Indoor CO₂ Monitoring: What the Numbers Mean

CO₂ monitoring — the simplest measure of whether you're ventilating enough.

Carbon dioxide is the cheapest, most useful number in indoor air quality — not because CO₂ itself is usually dangerous at indoor levels, but because it's an excellent proxy for whether a room is being ventilated enough. If CO₂ is high, your fresh-air supply is inadequate, and everything else (humidity, pollutants) is probably building up too.

People exhale carbon dioxide constantly, so indoor CO₂ rises whenever occupancy outpaces ventilation. That makes it a near-perfect tracer for ventilation adequacy: outdoor air is around 420 ppm, and the more that indoor CO₂ exceeds that, the less fresh air is reaching the space relative to the people in it. A CO₂ monitor is, in effect, a ventilation gauge.

What the numbers mean

Indoor CO₂ levels (approximate guidance)

CO₂ (ppm)	Interpretation
~420	Outdoor / fresh-air baseline
Below 800	Well ventilated — good fresh-air supply
800–1000	Acceptable, but ventilation is being stretched
1000–1500	Stuffy — inadequate ventilation; act on it
Above 1500	Poor — clear sign of insufficient fresh air

CO₂, sleep and cognition

At the levels found in poorly-ventilated homes, CO₂ is not acutely toxic — but elevated CO₂ is associated with stuffiness, drowsiness and measurable reductions in concentration and cognitive performance, and bedrooms with closed doors and windows routinely exceed 1500–2500 ppm overnight, which can affect sleep quality. Because CO₂ tracks ventilation, high CO₂ also means other pollutants and humidity are accumulating, compounding the effect.

Using a CO₂ monitor

Choose an NDIR (non-dispersive infrared) sensor — these measure CO₂ directly and accurately; cheaper 'eCO₂' sensors estimate it from VOCs and are unreliable.
Place it at head height in occupied rooms (living room, bedroom), away from direct breath and draughts.
Watch the trend, not just the spot value — see how CO₂ rises through an evening in the living room, or overnight in a bedroom with the door shut.
Use it to test your ventilation — open a trickle vent or run the MVHR boost and watch CO₂ fall; it's an instant feedback loop on whether your ventilation is working.

What high CO₂ tells you to do

The overnight bedroom problem

The room where CO₂ most often runs out of control is the bedroom, and it is the one people check least. A closed bedroom door, a shut window and one or two sleeping occupants in a small, increasingly airtight room is a recipe for steadily rising CO₂ through the night — readings of 1,500 to 2,500 ppm by morning are common, and poorly-ventilated rooms can climb higher. Because this happens while everyone is asleep, it goes unnoticed, yet it is exactly when ventilation matters for sleep quality and next-day alertness. A simple overnight CO₂ log is one of the most revealing IAQ measurements a homeowner can take: it frequently shows that the bedroom, not the living room, is the home's worst-ventilated space. The fixes are straightforward — an open trickle vent, a window on the vent latch, an undercut door to allow air transfer, or, in a tight home, ensuring the MVHR supplies the bedroom adequately.

Demand-controlled ventilation — letting CO₂ drive the fan

Because CO₂ tracks occupancy so faithfully, it is the ideal signal for demand-controlled ventilation: rather than running a fan at a fixed rate regardless of whether anyone is home, the system ramps ventilation up when CO₂ rises and eases it down when the space empties. This matches fresh-air delivery to actual need, saving energy when rooms are unoccupied while guaranteeing adequate ventilation when they are busy. Many modern MVHR systems and some advanced extract systems offer CO₂-led control, with sensors in the key occupied rooms. It is more sophisticated (and more expensive) than humidity-only control, but for spaces with variable occupancy — a home office, a busy living room, a bedroom — it delivers better air quality for less energy than running flat-out continuously. CO₂ is therefore not just a diagnostic; it is increasingly the control input that runs the ventilation itself.

Common questions from clients.

Why is my bedroom so stuffy in the morning?

Almost certainly rising CO₂ overnight. A closed door, a shut window and sleeping occupants in a small, fairly airtight room let CO₂ climb steadily — 1,500 to 2,500 ppm by morning is common, sometimes higher. It happens while you sleep, so it goes unnoticed, but it affects sleep quality and next-day alertness. An overnight CO₂ log usually reveals the bedroom as the home's worst-ventilated room; an open trickle vent, a vent-latched window, an undercut door, or adequate MVHR supply fixes it.

Can CO₂ control my ventilation automatically?

Yes — demand-controlled ventilation uses CO₂ sensors to ramp the fan up when a room is occupied and CO₂ rises, and down when it empties. Because CO₂ tracks occupancy so well, this matches fresh air to actual need, saving energy in empty rooms while guaranteeing ventilation in busy ones. Many MVHR systems and some advanced extract systems offer it; for spaces with variable occupancy it gives better air quality for less energy than running at a fixed rate.

Is the CO₂ in my home dangerous?

At the levels found in homes (typically a few hundred to a couple of thousand ppm), CO₂ isn't acutely dangerous — but it's a reliable sign of how well you're ventilating. Elevated CO₂ causes stuffiness and reduced concentration, can affect sleep, and indicates that humidity and other pollutants are also building up. It's valuable as a ventilation indicator rather than as a direct hazard.

What CO₂ level should I aim for?

Below about 800 ppm indicates good ventilation; 1000–1500 ppm suggests it's inadequate and you should act (open vents, run extract/MVHR); above 1500 ppm is poor. Outdoor air is around 420 ppm, so the closer you stay to that, the fresher the air. Bedrooms with doors and windows closed often exceed these at night, which is worth checking.

Which CO₂ monitor should I buy?

Choose one with a true NDIR (non-dispersive infrared) sensor, which measures CO₂ directly and accurately. Avoid cheap 'eCO₂' monitors that estimate CO₂ from VOC readings — they're unreliable. Place it at head height in the rooms you use most and watch the trend over time; it's the most informative low-cost IAQ instrument you can own.