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Indoor CO2 is only meaningful relative to the outdoor background, which sits near 420 ppm in most populated regions. Subtract that baseline and what remains is the ventilation signal: a room holding at 600 to 800 ppm is being flushed well, 1000 to 1500 ppm means the air is being recycled through lungs faster than it is being replaced, and anything above roughly 2000 ppm indicates a space that is barely ventilated at all. The absolute number matters less than the gap above outdoors and how quickly it falls when you open a window. A wall unit such as the Keenso wall mount CO2 monitor or the GOWENIC NDIR wall monitor gives you that number continuously; a particle counter such as the HENRUN portable particle counter answers a different question entirely and should not be read as a ventilation instrument.
Why outdoor CO2 is the number you subtract, not the number you chase
Every CO2 reading indoors is the sum of two things: the background concentration of the outside air, and whatever has been added by people, combustion and off-gassing inside the building. The background is not zero and it is not constant. In most populated regions the outdoor figure sits close to 420 ppm, having risen from a pre-industrial figure nearer 280 ppm. In a city centre on a still night it can run higher; on a windy coastal site it can run lower. A monitor reading 480 ppm in an empty, well-ventilated room is telling you about the outside air, not the room.
That is why the first useful thing you do with a CO2 meter is take it outside. Power it on, let it settle for a few minutes in fresh air away from an exhaust or a doorway, and note the figure. That number is your baseline for the day. Everything afterwards is read as a subtraction. A bedroom at 900 ppm with a 430 ppm baseline has about 470 ppm of accumulated human contribution; the same 900 ppm against a 600 ppm baseline in a polluted courtyard means the room itself is adding far less.
This baseline step also catches a common fault. NDIR sensors drift, and many domestic units offer a self-calibration routine that assumes the lowest reading it sees over a period is fresh outdoor air. If the monitor never leaves a occupied room, that assumption is wrong and the unit quietly calibrates itself to a stale value. Taking it outside occasionally, and comparing against a known outdoor figure, is how you find out whether the number on the screen is still trustworthy. There is a fuller treatment of this in why outdoor CO2 is useful for checking a monitor.
The indoor rise, and the arithmetic that turns it into a ventilation rate
People exhale CO2 at a rate that depends on their size and activity, roughly 0.005 litres per minute per kilogram at rest and several times that under exertion. In a sealed room, that exhalation accumulates. The rate of accumulation is set by the volume of the room and the rate at which fresh air replaces stale air. This is the whole mechanism, and it is why CO2 is such a good proxy for ventilation: you are not measuring air movement, you are measuring the dilution of a tracer gas that every occupant produces continuously.
The practical version of the arithmetic is a decay test. Get a room up to a high reading, note the number, open the window or start the mechanical system, and time how long it takes to fall by half. The half-life is a direct read on the air change rate. A room that halves in ten minutes is being ventilated vigorously; one that takes an hour is barely moving air at all. You do not need a formula to use this. You need a clock and a monitor that updates faster than the room changes, which is why the response time matters.
| Reading (ppm) | Approximate gap above a 420 ppm outdoor baseline | What it usually indicates |
|---|---|---|
| 420 to 500 | 0 to 80 | Fresh air, or an empty well-ventilated room |
| 600 to 800 | 180 to 380 | Good ventilation with occupants present |
| 800 to 1000 | 380 to 580 | Acceptable, but the top of the comfortable band |
| 1000 to 1500 | 580 to 1080 | Poor ventilation; occupants will notice stuffiness and reduced concentration |
| 1500 to 2000 | 1080 to 1580 | Very poor; headaches and drowsiness reported |
| Above 2000 | Above 1580 | Barely ventilated; treat as a fault in the space, not the meter |
The 1000 ppm line is the one most cited, and it is worth understanding where it comes from. It is not a health threshold in the toxicological sense; CO2 at that concentration is harmless. It is a comfort and performance threshold, and it corresponds roughly to the point at which a typical room has been rebreathing its own air enough for occupants to notice. The reasoning behind it is set out in what 1000 ppm CO2 means, and the higher band is covered in what 2000 ppm CO2 means indoors.
What a wall monitor actually shows you, and what it leaves out
A domestic CO2 monitor is a small NDIR instrument: an infrared source, a chamber, and a detector that measures how much light the CO2 in the chamber absorbs at a specific wavelength. It is a genuinely good sensing principle, selective for CO2 and stable over time. The wall units in this category combine that sensor with a display and, often, temperature and humidity. The GOWENIC wall mount monitor is a fair example of the type: an NDIR sensor with audible and visual alerts, wall mounted, tracking CO2 alongside temperature and humidity. The Keenso wall mount unit takes a different presentation route, showing four colour-coded stages of CO2 concentration rather than a single number, with a quoted response time of 1.5 seconds.
Those two design choices illustrate the trade-off in this class. A staged colour display is easy to read across a room and hard to misinterpret at a glance, but it throws away the resolution you need for a decay test. A numeric display gives you the arithmetic but asks you to remember what the numbers mean. Neither is wrong; they suit different users. What neither does is measure ventilation directly, and neither tells you anything about particles, volatile organic compounds or mould. If your concern is dust or smoke, the CO2 number is the wrong instrument, a point developed in CO2 versus PM2.5 for ventilation.
Where a listing is silent, treat it as silent. The Keenso bullets quote a temperature accuracy of plus or minus 1.0 degrees C but publish no CO2 accuracy figure; the GOWENIC bullets describe a high sensitivity NDIR sensor but publish no accuracy or range. Not stated in the listing bullets is the honest answer for both, and you should not assume a figure that is not printed.
Where a particle counter fits, and where it does not
It is easy to buy the wrong instrument here, because both categories are sold as air quality monitors. A particle counter draws a sample of air through a light scattering chamber and counts particles by size band. It tells you about dust, smoke, pollen and aerosols. It tells you nothing about CO2, and therefore nothing about ventilation in the sense this article means. The two measurements answer different questions and are not substitutes.
The portable counters in this category are built for a different job. The HENRUN portable particle counter is described for real time assessment in hospital ventilation systems, electronics workshops and post-renovation environments, with preset modes and automatic logging; that is a particulate survey tool. The generic handheld particle counter uses scattering technology and a 2000 mAh battery for on-the-go dust measurement. Neither publishes a CO2 specification, because neither measures CO2. If your question is whether a room is being ventilated, a particle counter will not answer it, however good it is at counting dust. The relationship between the size bands is covered in how to compare PM1, PM2.5 and PM10 readings.
There is one legitimate overlap. A CO2 decay test tells you the air change rate; a particle count before and after a ventilation change tells you whether the particles that were present have been cleared. Used together they describe both the dilution and the filtration. Used alone, each is half the picture.
The maintenance nobody mentions, and the point where CO2 stops being the right answer
An NDIR CO2 sensor is stable but not infallible. The two things that degrade a domestic unit are calibration drift and the assumption built into automatic baseline correction. Most units quietly assume that the lowest CO2 they see over a week is outdoor air at around 400 ppm. In a room that is never empty and never ventilated, that assumption pulls the reading down over time, and the monitor becomes optimistic exactly when you most need it to be pessimistic. The fix is simple: take it outside periodically and check it against fresh air. If it reads 800 ppm outdoors on a still day, it needs attention.
The other maintenance item is placement. CO2 mixes well in a room, but a monitor mounted above a radiator, in direct sun, or next to a sleeping person’s head will read the local condition rather than the room average. Wall units should sit at breathing height, away from heat sources and away from the direct output of a vent. The temperature correction circuitry in a unit like the Keenso helps with sensor drift, but it cannot correct for being mounted in the wrong place.
Finally, know where the measurement stops being useful. CO2 is a ventilation proxy, not a contamination measurement. It says nothing about carbon monoxide from a faulty appliance, nothing about formaldehyde from new furniture, and nothing about whether the air being drawn in is itself clean. A CO2 reading of 500 ppm in a room next to a busy road tells you the room is well ventilated and tells you nothing about what it is being ventilated with. For outdoor air quality the relevant instruments are elsewhere, as covered in the AirVisual outdoor review. And a CO2 monitor is not an alarm: it will not wake you for a gas leak, and it should never be treated as a safety device for combustion.
Reading the number rather than reacting to it
Take the outdoor baseline first, subtract it from every indoor reading, and watch the rate of change rather than the absolute figure. A room that falls quickly when a window opens is ventilated; one that holds its number is not, whatever the display says. Buy an NDIR CO2 monitor if ventilation is your question, and buy a particle counter if particles are, and do not expect either to do the other’s job.
Frequently Asked Questions
What is a normal outdoor CO2 level to compare against?
Close to 420 ppm in most populated regions, though it varies with local sources and weather. Take your own reading outside on the day rather than relying on a published figure, and use that as the baseline you subtract.
Is 1000 ppm CO2 dangerous?
No. CO2 at 1000 ppm is not toxic. The threshold is used because it marks the point where a typical occupied room has been rebreathing its own air enough for people to notice stuffiness and reduced concentration. It is a ventilation indicator, not a health limit.
Why does my monitor read high in an empty room?
Either the outdoor background is genuinely elevated, or the unit has drifted and its automatic baseline correction has pulled it off. Take it outside and let it settle; if it still reads several hundred ppm above fresh air, it needs recalibration or replacement.
Can a particle counter tell me about ventilation?
Not directly. A particle counter measures dust, smoke and aerosols, not CO2. It can show you that particles cleared after a ventilation change, but it cannot tell you the air change rate the way a CO2 decay test can.
How accurate are the CO2 figures on a wall monitor?
The listings for the wall units here do not publish a CO2 accuracy figure, so Not stated in the listing bullets is the honest answer. NDIR sensors are selective and stable, but treat the absolute number as indicative and the trend as reliable.
Will a CO2 monitor warn me about a gas leak?
No. A CO2 monitor measures carbon dioxide and is not a combustible gas or carbon monoxide alarm. It should never be used as a safety device for combustion appliances.