Air quality
Indoor CO₂: the simplest indicator of your air quality
CO₂ does not poison you at the levels found in a home. It tells you something else, far more useful: whether the air is being renewed.
Published on · 7 min read
CO₂ sensors have become commonplace, which is a good thing — provided you understand what they actually measure. Many people believe they are monitoring a pollutant. In reality they are measuring a tracer.
That distinction changes how the numbers should be read, and it is what makes the tool useful.
A tracer, not a poison
The CO₂ you measure in your living room comes essentially from the occupants' breathing. At the concentrations found in a home it is not toxic: health effects concern far higher levels, encountered in confined industrial settings, not in a bedroom.
Its value lies elsewhere. Because it is produced continuously by occupants and removed only by air renewal, its concentration directly measures how effective that renewal is. If it rises, the air is not being renewed enough — and what CO₂ then signals is that everything else is accumulating too: moisture, volatile organic compounds, odours, particles.
What CO₂ does not see
Reading the numbers
| Concentration | Common interpretation |
|---|---|
| ≈ 420 ppm | Current outdoor air, the floor value indoors |
| Up to ~800 ppm | Satisfactory air renewal for current occupancy |
| 800 to 1,000 ppm | Watch zone: air renews more slowly than it loads |
| 1,000 to 1,400 ppm | Insufficient renewal, worth correcting |
| Above 1,400 ppm | Marked stuffiness, ventilation clearly falling short |
Where and when to measure
A single spot reading tells you almost nothing. What matters is the curve over several days — because CO₂ narrates the home's occupancy, and occupancy varies.
The bedroom is the most revealing measurement point: door closed, two people, eight hours. If ventilation is insufficient, the curve climbs steadily all night and drops sharply when the door opens in the morning. That sawtooth profile is the most reliable signal of inadequate air renewal.
Reading a curve: three typical profiles
- Continuous overnight climb to waking, sharp morning drop → insufficient extraction or air supply in the bedroom. The most common profile.
- A stable plateau even with several occupants → renewal matches production. This is the behaviour you want.
- Short evening peaks then a quick return to normal → a demand-controlled system reacting correctly, or effective manual airing. Nothing to fix.
What measurement lets you decide
The practical value of CO₂ is that it turns an impression into data. “The bedroom feels stuffy in the morning” becomes a curve, and a curve can be compared before and after an intervention.
It also lets you rank spending. If the curve flattens after simply clearing a transfer gap under the bedroom door, you have just avoided a four-figure investment. If it does not move, you know the problem is elsewhere — and you look in the right place.
- 1Place the logger in the bedroom at bed height, away from a window and out of the direct airflow from a valve.
- 2Leave it running for at least three consecutive nights, living normally.
- 3Note significant events: two people overnight, window open, absence.
- 4Compare the curve with the markers above.
- 5Change one parameter at a time, then measure again.
Frequently asked questions
Are cheap CO₂ sensors reliable?
Two technologies coexist. NDIR sensors genuinely measure CO₂ and give usable readings. So-called “eCO₂” sensors infer a value from other compounds and can depart sharply from reality. Check for the NDIR designation before buying: that is what separates an instrument from a gadget.
Does the sensor need calibrating?
Most consumer NDIR sensors self-calibrate by taking the lowest value seen over several days as a reference, assuming it corresponds to outdoor air. For that to work, the device must see fresh air from time to time. A sensor left for months in a room that is never ventilated will eventually drift.
Is a high level dangerous to sleep in?
At the levels found in a home, CO₂ itself is not dangerous. What a high level indicates is that the air is not being renewed — so moisture and other pollutants are accumulating too. Research has associated elevated levels with reduced sleep quality and cognitive performance; the prudent response is to address air renewal, without dramatising the gas itself.