Of all the numbers an indoor air monitor can show you, CO₂ is one of the most useful. That's not because it's dangerous at typical indoor levels, but because it's one of the clearest signs of how well a room is ventilated.
Where indoor CO₂ actually comes from
Every person in a room exhales carbon dioxide. In a well-ventilated space, that CO₂ mixes with fresh air coming in and gets carried back out. In a closed room with several people, or a bedroom overnight with the door and windows shut, it simply has nowhere to go. It builds up steadily, hour by hour, until a window opens or a door lets fresh air back in.
That makes CO₂ less a pollutant to fear and more a stand-in for something that's much harder to measure directly: how much fresh air is reaching a room, and how quickly it's being replaced.
Roughly what the numbers mean
Outdoor air typically sits at around 400 to 450 parts per million (ppm). Indoors, the number climbs from there depending on occupancy and ventilation. As a general guide:
| About 400–800 ppm | Close to outdoor levels. A sign of good ventilation for the number of people in the room. |
|---|---|
| About 800–1,200 ppm | Common in an occupied room with average ventilation. Not alarming, but a reasonable point to consider airing the room out. |
| About 1,200–2,000 ppm | Often linked with the "stuffy room" feeling. People notice it at different levels, but it's a good time to let fresh air in. |
| Over 2,000 ppm | A clear sign the room needs fresh air. Sustained levels this high are usually a ventilation problem worth fixing, not just airing out once. |
These bands are a general guide, not a medical threshold. People vary in how they notice CO₂ build-up, and the point where a room feels stuffy isn't the same number for everyone.
Why it climbs faster than you'd expect
A small, well-sealed bedroom with the door shut and two people asleep in it can raise CO₂ surprisingly quickly overnight. Modern, well-insulated homes are good at keeping heat in, which also means they're good at keeping stale air in. A home office with the door closed on a video call, or a meeting room after an hour with several people in it, follows the same pattern on a shorter timescale.
What to actually do when it rises
- Open a window, even briefly. A few minutes of cross-ventilation resets CO₂ far faster than most people expect.
- Crack a door as well as a window if you can, to let air actually flow through rather than just in.
- Look at trickle vents and extractor fans. If a room consistently runs high regardless of how often you air it out, that's a sign the room's baseline ventilation may need attention, not just a one-off airing.
- Use the trend, not just the number. A CO₂ reading that's climbing steadily tells you more than a single high reading — it shows you the room isn't keeping up with however many people are in it.
Why we put it front and centre
CO₂ is the main reading on the Atmosense Core CO₂. It's measured with an NDIR sensor and shown in real time alongside temperature and humidity, with readings and alerts on your phone over Wi-Fi. It's one of the few readings that responds quickly and visibly to something as simple as opening a window, which makes it a genuinely useful number to glance at through the day.
Watch your CO₂ in real time
The Atmosense Core CO₂ shows CO₂, temperature and humidity on one display, with phone alerts when a room needs fresh air.
Explore Core CO₂