Since 2022 a citizen science project called SAMHE, Schools’ Air quality Monitoring for Health and Education, has put low cost air quality monitors into classrooms across the UK, with pupils and teachers reading the data themselves as part of the project rather than researchers simply collecting it from a distance. Two papers published from that dataset over the past eighteen months, one covering fine particulate matter across 490 schools and the other covering ventilation across 322, give the clearest national picture yet of what is actually in the air where children spend a large share of their day. Neither points mainly to what happens minute to minute in a classroom. Both point to things a school has far less control over, where the building sits and how well it is funded.

The Outdoor Air Problem a Window Cannot Fix

The larger of the two studies, led by Alice Handy and colleagues and published in Science of the Total Environment in 2025, analysed fine particulate matter, PM2.5, readings from 490 schools across the UK monitored through the 2023 to 2024 academic year. Its central finding is that indoor PM2.5 concentrations in schools closely track outdoor ambient levels, with outdoor air acting as the dominant source of the particulate matter measured inside. Short, episodic spikes in outdoor pollution, the kind of day when outdoor air quality is noticeably worse than usual, contributed a disproportionate share of a school’s total exposure across the year, rather than exposure building up steadily and evenly.

That finding matters because it complicates the usual advice about improving indoor air, opening windows, upgrading ventilation systems, running air cleaners. All of those measures help with pollutants generated inside a building, but a school sitting near a busy road or in an area with generally poorer air quality is drawing much of its indoor pollution problem in from outside, whatever it does internally. Fixing that runs into questions of where schools are built and how local air quality is managed, questions that sit well outside any individual school’s control.

Ventilation, Deprivation and a Threshold Most Schools Meet, Except in the Cold

The second study, by Samuel Wood, Alice Handy, Katherine Roberts and Henry Burridge, published in Developments in the Built Environment in October 2024, used carbon dioxide readings as a proxy for ventilation rate, working from around 9,406 school days of data collected across 322 schools during autumn 2023, with 190 schools included in the final analysis. The overall mean ventilation rate came out at 5.3 litres per second per person, though that figure moved substantially with the weather, rising to 6.8 in warmer conditions and falling to 3.8 when it turned cold. Daily mean CO2 levels stayed below the government’s Building Bulletin 101 guidance threshold of 1,500 parts per million in the majority of schools, and nearly half never exceeded it at all across the whole study period. The exceptions clustered predictably around temperature: once outdoor conditions dropped to around 5 degrees Celsius, the share of school days exceeding the threshold rose to roughly one in five, consistent with windows being kept shut against the cold rather than for air quality reasons.

What stood out once outdoor temperature was accounted for was who was left with the higher readings. State funded schools showed higher CO2 concentrations than private schools, and schools in more deprived areas showed higher levels than those in less deprived ones. Secondary schools ran higher than primary schools too, though that gap narrowed considerably once the researchers adjusted for the fact that older pupils simply produce more CO2 per body than younger ones. Schools operating above their nominal pupil capacity consistently showed higher concentrations than those within it. None of these differences point to how conscientiously staff were managing their classrooms. They point to building age, funding and occupancy, factors that track a school’s resources rather than its day to day practices.

A National Baseline That Did Not Exist Before

What makes both studies notable is less any single figure than the dataset behind them. Most previous UK research on classroom air quality covered a handful of schools at most, monitored for a short period. SAMHE is led by the Stockholm Environment Institute’s York centre at the University of York, working with Imperial College London, the University of Cambridge, the University of Surrey and the University of Leeds, funded by the Department for Education and the Engineering and Physical Sciences Research Council. It is running from 2022 to 2029 and has already produced two papers covering hundreds of schools simultaneously across a full academic year, drawing on the same shared monitoring network and overlapping research teams. Since those papers were published, the government has itself updated its practical guidance for schools, issuing new advice on ventilation and air quality in England’s education and childcare settings in February 2026 that keeps the same 1,500 parts per million daily mean ceiling the SAMHE studies measured against, while adding a lower 800 parts per million marker below which ventilation is described as good.

Taken together, the two studies reframe what improving classroom air quality actually requires. Better ventilation habits and cleaner outdoor air both matter, and the evidence linking CO2 levels to pupil concentration and PM2.5 exposure to respiratory health is well established elsewhere. But the specific pattern SAMHE has found, outdoor pollution driving indoor exposure regardless of what a school does internally, and CO2 levels tracking a school’s funding and location as much as its ventilation practices, means the fix is not simply a matter of better habits inside the classroom. It runs into decisions about where schools are sited, how they are funded and how crowded they are allowed to become, questions that go well beyond what any single teacher or head can control.

Sources: Science of the Total Environment (Handy et al, 2025); Developments in the Built Environment (Wood, Handy, Roberts and Burridge, 2024); Stockholm Environment Institute (SAMHE project).