What a CO₂ monitor can (and can't) tell you about your home

A CO₂ monitor tells you one thing well: how much outside air is reaching a room while people are in it. It doesn't measure every pollutant, it isn't a mold test, and it isn't a medical test. For a bedroom, there's now a clear number to compare it with: 800 ppm or less, on average, while you sleep.

Why CO₂ tracks fresh air

Every time you breathe out, you add a little carbon dioxide (CO₂) to the room. Outside air coming in through windows, doors, gaps and ventilation systems dilutes it and carries it away. When a room gets plenty of outside air for the number of people in it, CO₂ stays fairly close to outdoor levels. When it doesn't, CO₂ builds up.

That makes CO₂ a handy stand-in for ventilation. ASHRAE, the engineering society behind US ventilation standards, calls indoor CO₂ "at best an indicator of outdoor air ventilation rate per person." The US EPA puts it in similar terms: indoor CO₂ can give you information about ventilation, but it needs to be interpreted carefully.

Outdoor air is the starting point. Global outdoor CO₂ passed 400 ppm (parts per million) in 2013 and reached about 426 ppm in 2024, according to ASHRAE; NOAA's monthly readings at Mauna Loa were around 428 ppm in mid-2026. For everyday purposes, think of outdoor air as about 425 ppm and slowly rising.

What it can't tell you

A CO₂ number is narrow on purpose. Keep these limits in mind.

• It doesn't measure other pollutants. ASHRAE notes that indoor CO₂ is "not necessarily well correlated" with pollutants such as mold, formaldehyde, carbon monoxide and airborne particles. Many of those come from sources that have nothing to do with how many people are in the room, so a room can have low CO₂ and still have other air problems.
• It isn't a mold test. A low reading doesn't mean a room is free of mold, and a high one doesn't mean it has any. Visible mold or a musty smell needs its own investigation.
• It isn't a carbon monoxide alarm. CO₂ and carbon monoxide (CO) are different gases. A CO₂ monitor won't warn you about CO from a furnace, water heater or attached garage. The US Consumer Product Safety Commission recommends CO alarms on each level of the home and outside sleeping areas.
• It isn't a medical measurement. ASHRAE's position document describes the evidence for direct effects of CO₂ itself on health, sleep and performance at commonly observed indoor levels as "inconsistent." Sleep studies do find people sleep a little worse in poorly ventilated bedrooms, but that's about the fresh air the number stands for. A CO₂ reading describes the room, not the people in it.

The UK Health and Safety Executive (HSE) sums it up well: CO₂ measurements are "only a broad guide to ventilation rather than demonstrating 'safe levels'."

True CO₂ sensors and estimated ones

Not every device that shows a CO₂ number actually measures CO₂. There are two main kinds, and the difference matters.

NDIR sensors (non-dispersive infrared) detect CO₂ directly, using the way CO₂ absorbs infrared light. HSE recommends NDIR monitors as the most suitable type for checking ventilation.

Estimated CO₂ sensors measure other gases and calculate a CO₂-like number from them. The reading is usually labeled eCO₂, CO₂eq or "equivalent CO₂", often next to a TVOC (total volatile organic compounds) figure. The datasheet for one common chip of this type, Sensirion's SGP30, says its CO₂eq signal is calculated from ethanol and hydrogen measurements. Because the number is derived from other gases, it can move for reasons that have nothing to do with fresh air.

If your monitor's box, manual or listing doesn't mention NDIR, check before you rely on its CO₂ readings. HouseExhale's room reports assume a true CO₂ sensor.

What the numbers can show you

With a true sensor, two things are worth watching: how far the room climbs above outdoor air while people are in it, and whether that happens night after night. Lower means more outside air per person. Higher means less.

Pattern matters more than the peak. HSE cautions that single or "snapshot" readings can be misleading, because the number changes with how many people are in the room, what they're doing and whether windows or doors are open. A few nights of readings, taken the same way each time, tell you far more than one glance at the screen.

The number to aim for in a bedroom

For a bedroom, aim for 800 ppm or less, averaged over the hours you're asleep. That's the recommendation of a 2025 review of 17 sleep studies carried out for ASHRAE: sleep was measurably more disturbed once bedroom CO₂ reached about 1,000 ppm, and no effect showed at 850 or below. HouseExhale scores each night against that goal. Why 800, and not 1,000 explains the numbers, and the research behind the goal goes study by study.

Getting trustworthy readings

A few habits make any monitor more useful:

• Keep it at about head height and at least 2 ft (60 cm) from anyone's face, since breath near the sensor pushes the number up.
• Keep it out of direct drafts from windows, doors and vents.
• Use the same spot every time, so one night can be compared with the next.
• Write down who was in the room and whether the door and window were open or shut.

Do that, and a CO₂ monitor becomes a simple, honest answer to one question: is this room getting enough outside air for the people using it?