline.
A Danish university campus recorded a 27% annual HVAC reduction with DCV while maintaining ASHRAE air-quality compliance.
The US Department of Energy’s PARC programme found that commercial buildings are routinely over-ventilated by up to six times the required minimum — a vast amount of which is being heated, cooled and re-supplied for nobody.

Pair DCV with heat-recovery ventilation and the heat penalty effectively disappears: a modern single-room ERV like Optim Vent recovers around 97% of the heat from outgoing stale air into incoming fresh air, runs at 7.8 W (about €10/year on the school’s electricity bill), and sits below 33 dB — quiet enough to meet UK BB93 and Irish TGD-021-5 acoustic requirements for classrooms. Compare single-room ERV against whole-house MVHR for the design choice; the heat-recovery ventilation explainer covers the underlying physics.

This is one of the rare interventions where doing the right thing for pupils and doing the right thing for the heating bill point in exactly the same direction.

What a School Can Actually Do

In rough order of effort and cost:

1. Get the audit. The SEAI Support Scheme for Energy Audits voucher covers the audit cost where the school qualifies — eligibility is confirmed upfront. The school keeps the report regardless of next steps. The audit is the cheapest, lowest-risk action a school board can take.

2. Deploy continuous CO₂ sensors that log and alert. Not the portable monitors — sensors that record historic data, send the principal an alert when a classroom breaches threshold, and feed a dashboard the Board can see. This step alone moves the school from aware of the problem to measuring it, and it produces the documented baseline future grant rounds reward.

3. Retrofit DCV into the most-occupied rooms first. This is rarely a one-shot capital project. The pragmatic approach is to start with the rooms that show the highest CO₂ peaks in step 2 — typically the senior classrooms and any room with low ceiling height — and grow from there. Pay-from-savings arrangements exist for schools where capex is constrained.

The full deployment story — sensors, controls, dashboard, the SEAI Monitoring & Reporting layer underneath — is on our schools use-case page. That page covers the wider stack (energy sub-metering, lighting automation, water-leak detection, building-safety contact sensors, the Optim Vent ERV unit specifically) — but if classroom IAQ is the entry point, the audit and the CO₂ sensors are where most schools start.

In Summary

The Irish data is now in. Peer-reviewed studies find most primary classrooms breach national CO₂ thresholds, and the picture is consistent with international data.
The cognitive evidence is reproducible. Harvard, Cambridge, Aarhus and Berkeley researchers reach the same conclusion across four countries and two decades: high CO₂ in classrooms measurably reduces learning. A 50% cognitive drop at 1,400 ppm is not a fringe finding.
The regulatory floor identifies the problem; it doesn’t solve it. “Open the windows” works in May. It does not work in an Irish January, which is exactly when the problem peaks.
The portable monitors don’t act. They are conscience devices. The fix is automated demand-controlled ventilation tied to continuous CO₂ sensing.
The trade-off principals fear doesn’t exist. DCV plus heat-recovery ventilation produces better air quality and lower heating bills simultaneously — one of the rare retrofits with no comfort/efficiency tension.

If you’re on a school board reading this, the right next move is the audit. The board gets a written report regardless of what’s done with it. The full schools deployment is on our schools use-case page; the audit booking is on the contact page below.