Indoor CO2 Concentration

Indoor CO2 concentration is the amount of carbon dioxide in the air inside a room, measured in parts per million (ppm). The Slovak term is koncentrácia CO2 vo vnútornom prostredí. Occupants are usually the main source: every breath exhales CO2, so the level rises when rooms are occupied and falls when fresh outdoor air dilutes it.

What is indoor CO2 concentration and where does it come from?

The indoor reading is not a climate measure. The greenhouse effect concerns CO2 in the global atmosphere, and a higher reading in a room does not change the local climate in any way that matters for the building. The number is a tracer: it shows how much of the room's air has been breathed by people rather than replaced with outdoor air.

Because it is easy to measure, CO2 is the standard proxy for ventilation adequacy. It does not measure every pollutant, which is why it sits within the wider topic of indoor air quality.

Why is CO2 a good indicator of ventilation adequacy?

CO2 balances occupant output against air exchange. Occupants add CO2, and ventilation removes it by bringing in outdoor air. High air exchange keeps the reading near the outdoor background. Low air exchange lets it climb for as long as people stay in the room.

The reading responds to real occupancy rather than to a schedule, so a bedroom used only at night shows what actually happened there. Its limitation is lag: after people leave, CO2 falls gradually, so a high value does not prove the room is occupied right now.

Reading or reference	What it tells you	What it does not tell you
Outdoor background	The starting point for any indoor reading, commonly cited as about 400 ppm	A limit for indoor spaces
Indoor reading rising during occupancy	Air exchange is falling behind the number of people	Which pollutant is harmful, or how harmful
Indoor reading falling after people leave	Ventilation is diluting the room	Whether radon, VOCs or moisture are under control
Pettenkofer reference, 1000 ppm	A 19th-century hygiene benchmark still quoted in design texts	A current Slovak legal requirement

Which reference values are commonly cited, and which are binding?

The two figures most often cited are the outdoor background, commonly given as about 400 ppm, and the Pettenkofer reference of 1000 ppm. Outdoor levels vary by place and season, so treat the first as an approximate starting point.

The second is named after Max von Pettenkofer, a 19th-century hygiene researcher who used indoor CO2 to judge whether rooms were fresh enough. His benchmark was linked to occupant odour. CO2 is not considered toxic at typical indoor levels, so 1000 ppm is a design reference, not a toxicity threshold.

This article does not cite a Slovak legal limit, and none should be inferred from these figures. For any binding requirement, check the current regulations and the design documentation for the specific building.

Why do bedrooms in new airtight houses show high CO2?

A closed bedroom is the classic case. It is small, occupied for many hours, often by two people, and its windows stay shut through the night. In a new airtight house the natural leakage that once diluted the air is much smaller, as described in the entry on airtightness, so the room depends on the ventilation system or on opening a window. A reading that climbs all night and drops within minutes of opening a window points to the closed-window pattern, not to a sensor fault.

Where a heat recovery ventilation system is installed, check whether supply air actually reaches the bedroom and whether the fan runs at an appropriate speed while the room is occupied. Without a mechanical system, a closed bedroom in an airtight building will not ventilate itself.

How do you use a cheap NDIR CO2 sensor as a diagnostic?

Most affordable sensors use non-dispersive infrared (NDIR) measurement, which infers concentration from how much infrared light the gas absorbs. It is good enough for diagnosis, but low-cost units drift, are affected by temperature, and may use automatic calibration routines that assume the room regularly reaches fresh outdoor air. Treat one as a pattern tool, not a certified instrument.

Run a simple protocol rather than trusting one reading:

Step	What to do	What it shows
Placement	Mid-room, at about breathing height, away from the window, supply valve and bed	Keeps one person's breath from dominating the reading
Baseline	Check the sensor in fresh outdoor air first	Confirms it is in a plausible range
Night log	Record a full night with doors and windows as normal	The overnight peak and the morning value
Window test	Open the window for a few minutes	How quickly the room actually exchanges air
System comparison	Repeat at normal and higher fan settings	Whether the system responds to the load

The result shows whether the room is keeping up, not what the correct design flow is. Turning a pattern into a fix is a decision for the ventilation designer.

How does demand-controlled ventilation use CO2 readings?

A demand-controlled ventilation system uses the same signal as a control input. The controller raises fan speed as CO2 climbs and lowers it as the room empties, so air exchange follows the actual load instead of running at a fixed rate. The saving depends on the building and its occupancy pattern, so it should be estimated for the specific house.

The logic has traps. CO2 lags occupancy, so the fan reacts after people arrive and runs briefly after they leave. CO2 alone also misses moisture from showers and cooking, which is why many systems add humidity control.

How is indoor CO2 different from indoor air quality?

Indoor CO2 is one signal within indoor air quality, not a substitute for it. Indoor air quality covers particles, volatile organic compounds from materials, radon from the ground, moisture and mould risk, and odours. CO2 tracks occupant-generated pollution and the ventilation rate, but it says nothing direct about radon, formaldehyde or fine particles.