A small room in an old block of flats, Christmas, me, my wife and our child, window shut. In the morning the meter read 4500 ppm of CO₂. At breakfast I nearly put salad into my coffee. It’s not a joke, just a good picture of what a night in used air does to your head.
I have been dealing with carbon dioxide in buildings for more than ten years: measurements, ventilation controls, hundreds of sensors in offices, thousands of measuring points. This compendium sums up what that practice means for ordinary people: what CO₂ tells you about the air, what it does to sleep and thinking, where levels are highest and how to fix it without wasting heat. The basic indoor air parameters are covered in Main indoor air quality parameters.
🫁 CO₂ is a breath counter for the room
Outdoor air today contains about 420–430 ppm of CO₂ (ppm = parts per million). The air we breathe out contains tens of thousands of ppm (about 4%). So every ppm above the outdoor background tells you directly how much of the room’s air has already been exhaled by someone.
At typical indoor levels CO₂ itself won’t poison you. It is, however, the best and cheapest-to-measure indicator of whether ventilation keeps up with the people in a room. Along with it come moisture, odours, volatile organic compounds and the aerosols we exhale. And CO₂ has no smell — your nose won’t warn you, you’ll only feel the effects.
📏 The CO₂ scale from practice
| Level | What it means in practice |
|---|---|
| ~420 ppm | outdoor air; a meter showing 200–300 ppm outdoors needs calibration |
| up to 600 ppm | fresh air |
| up to 800 ppm | nobody complains — I found this empirically at thousands of measuring points |
| 800–1000 ppm | you start noticing smells, the first “stuffy” impression |
| 1000 ppm | the commonly accepted limit of good ventilation — time to air the room |
| ~1500 ppm | clearly stale, like a crowded shopping mall; drowsiness, poorer concentration |
| 2000–3000 ppm | heavy head, headache, reaching for yet another coffee |
| 5000 ppm | occupational exposure limit (8 h); most home meters only measure up to this value |
💻 One hour of home office in a closed room
I ran a simple experiment: one person, an ordinary room in a flat with passive stack ventilation, door closed, window trickle vents closed — the way most people have it in winter, because “it’s draughty”. Start from a well-aired flat.
- after about 20 minutes — 800 ppm, end of full comfort,
- after about 40 minutes — over 1000 ppm, time to ventilate,
- after one hour — about 1300–1500 ppm, headache, fuzzy head, slow thinking.
A few minutes of wide-open airing brought the level back close to outdoor air. One person, one hour. With more people at home I regularly see 2000–3000 ppm in flats.
🚪 Why ventilation “exists” but doesn’t work
A grille in the bathroom or kitchen — passive or with a fan — only extracts. To extract, air must come in from somewhere: through window or wall trickle vents, leaks, and then under the bathroom door. New airtight windows, taped-up vents, too small a gap under the door — and air exchange practically stops. In winter, stack ventilation with tightly closed windows simply performs poorly.
☀️ Summer is worse: warm outside and air conditioning
Passive stack ventilation is driven by the temperature difference between the flat and the outdoor air. In winter the difference is large and the draught in the stack is strong. The warmer it is outside, the weaker the draught – it clearly drops in spring and autumn, and in a heatwave it can stop completely or even reverse.
On top of that, in summer we close the windows and switch on the air conditioning. But a typical wall-mounted split unit does not exchange air – it cools and recirculates the same used room air. The result: with the windows shut and the AC running, CO₂ builds up even worse than in winter. Cool doesn’t mean fresh – if you air-condition the bedroom, keep the trickle vent open or provide mechanical ventilation.
🛏️ The bedroom: eight hours in the same air
The bedroom is the worst place, because we spend eight hours there without airing. A simple calculation: a 30 m³ room, two sleeping people (about 25 litres of CO₂ per hour together), airtight windows letting in maybe 6 m³ of air per hour. After eight hours you get close to 4000 ppm. That matches what I measure: I had 4000–4500 ppm, and a friend in a small bedroom in an old building, with his wife and child, measured 7000 ppm with a professional meter and was sure it was broken.
Research confirms this. In a Danish dormitory study (Strøm-Tejsen et al., Indoor Air 2016), opening a window or adding a fan lowered night-time CO₂ from about 2400 to about 800 ppm. Participants slept deeper, felt more rested and did better on a logical thinking test the next day.
🧠 Meetings, offices, schools — thinking in used air
At a university I used to work at, we had a Monday meeting in a small room of an old building. I always took a coffee and still got sleepy halfway through — and I like my job. Eventually I brought a meter: 2000–3000 ppm. Opening a window in the adjacent room (so it wouldn’t be draughty) and leaving the door open was enough. Three coffees per meeting were no longer needed.
In the Harvard COGfx study (Allen et al., Environmental Health Perspectives 2016), participants in “green” office conditions scored about 61% higher on decision-making tests, and with enhanced ventilation about twice as high, as in a conventional office with CO₂ around 950 ppm. To be fair, when pure CO₂ was added in a chamber without people, some studies found no effect (Zhang, Wargocki, Lian, Indoor Air 2017). Most likely the whole “package” of used air is to blame. For you the conclusion is the same: high CO₂ in a room full of people is the signal to ventilate.
Children in schools have it worst: thirty people in a classroom, windows closed, breaks too short to air the room. Over 2000 ppm is a common reading in classrooms — and thinking is exactly what school is for.
On the other hand, air in modern office buildings is usually better than at home. I have access to hundreds of sensors in different offices — air handling units have filters, intakes high above the street and mechanical supply. My own meter shows it every day: high at night at home, low from the moment I enter the office.
🦠 How much of someone else’s breath you inhale
CO₂ lets you calculate one more thing. The share of air already exhaled by others is roughly (C − C₀) / Cexhaled, where Cexhaled ≈ 38,000 ppm (Rudnick and Milton, Indoor Air 2003). At 1000 ppm that’s about 1.5% of every breath, at 2000 ppm about 4% — roughly every twenty-fifth breath. A CO₂ meter doesn’t see viruses, but it shows how much air we share. That’s why CO₂ meters went into schools and offices during the pandemic.
🚗 The car: recirculation
The car is the tightest space: a few cubic metres of air. I checked: one person, recirculation on — over 1000 ppm after about 5 minutes of driving, and before the tenth minute conditions are really bad. With two, three or four people it happens proportionally faster. The same goes for an electric car with ventilation switched off.
Once, driving with my family, I got drowsy and was already looking for a petrol station to get coffee. It turned out I had switched recirculation on by accident. Some automatic climate systems switch to recirculation on their own and stay there until you switch back manually. The advice to close the air intake in traffic “to avoid fumes” just swaps fumes for your own breath. Invest in a good activated-carbon cabin filter and drive on fresh air.
🏠 What to do at home
- Supply air in rooms — open window or wall trickle vents, especially in the bedroom. Don’t tape them up for winter.
- Extract in the kitchen and bathroom — clear grilles, and doors with an undercut or a grille so air can flow through.
- At night — a trickle vent or a window ajar in the bedroom; bedroom door ajar rather than closed.
- In winter — short, wide-open airing instead of a window permanently ajar. A few minutes is enough to get back to background level.
- Best of all — mechanical ventilation with heat recovery controlled by a CO₂ sensor: fresh air when you’re there, without ventilating an empty house.
🌬️ Heat recovery is about comfort and health, not savings
Mechanical ventilation with heat recovery (MVHR) is usually sold as a way to lower your bills. Heat recovery does exist – but it isn’t the main point. The main point is fresh air in the bedroom all night with the windows closed, no draughts, no street noise and no smog thanks to filters, in winter and in summer, regardless of whether the stack is pulling or not.
- 😴 Sleep – bedroom CO₂ around 800 ppm instead of 3000–4000 ppm,
- ❤️ Health – less of other people’s breath, moisture and pollutants, filtered air,
- 🧠 Productivity – a head that works instead of a third coffee,
- 🛋️ Comfort – closed windows without stuffiness or cold draughts.
That’s why ventilation is really not the place to cut costs – not when building, not when choosing the unit, not in the settings. You can save on many things at home, but not on the air you breathe for a third of your life.
And don’t overdo it the other way. Ventilating just for the sake of it has nothing to do with energy efficiency — 400 ppm at any cost means throwing heat out of the window in winter. Aim for around 800 ppm, and treat 1000 ppm as the signal to act.
📟 Monitoring CO₂ is really important
Without a meter you don’t know what you’re breathing — and a good one is cheap today. What to look for:
- NDIR sensor (infrared) — it really measures CO₂. Avoid cheap “eCO₂” sensors that estimate CO₂ from volatile compounds.
- Accuracy of ±50–70 ppm is normal for these sensors — don’t worry if two meters differ by a few dozen ppm.
- Calibration — outdoors a meter should show about 420 ppm. Shows 200–300? Put it outside and calibrate it (better models calibrate themselves).
- Warning at 800 ppm — gives you time to react before it gets stuffy.
- Placement — at head height, away from windows, doors and your own breath.
- Take it around for a few days — the child’s room, the bedroom, your grandparents’ flat. The result is often surprising.
Meters I have tested and reviewed:
- Inkbird IAM-T2 — review
- IQAir AirVisual Pro — review after years of use
- Dienmern DM125 — cheap air quality monitor
- Cheap, small and good CO₂ monitor
- The cheapest air quality monitor
Instead of another coffee, open the window. Stay fresh!
🛠️ The air quality monitors I recommend
- 📲 Inkbird IAM-T2 – CO₂, temperature and humidity, Bluetooth and an app, a battery that lasts over a month. I also recommend its twin with a more readable display, the Inkbird IAM-T1: IAM-T2 on Amazon | IAM-T1 on Amazon
- 🏠 Tuya WiFi 8-in-1 – CO₂, PM2.5 particulates, formaldehyde, TVOC, temperature and humidity, among others. I use it myself; it is good enough to recommend as an alternative to the Dienmern DM125: Amazon | AliExpress
- 🏆 IQAir AirVisual Pro – the best of the best, and the most expensive: Amazon
These are affiliate links: if you buy through them I get a small commission, and the price for you does not change.
Sources: Strøm-Tejsen P. et al., The effects of bedroom air quality on sleep and next-day performance, Indoor Air 2016; Allen J.G. et al., Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and VOC Exposures in Office Workers, Environ Health Perspect 2016; Zhang X., Wargocki P., Lian Z., Physiological responses during exposure to carbon dioxide and bioeffluents at levels typically occurring indoors, Indoor Air 2017; Rudnick S.N., Milton D.K., Risk of indoor airborne infection transmission estimated from carbon dioxide concentration, Indoor Air 2003.
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