Carbon dioxide (CO2)

AIR

Under 800 ppm sleep target (bedroom overnight)

CO2 reads how well your room is breathing, and a closed bedroom door is usually the whole story.

A comfort parameter, not a contaminant

Stasis Home Research Every claim on this page is graded against peer-reviewed health research and linked to its primary source.Updated July 2026

Carbon dioxide (CO2) is not dangerous at household levels, but it is a readout of how well a room is breathing. Aim for under 800 ppm, or parts per million, in the bedroom overnight and under 600 ppm where you work; the sleep studies show deep sleep degrading above roughly 920 ppm. A closed-door bedroom with two sleeping adults routinely passes 1500 ppm by morning. The fix is mostly free: open the door, crack a window on clean-air nights.

Carbon dioxide (CO2): what is it, and where does it come from?

Carbon dioxide (CO2) is a colorless, odorless gas produced by normal metabolism in every breathing person, and in a home it comes from the people in it. Indoor CO2 is not driven by outdoor CO2. It is driven by occupants breathing in a space against the rate at which outdoor air comes in to dilute it. That is why ASHRAE, EPA, WHO, and the wider building-science consensus treat indoor CO2 as a ventilation marker rather than a regulated pollutant. At the levels found in homes, very roughly 400 to 3000 ppm, CO2 is not a contaminant in the toxicological sense. It does not damage tissue, does not build up in the body, and is not a foreign substance the way lead or PFAS are. Your lungs and kidneys clear it. The reason it matters now is the building, not the molecule. Since the 1970s, homes have been sealed progressively tighter for energy efficiency, and ventilation systems did not scale in proportion. The air leaks out more slowly than it used to, so the CO2 you breathe out accumulates faster.

Carbon dioxide (CO2): why does it matter?

Carbon dioxide matters at home because of sleep and focus, not toxicity. It is harmless to your tissues at household levels, but it diffuses freely into the brain and shifts the chemistry that regulates your breathing drive and blood flow. During sleep that shows up as degraded sleep architecture: the slow-wave deep sleep that does the heavy lifting on recovery shrinks, sleep efficiency drops, and you wake more. In one study, deep sleep fell by roughly 4 percent for every 100 ppm of overnight CO2. Those numbers sit exactly where a real bedroom lands. The outdoor air you would breathe on a walk is about 431 ppm, while a closed-door master bedroom with two sleeping adults in a post-2008 build commonly reads 1500 to 2200 ppm by 5 or 6am. That is the gap, and it is why people wake up groggy in a room that felt fine when they went to bed. For daytime focus, the laboratory cognition work points lower still, toward keeping occupied work spaces under 600 ppm, though that evidence carries named caveats and we treat it as aspirational rather than settled.

A controlled bedroom study stepping CO2 through roughly 680, 920, and 1350 ppm found the deep slow-wave fraction of sleep dropped at each step, from about 20 percent down to 14 percent, with subjective sleep quality and breathing comfort declining alongside it.Wang et al., 2023, The Influence of Bedroom CO2 Concentration on Sleep Quality (Buildings, MDPI)
In a simulated-bedroom trial, sleep efficiency fell and time awake rose as CO2 climbed from 750 to 1300 ppm, and at 1300 ppm deep sleep dropped while a salivary stress hormone increased.Kang et al., 2024, CO2, sleep efficiency, and cortisol in simulated bedrooms
A field study of Danish bedrooms during the heating season recorded a median overnight CO2 of about 1120 ppm, with subjective sleep quality tracking perceived air quality and improving when occupants opened a window or door.Liao et al., 2022, Bedroom ventilation and sleep quality in Denmark during the heating season (Build Environ 224:109557)
The 2022 ASHRAE Position Document on indoor CO2 states that indoor CO2 concentrations are not overall indicators of air quality, but can be a useful tool when their limitations are understood, framing CO2 as a ventilation proxy rather than a regulated contaminant.ASHRAE Position Document on Indoor Carbon Dioxide, 2022

Carbon dioxide (CO2): what do we grade it against?

Carbon dioxide (CO2) has a health-based level but no enforceable legal limit. The health-based level for Outdoor baseline (ppm) is ~431 (the physical floor; indoor air cannot drop below outdoor), from NOAA Mauna Loa, April 2026. The table below lists every level on this page next to the source that set it.

Where the health research draws the line, versus the legal limit.
Contaminant	Health-based level	Legal limit	Source (health-based)
Outdoor baseline (ppm)	~431 the physical floor; indoor air cannot drop below outdoor	none set	NOAA Mauna Loa, April 2026
Sleep target, bedroom overnight (ppm)	Under 800 Stasis operational target; margin below where deep-sleep and efficiency declines first appear in studies	none set	derived from Wang 2023 and Kang 2024
Cognition target, occupied work periods (ppm)	Under 600 aspirational; matches the WELL v2 high-performance threshold	600 CA Title 24-2025 adopted 600 ppm for nonresidential and multifamily common areas only, not for the bedroom you sleep in	WELL Building Standard v2 (A08)
Acceptable ceiling (ppm)	Under 1000 the institutional comfort threshold; WHO recommends staying below it	none set	WHO Indoor Air Quality guidance, 2023
Workplace limit, 8-hr (ppm)	5000 asphyxiation and metabolic-stress limit, not a comfort threshold; roughly 5x above where the sleep effects begin	5000 OSHA permissible exposure limit, 8-hour TWA	OSHA Chemical Sampling, Carbon Dioxide

Health-based levels come from peer-reviewed research and government risk scientists working without cost constraints, while legal limits are enforceable compromises. Your report grades to the health column.

What helps against Carbon dioxide (CO2)?

5 approaches below reduce Carbon dioxide (CO2) in a home, ordered from the most direct fix to the smallest change in habit. The most direct is Open the bedroom door at night. Each entry also names what it cannot do, because no single fix covers every exposure route.

Bigger retrofits

Mechanical ventilation with filtered intake (ERV/HRV)
For a home with chronically severe overnight CO2 and chronically poor outdoor air, a heat- or energy-recovery ventilator with a MERV-13 or HEPA-grade intake filter resolves the open-window tension structurally, bringing in filtered outdoor air continuously.
This needs an HVAC contractor, and we do not install it.

Free and behavioral

Open the bedroom door at night
This is the single largest lever in the overnight math. A closed door isolates a small pocket of air from the rest of the home's ventilation; opening it, or cracking it six inches or more, roughly quadruples the air the room can draw on and commonly pulls a tight bedroom back under 800 ppm.
Only works if the rest of the house is itself ventilated. It moves the problem to the home's whole-house exchange rate, which is the next constraint.
Crack a window when outdoor air is clean
Outdoor air exchange is the only thing that lowers CO2, and on a clean-air night a one-inch crack on a quiet street produces a dramatic overnight improvement. We tie this to the nearest regional PM2.5 reading so it is a clean-day recommendation.
On smog, wildfire, or high-particle days this pulls outdoor PM2.5 and PM10 indoors. On those nights the call reverses: close the windows, run the bedroom HEPA, and accept the higher CO2 rather than the particle load.
Run the bedroom HEPA you already have for PM2.5
A HEPA unit running low to medium overnight moves and mixes room air, which supports the open-door effect and reduces stratification. You already run it for particles, so it costs nothing new.
The HEPA does not remove CO2. No consumer filter does. It helps only by circulating air, never by scrubbing the gas, and on its own with the door shut it will not bring the level down.
Morning cross-ventilation flush
Opening windows on opposite sides of the house for 15 to 30 minutes before the AC takes over clears the overnight buildup before the day starts. The reliable clean window is whatever time the outdoor particle reading is lowest; in coastal areas, for example, the morning marine layer often keeps outdoor particles down.
Depends on operable windows and a layout that cross-ventilates. In dense newer subdivisions where windows face a neighbor's wall, the air may not move, and the recommendation has to adapt to what the home can do.

See how well each fix works

Method

Mechanical ventilation (ERV / HRV)

The molecule is fine. CO2 is not poisoning you. What it tells you is whether your room is breathing, and a closed bedroom door in a sealed new build is usually the whole story behind a groggy morning. Open the door, and crack a window when the outdoor air is clean. California now requires a ventilation system in your new home and verifies its airflow, but it does not require anyone to measure the CO2 that system actually leaves in your bedroom. Your home is the unit of control you have, and most of the control here costs nothing.

Common questions about Carbon dioxide (CO2)

Is CO2 in my home dangerous?

Carbon dioxide in your home is not dangerous at the levels a house reaches. CO2 does not damage your tissues or build up in your body the way lead or PFAS do; it is the gas you exhale, and your lungs and kidneys clear it. The concern at home levels is sleep and focus. Elevated overnight CO2 is a signal that the room is poorly ventilated, and the studies link that to lighter, less restorative sleep.

Why is my bedroom CO2 so high in the morning?

Bedroom CO2 peaks in the morning because the door was closed all night and the room is sealed tighter than older homes were. A closed-door bedroom with two sleeping adults in a post-2008 build commonly climbs to 1500 to 2200 ppm by 5 or 6am, against an outdoor baseline near 431 ppm. The CO2 you breathe out accumulates faster than the slow-leaking modern envelope can dilute it. Opening the door, which roughly quadruples the air the room can draw on, is usually enough to bring it back under 800 ppm.

Will a houseplant or an air purifier lower my CO2?

No. Neither houseplants nor air purifiers lower indoor CO2. Per-plant CO2 uptake is dwarfed by what a sleeping person breathes out, so plants are decorative. A HEPA purifier does not remove CO2 either; no consumer filter does. The only thing that lowers indoor CO2 is more outdoor air exchange, which is why the recommendations are about doors and windows, not devices.

What is a good indoor CO2 level, and what counts as safe?

A good indoor CO2 level is under 800 ppm in a bedroom overnight and under 600 ppm in the spaces where you work. Read those as a ladder above the outdoor baseline of about 431 ppm, the floor a room can physically reach. The 800 ppm bedroom number is our operational sleep target, set to keep margin below where the studies first see trouble; the 600 ppm number matches the WELL high-performance threshold and is aspirational. Between roughly 920 and 1000 ppm, deep sleep and sleep efficiency start to slip, and 1000 ppm is the comfort ceiling the WHO recommends staying under. California adopted that same 600 ppm target for commercial and shared multifamily spaces this year, but explicitly not for the bedroom you sleep in.

Is 1000 ppm of CO2 bad indoors?

1000 ppm indoors is not dangerous in any toxic sense, but it is above where you want a room you sleep or work in. It sits right at the comfort ceiling the WHO recommends staying under, and it is where the sleep evidence starts to show up: in a simulated-bedroom trial, sleep efficiency fell and time awake rose once CO2 reached 1000 ppm. Our bedroom sleep target is under 800 ppm to hold margin below it. Treat 1000 ppm not as harmful but as a signal the room needs more outdoor air, which for most people means opening the door.

CO2 says open the window but PM2.5 says close it. Which is right?

Which call is right depends on the outdoor air that day, and we tie the window recommendation to the nearest regional PM2.5 reading. On clean-air nights, crack the window and accept the air exchange to clear the CO2. On smog, wildfire, or high-particle days, close the windows, run the bedroom HEPA, and accept the higher overnight CO2 rather than pull outdoor particles indoors. If both problems are chronic, that is the case for a mechanical ventilator with a filtered intake.

What is a normal CO2 level in a house?

A normal house runs very roughly 400 to 3000 ppm of CO2, depending on how many people are inside and how much outdoor air is getting in. The floor is the outdoor baseline of about 431 ppm, which indoor air cannot drop below. Occupied rooms sit above it, and a closed-door bedroom with two sleeping adults commonly reaches 1500 to 2200 ppm by morning. Normal is not the same as good: our bedroom target is under 800 ppm overnight.

Does CO2 affect sleep quality?

Yes, and sleep is the best-replicated effect at household CO2 levels. A controlled bedroom study stepping CO2 through roughly 680, 920, and 1350 ppm found the deep slow-wave share of sleep dropping at each step, from about 20 percent to 14 percent, with subjective sleep quality declining alongside it. A separate simulated-bedroom trial saw sleep efficiency fall and time awake rise as CO2 climbed from 750 to 1300 ppm. One field study measured roughly 4 percent less deep sleep for every 100 ppm of overnight CO2.

Does CO2 make you tired?

Elevated bedroom CO2 is linked to worse sleep, and worse sleep is why people wake up groggy, but the evidence for a direct next-day effect is thinner than the sleep evidence itself. Controlled and field studies consistently show deep sleep and sleep efficiency dropping as overnight CO2 rises. A single-blind bedroom ventilation trial found that same shorter deep sleep in the low-ventilation condition but no measurable next-day cognitive difference. The evidence links CO2 to worse sleep, and we say so when a study finds no next-day effect.

Does opening a window lower CO2?

Yes. Outdoor air exchange is the only thing that lowers indoor CO2, and on a clean-air night a one-inch crack on a quiet street produces a dramatic overnight improvement. It cannot push a room below the outdoor baseline of about 431 ppm, which is the floor. The real limit is the outdoor air, not the window: on smog, wildfire, or high-particle days, opening it pulls PM2.5 and PM10 indoors, and the better call is to close up and run the bedroom HEPA.

What CO2 level is dangerous?

The level treated as hazardous is 5000 ppm averaged over eight hours, the OSHA workplace limit, and it is set for asphyxiation and metabolic stress rather than comfort. Homes do not reach it. That limit sits roughly five times above where the sleep effects begin, which is the point: the numbers a real bedroom produces, 1500 to 2200 ppm by morning, describe a ventilation problem, not a poisoning problem.

How do I measure CO2 at home?

Measure CO2 with an NDIR sensor, the non-dispersive infrared type, which reads the gas directly and holds its factory calibration for years. Put it in the master bedroom first and read the overnight curve rather than a single spot number, since the peak arrives around 5 or 6am. Avoid cheap monitors that estimate CO2 from total VOCs instead of measuring it; those are not CO2 readings. Consumer sensors also self-calibrate against their lowest recent reading, so in a home that never airs out they drift low. Taking the sensor outdoors for 10 to 15 minutes resets that reference.