A CO₂ monitor spikes when people enter because exhaled breath is about 40,000 ppm, roughly a hundred times room air. Someone walking past or talking near the sensor sends a plume of breath over it and the reading jumps by hundreds of ppm within seconds. That spike is real CO₂ but it is not the room’s concentration. A monitor placed a meter or more from people, away from the door, reads the room instead.
Two different things a monitor sees
The breath plume. Every exhalation is a cloud of warm, moist, CO₂-rich air that spreads a meter or two before it mixes into the room. A monitor inside that radius reads the plume: fast rise, fast fall, repeating with each breath. It is most obvious when someone leans over the monitor to look at the number.
The room. Once breath has mixed with the room air, the concentration rises slowly and steadily, a few ppm per minute per person, and falls just as slowly when ventilation improves. This is the number you want.
Telling them apart
- A spike that arrives within seconds of someone approaching and fades within one to three minutes of them moving away is breath.
- A rise that continues over 20 to 30 minutes after a group sits down is occupancy.
- A reading that changes when you lean in to read it is telling you about your own breath.
Monitors with a history graph make this obvious: breath spikes look like sharp needles, occupancy looks like a ramp.
Other sources of sudden jumps
- Door opening from a stuffier room. A hallway or adjacent room at 1,500 ppm pushes a slug of stale air in.
- Warm-up. Some sensors read high or low for the first minute or two after power-on.
- Sensor sampling interval. Monitors that sample every 5 or 10 minutes can appear to jump because they skip the in-between values.
- Kettles, gas hobs and candles. Combustion produces CO₂ and a monitor in a kitchen sees the hob directly.
Placement that reads the room
- Breathing height, roughly 1 to 1.5 m from the floor.
- At least a meter from the nearest chair, bed or standing spot. A desk monitor should be at the far edge of the desk, not beside the keyboard.
- Away from the door swing and from supply vents and windows, which deliver fresh air before it has mixed.
- Not in direct sun or on a warm appliance; temperature affects some sensors.
In a classroom or meeting room, an interior wall halfway along the room is usually representative. The best CO₂ monitors guide includes placement notes for each pick, and CO₂ monitor accuracy explained covers why placement error dwarfs sensor error.
Using the spike deliberately
A breath spike is a free functionality test. Breathe gently toward the monitor from arm’s length: the reading should rise within a few seconds and recover within a few minutes. A monitor that does not react is either estimating CO₂ from a VOC sensor, still warming up, or faulty. See how fast a CO₂ monitor should respond.
Recommended Tools
Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.
Frequently Asked Questions
How much CO₂ is in exhaled breath?
About 35,000 to 45,000 ppm, roughly 4 percent. A single breath directed at a monitor from a meter away can raise its reading by several hundred ppm for a minute or two.
Is a spike when people arrive a sign the monitor is faulty?
No. A short spike that settles within a few minutes is breath reaching the sensor. A sustained climb over the following half hour is the real effect of extra people on the room.
Where should the monitor go to avoid spikes?
On a shelf or interior wall at about 1 to 1.5 m height, at least a meter from the nearest seat and away from the door swing, vents and windows.
Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.