Can I trust my CO₂ monitor? Four checks that take one evening

If your monitor has a true NDIR sensor and settles near outdoor air when you take it outside, it's accurate enough for a bedroom. Good home monitors are within about 50 to 90 ppm at 800, and the bedroom goal has a margin built in for exactly that reason. What matters more than the last few ppm is using it the same way every night.

Check 1: Is it measuring CO₂ at all?

Not every device with a CO₂ number measures CO₂. NDIR (non-dispersive infrared) sensors detect it directly, from the way CO₂ absorbs infrared light. The UK Health and Safety Executive recommends NDIR monitors as the most suitable type for checking ventilation.

Other devices estimate a CO₂-like number from other gases, usually labeled eCO₂ or CO₂eq, often next to a TVOC figure. The datasheet for one common chip of this kind, Sensirion's SGP30, says its CO₂eq signal is calculated from ethanol and hydrogen measurements. It can move for reasons that have nothing to do with fresh air, so it won't give you a fair bedroom answer.

Look for NDIR on the box, in the manual or on the product page. The Aranet4 HOME and SwitchBot Meter Pro CO₂ both use NDIR sensors.

Check 2: How close is close enough?

Makers state accuracy as a fixed amount plus a percentage of the reading. The Aranet4 lists ±(30 ppm + 3%), and the SwitchBot Meter Pro CO₂ ±(50 ppm + 5%). Here's what that means at the numbers that matter for a bedroom:

That's one reason the goal sits at 800. The 2025 review behind it found sleep measurably more disturbed from about 1,000 ppm, with no effect at 850 or below, and set the goal at 800 to allow for sensor accuracy and a safety margin. Even at the edge of its stated accuracy, a good monitor reading 800 is well short of 1,000. Why 800, and not 1,000.

Check 3: The outdoor check

Outdoor air is the one reference everyone has. Take the monitor outside to a shady spot away from people, cars and exhaust vents, set it down, step back, and give it 15 to 30 minutes.

• About 400 to 450 ppm: fine. NOAA's global average was about 428 ppm in mid-2026, but some sensors are set to treat fresh air as 400.
• Up to about 500: SwitchBot treats 400 to 500 ppm outdoors as normal for its CO₂ monitor.
• Well above 500: try a different spot first. AirGradient notes that a monitor near a busy road or an exhaust vent can sit hundreds of ppm above background. If it still reads high in clean air, follow the maker's calibration steps; SwitchBot recommends recalibrating in its app when the outdoor reading exceeds 500.

Check 4: Give a new monitor time to settle

Many CO₂ sensors correct themselves automatically by treating the lowest readings they see as fresh air. AirGradient explains that the sensor in its monitors uses a rolling window of about 180 hours, a little over a week. Aranet says the Aranet4 is calibrated at the factory, so a new one doesn't need calibrating right away; in automatic mode it needs about 8 hours of fresh air near 420 ppm in each 30-day period, and Aranet recommends calibrating once a year.

So a brand-new monitor may read a little high or low for its first week or two. Self-correction works best when the monitor sees fresh air now and then, for example when the room is aired out during the day. You can start measuring right away; run the outdoor check, and if something looks odd, check again after a week.

Why two monitors disagree

Put two good monitors side by side and they'll rarely match exactly. SwitchBot's own help page says "different brands use different sensors and technologies, so direct comparisons may not provide meaningful results." The accuracy figures above add up too: at 800 ppm, an Aranet4 and a SwitchBot can both be within spec and still be about 140 ppm apart.

If you want to compare them fairly, do it outdoors, side by side. SwitchBot suggests leaving at least 1 cm between the devices and letting them sit undisturbed for five minutes before reading. Comparing one on the dresser with another by the pillow tells you about placement, not accuracy.

What matters more than accuracy

Placement and consistency. A monitor beside your pillow catches your breath directly; HSE notes that breath close to the sensor can make readings misleadingly high. Keep it at about head height, at least 2 ft (60 cm) from anyone's face, about 3 ft (1 m) from windows, doors and vents, and out of direct sun. Where to put your monitor.

Then use the same monitor in the same spot every night. If yours reads a little high, it reads a little high every night, so a before-and-after comparison still shows whether a change helped. That's how HouseExhale compares nights: same room, same monitor, one change at a time, each night scored by its average while you slept.