Carbon dioxide (CO2)

AIR

Under 800 ppmsleep target (bedroom overnight)

CO2 is not a poison. It is a direct readout of how well your room is breathing, and a closed bedroom door is usually the whole story.

A comfort parameter, not a contaminant

Stasis Home ResearchEvery claim on this page is graded against peer-reviewed health research and linked to its primary source.Updated 2026-06

Indoor CO2 is the gas you exhale, and its level tracks one thing: how much outdoor air is reaching the room against how much you are breathing out. It is not a contaminant at home concentrations. What it signals is ventilation. A closed-door bedroom with two sleeping adults routinely climbs past 1500 ppm by morning in a tight modern build, and the sleep evidence says deep sleep starts measurably degrading above roughly 920 ppm. The fix is mostly free: open the door, crack a window when the outdoor air is clean.

What it is, and where it comes from

Carbon dioxide is a colorless, odorless gas produced by normal metabolism in every breathing person. At the levels found in homes, very roughly 400 to 3000 ppm, it 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. Indoor CO2 is also not driven by outdoor CO2. It is driven by people breathing in a space against the rate at which outdoor air comes in to dilute it, which is why ASHRAE, EPA, WHO, and the wider building-science consensus treat indoor CO2 as a ventilation marker rather than a regulated pollutant. 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.

Why it matters

CO2 is harmless to your tissues at home levels, so the concern is sleep and focus, not toxicity. The gas diffuses freely into the brain and shifts the chemistry that regulates your breathing drive and blood flow, and during sleep that shows up as degraded sleep architecture. Controlled bedroom studies find that as overnight CO2 rises, the slow-wave deep sleep that does the heavy lifting on recovery shrinks, sleep efficiency drops, and you wake more. One field study put the effect at roughly 4 percent less deep sleep 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. 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

What we grade it against

Where the health research draws the line, versus the legal limit.
Contaminant	Health-based level	Legal limit	Source (health-based)
Outdoor baseline (ppm)	~431the physical floor; indoor air cannot drop below outdoor	none set	NOAA Mauna Loa, April 2026
Sleep target, bedroom overnight (ppm)	Under 800Stasis 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 600aspirational; matches the WELL v2 high-performance threshold	600CA 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 1000the institutional comfort threshold; WHO recommends staying below it	none set	WHO Indoor Air Quality guidance, 2023
Workplace limit, 8-hr (ppm)	5000asphyxiation and metabolic-stress limit, not a comfort threshold; roughly 5x above where the sleep effects begin	5000OSHA 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. Legal limits are enforceable compromises. Your report grades to the health column.

What helps

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 is a capital retrofit in HVAC-contractor territory, not the Stasis default lane. We name the category for households whose readings warrant it and refer out; we do not push the install.

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. It is already in the recommendation set for particles, so it is a reuse, not an addition.
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.

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. The fix is mostly free. Open the door, crack a window when the outdoor air is clean, let the HEPA you already own move some air. 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

Is CO2 in my home dangerous?

Not at the levels found in homes. 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, not toxicity. 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?

Because the door was closed 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. Per-plant CO2 uptake is dwarfed by what a sleeping person breathes out, so plants are decorative, not mitigation. 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 number should I aim for?

For sleep, under 800 ppm in the bedroom overnight, which leaves margin below where deep sleep and sleep efficiency first start to slip in the studies, around 920 to 1000 ppm. For focused daytime work, under 600 ppm is the aspirational target, matching the WELL high-performance threshold. California adopted that same 600 ppm for commercial and shared multifamily spaces this year, but explicitly not for single-family bedrooms.

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

It 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.