Air purifier vs ventilation in classrooms: which does what
"Should we open windows or buy air purifiers?" is the wrong question — the evidence says do both, because they solve different halves of the same problem. This guide explains the difference, and then looks at what two of the largest real-world school studies — one in the United States, one in Italy — actually measured.
What ventilation does
Ventilation replaces stale indoor air with outdoor air. It lowers carbon dioxide, dilutes airborne particles and aerosols, and is the reason CO2 monitors are useful — they tell you when fresh air isn't keeping up. Its limits are practical: in a British winter you can't leave every window open all day, internal rooms have no external wall, and busy roads or noise make full ventilation impractical in many schools.
What an air purifier does
A portable air purifier removes fine particles — PM2.5, pollen, smoke and the aerosols that carry viruses — from the air already in the room, without letting the heat out. It runs continuously and quietly in the background, so the room stays clean between window purges and through the cold months when ventilation drops. It does not lower CO2 — only fresh air does that — which is exactly why it complements ventilation rather than replacing it.
The simple rule: ventilation brings CO2 down; filtration keeps particles and aerosols down the rest of the time. A classroom needs both, and the CO2 monitor tells you when to reach for the first.
Why the classroom is the key battleground
Schools don't spread respiratory viruses evenly. The enclosed, densely occupied classroom — not the corridor or the playground — is repeatedly identified as the primary transmission site, because that's where 30 people share the same air for an hour at a time. That's why the intervention that matters most is the one applied inside the teaching room itself.
What the school studies actually found
United States — CDC study of Georgia elementary schools
A CDC study of 169 Georgia K–5 schools (autumn 2020) compared COVID-19 incidence against the prevention measures each school used. Improving ventilation was associated with 39% lower incidence. Dilution methods alone — opening windows and doors or using fans — gave a 35% reduction; combining that dilution with HEPA filtration lifted the reduction to 48%. In other words, adding filtration on top of ventilation measurably out-performed ventilation on its own.
Italy — over 10,000 classrooms in the Marche region
A retrospective cohort study covering 10,441 classrooms in Italy's Marche region (September 2021–January 2022) compared rooms with mechanical ventilation against naturally ventilated ones. The protective effect scaled directly with the air-change rate: infection risk fell 40% at 2.4 air changes per hour, 66.8% at 4, and 82.5% at 6. Ventilation of 10–14 litres/second per student cut students' infection likelihood by around 80% versus natural ventilation alone.
The practical lesson for a UK school is the number six: reaching roughly six air changes per hour is where the protection becomes dramatic — and a portable purifier sized to the room adds air changes without any building work, on top of whatever the windows deliver.
Size the right units for your rooms
We'll match whisper-quiet MERV-13 units to your classroom sizes to add the air changes the evidence points to — and send school or trust pricing within one working day.
Get a school quote or buy online →Frequently asked questions
Is an air purifier as good as opening windows?
They do different jobs. Windows lower CO2 and bring in fresh air; a purifier removes particles and aerosols from the air already in the room. The Georgia school study found filtration added to ventilation out-performed ventilation alone, so the best answer is to use both.
How many air changes per hour does a classroom need?
The Italian Marche study found protection rose sharply with air-change rate — about 40% at 2.4, 67% at 4 and 82% at 6 changes per hour. Aiming for roughly 4–6 air changes, combining ventilation with a correctly sized purifier, is a sensible target.
Do purifiers lower CO2?
No. Only fresh air lowers CO2. Purifiers remove particles and aerosols. That's why filtration and ventilation are complementary — pair a purifier with a CO2 monitor.
Sources: CDC MMWR, Georgia elementary schools study (2021); Frontiers in Public Health, Marche region schools study (2022); The Lancet COVID-19 Commission.