Carbon dioxide (CO₂) is a colorless, odorless gas that people exhale with every breath. Outdoors it sits near 420 parts per million (ppm). Indoors it rises as people breathe and falls as fresh air comes in, which makes it the most practical measure of ventilation available. Under 800 ppm is well ventilated; above 1,000 ppm most guidance says increase fresh air.
What CO₂ is
CO₂ is one carbon atom bonded to two oxygen atoms. It is produced by combustion, by fermentation and by every animal that breathes, including us. An adult at rest exhales roughly 0.3 liters of CO₂ per minute, more during activity. It is heavier than air but mixes readily, so in an occupied room it is spread fairly evenly rather than pooling at the floor.
Where indoor CO₂ comes from
In homes and offices the source is almost entirely people. Gas cooking, candles and unvented heaters add some. Because the source is so predictable, the concentration in a room is a direct function of two things: how many people are in it and how quickly outside air is replacing the air they have breathed. That second factor is ventilation, and it is what a CO₂ reading reveals.
Why the number matters
At indoor levels CO₂ itself is not harmful; occupational limits are in the thousands of ppm. What matters is what a high reading implies:
- Everything else people exhale is also concentrated. Respiratory aerosols, odors and moisture rise and fall with CO₂. A low CO₂ reading means good dilution of anything airborne.
- Cognitive effects. Several controlled studies have associated readings above about 1,000 to 1,500 ppm with slower decision-making and drowsiness in some participants. The effect is modest and not universal, but it is consistent with the stuffy-meeting-room experience everyone recognizes.
- It is measurable. Unlike ventilation rate, which needs instruments and calculation, CO₂ can be read from a small monitor in real time.
What the numbers mean
| Reading | Meaning |
|---|---|
| 400 to 600 ppm | Close to outdoor air; ventilation exceeds occupancy |
| 600 to 800 ppm | Good; typical of a well-ventilated occupied room |
| 800 to 1,000 ppm | Acceptable; the point where many guidelines say to add fresh air |
| 1,000 to 1,500 ppm | Poor; drowsiness and complaints become common |
| Above 1,500 ppm | Ventilation is failing to keep up; act |
The bands are guidance drawn from common ventilation standards, not medical limits. See what a good indoor CO₂ level is for the detail behind them.
CO₂ is not carbon monoxide
The two are often confused. Carbon monoxide (CO) is a toxic product of incomplete combustion and needs a dedicated alarm. A CO₂ monitor cannot detect it. See CO₂ vs CO.
How to measure it
A monitor with an NDIR sensor measures CO₂ directly; devices that report “eCO₂” estimate it from other gases and are unreliable. Our best CO₂ monitors guide covers the choices, and the CO₂ level checker interprets a reading.
Frequently Asked Questions
Is CO₂ dangerous indoors?
Not at the levels found in occupied buildings. Health limits for CO₂ itself are in the thousands of ppm over a working day. The reason to watch it is what it indicates: stale air and everything else people exhale.
What is the outdoor CO₂ level?
About 420 ppm as a global average in 2026, a little higher in cities and near roads. That is the baseline a monitor should return to in an empty, ventilated room.
Do plants lower indoor CO₂?
Only trivially. A room full of houseplants absorbs a small fraction of what one person exhales. Ventilation moves CO₂; plants do not, in any practical sense.