Why a CO2 Monitor Matters at Home (and What the 1,000 ppm Rule Really Is)

What indoor CO2 tells you

CO2 builds up in proportion to how many people are in a room and how little outdoor air reaches it. That makes it a good stand-in for stale air and body odor, which is how it has been used for over a century. ASHRAE's position document is clear about the limits: many important pollutants, such as those from building materials or from outdoors, do not track CO2, and processes that remove particles do not change CO2. So a low CO2 reading does not prove the air is clean; it shows the room is well ventilated for the people in it.

Where 1,000 ppm comes from

The 1,000 ppm figure dates to the nineteenth century, when Max von Pettenkofer proposed it as a marker of inadequate ventilation (and 700 ppm for bedrooms), according to ASHRAE's history. The 1989 edition of ASHRAE's ventilation standard included a 1,000 ppm CO2 limit, which was removed from later editions because it was so often misread. ASHRAE states that Standard 62.1 does not provide a limit value for indoor CO2, despite many claims to the contrary.

Other bodies use CO2 differently. ASHRAE notes that national indoor limits in its review tend to be around 1,000 ppm but range up to about 1,500 ppm, and that the European standard EN 16798-1 uses CO2 above outdoor levels as an indicator of ventilation per person.

CO2 reference points, from the sources named. Indoor levels in homes are far below the workplace limits.

Figure	What it is	Source
426 ppm	Mauna Loa monthly mean, September 2026	NOAA
1,000 ppm	Historical ventilation marker; no longer an ASHRAE limit	ASHRAE position document
550 / 800 / 1,350 ppm above outdoors	EN 16798-1 categories I, II and III-IV	Cited by ASHRAE
5,000 ppm	8-hour workplace limit (OSHA PEL, NIOSH REL)	NIOSH Pocket Guide
30,000 ppm	15-minute NIOSH short-term limit	NIOSH Pocket Guide
40,000 ppm	Immediately dangerous to life or health	NIOSH Pocket Guide

Does CO2 itself affect how you think or sleep?

The research is mixed. ASHRAE's review counts six studies that reported an association between CO2 and decreased cognitive performance at concentrations around 1,000 ppm, while other studies showed no cognitive effects. ASHRAE treats this as an open research question. For a home, the practical point is the same either way: high CO2 means the room needs more outdoor air, and more outdoor air also dilutes the other things people and furnishings emit.

Where a CO2 monitor earns its keep

• Bedrooms. Two people in a closed bedroom overnight are the classic case. Watch how high the reading climbs by morning with the door shut, then with the door ajar or a window cracked.
• Home offices. A small room with a closed door can rise through the workday. A reading that keeps climbing tells you to take a break and air it out.
• Kitchens with gas stoves. Burning gas also produces CO2, so the reading shows whether the range hood is pulling fumes out.
• Tight or new homes. Well-sealed homes depend on mechanical ventilation. A monitor shows whether it keeps up when the house is full.

For placement, keep the monitor at breathing height and away from your face, windows and vents; see where to place an air quality monitor.

NDIR vs eCO2: make sure it measures CO2

Sensor types, from the makers' documentation.

Sensor type	How it works	Example	Stated accuracy
NDIR	Measures infrared light absorbed by CO2	Aranet4; Airthings Wave Enhance	Wave Enhance: ±50 ppm ±5% (Airthings)
NDIR	Same principle	IQAir AirVisual Pro	±40 ppm or ±3% (IQAir)
Thermal conductivity	Infers CO2 from how air conducts heat	Atmotube PRO 2 (Sensirion STCC4)	±(100 ppm + 10%) (Atmotube)
eCO2 (metal oxide)	Calculated from VOC and hydrogen signals, not a CO2 measurement	Sensors such as Sensirion's SGP30	Not a CO2 accuracy figure

Sensirion's SGP30 datasheet says its CO2eq signal is calculated from ethanol and hydrogen measurements with internal algorithms. An "eCO2" reading rises with VOCs from cooking or cleaning even when CO2 has not changed, so it cannot tell you whether a room is ventilated. If a listing says eCO2 or CO2eq, it is not a CO2 monitor. Our guide to how monitors work covers every sensor type.

What to do when CO2 is high

1. Bring in outdoor air. Open a window a few inches, prop the door, or run a bathroom or kitchen exhaust fan vented outside.
2. Check your mechanical ventilation. If the home has an HRV, ERV or a fresh-air intake on the HVAC system, make sure it is on and its filters are clean.
3. Do not expect a purifier to help. HEPA and carbon filters do not remove CO2. CDC's NIOSH guidance counts portable air cleaners toward clean air delivery for particles, but only outdoor air lowers CO2.
4. Watch the trend, not a threshold. Since there is no ASHRAE limit, use the monitor to compare: door open vs closed, window cracked vs shut, fan on vs off.

For ranked picks, see the best CO2 monitors. If you want particles too, see the best air quality monitors, and for accuracy data, our guide to monitor accuracy.