What Is CO₂? Understanding Carbon Dioxide Levels Indoors
CO₂ is one of the easiest air quality metrics to measure indoors, and one of the most useful — not because it’s dangerous at typical indoor levels, but because it tells you how well a space is being ventilated.
What Is CO₂, and Why Do We Track It Indoors?
Carbon dioxide is a natural part of the air we breathe, and outdoor levels sit at a fairly stable baseline. Indoors, CO₂ rises simply because people exhale it — the more people in a space, and the less fresh outdoor air being brought in, the higher the concentration climbs.
That relationship is exactly what makes CO₂ useful as a proxy measurement. It doesn’t directly measure other pollutants like PM2.5 or VOCs, but a rising CO₂ level is a reliable sign that a room isn’t getting enough fresh air — which usually means other pollutants are building up too.
Illustrative ranges — actual levels vary with occupancy, room size, and ventilation rate.
What Do the Numbers Actually Mean?
- ~420 ppm — current global outdoor background level.
- 600–800 ppm — generally indicates a room is getting adequate fresh-air exchange for its occupancy.
- 1,000–1,200 ppm — common in occupied offices and classrooms; not acutely harmful, but often associated with reports of stuffiness and reduced alertness.
- 2,000 ppm and above — suggests ventilation is struggling to keep up with occupancy; a signal to increase fresh-air supply, not a health emergency in itself at these levels.
CO₂ Is a Ventilation Indicator, Not a Toxicity Alarm
At the concentrations typically found indoors — even in poorly ventilated rooms — CO₂ itself is not considered acutely toxic. Its real value is as an early, easy-to-measure signal that a space needs more fresh air, well before other pollutants like VOCs or particulate matter become a bigger concern.
What Raises Indoor CO₂ Levels?
- Occupancy — more people breathing in the same space raises CO₂ faster.
- Room size and ceiling height — smaller volumes concentrate CO₂ more quickly for the same number of occupants.
- Ventilation rate — how much outdoor air is being exchanged with indoor air, whether through mechanical systems or open windows.
- Time — CO₂ builds up gradually over a meeting, class period, or overnight in a closed bedroom.
How Is CO₂ Measured?
Most consumer and commercial indoor CO₂ monitors use non-dispersive infrared (NDIR) sensors, which are generally considered accurate and stable for this purpose. Cheaper sensors sometimes estimate CO₂ indirectly from other measurements (VOC-based estimation), which tends to be far less reliable — when precision matters, an NDIR-based monitor is the more dependable choice.
What Should You Do With CO₂ Readings?
- Use it as a trend, not a single snapshot. Watching how CO₂ rises and falls over a meeting or a school day tells you more than one isolated reading.
- Increase ventilation when levels climb. Opening a window, adjusting HVAC settings, or reducing occupancy density can bring levels back down.
- Pay attention in bedrooms. Closed doors and windows overnight are a common source of surprisingly high CO₂ by morning.
- Don’t treat CO₂ as a complete air quality score. It says nothing directly about PM2.5, VOCs, or humidity — those need their own measurements.
Key Takeaways
- Indoor CO₂ rises with occupancy and falls with better ventilation, making it a useful proxy for how fresh a space’s air is.
- Outdoor air sits around 420 ppm; well-ventilated indoor spaces are typically 600–800 ppm.
- Levels above roughly 1,000–1,200 ppm are common but often correlate with stuffiness and reduced alertness.
- CO₂ itself isn’t acutely dangerous at typical indoor levels — it’s valuable mainly as an early ventilation signal.
- NDIR sensors are the more reliable technology for accurate CO₂ monitoring.
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Continue Exploring
Continue learning: Learn how NDIR CO₂ sensors work, what affects their accuracy, and how calibration works, then explore the NDIR CO₂ sensor catalog.
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