CO₂ levels in a bedroom: when the number is too high

CO₂ levels in a bedroom are too high once they pass about 1,000 ppm, and a bedroom getting enough outdoor air stays under 800 ppm. The air outside sits near 428 ppm, so a closed bedroom is often running two to five times the outdoor number by morning. That climb is why a room can feel fine at 11pm and closed in by 3am with nothing in the house having changed except a shut door.

The rest of this post is where those two thresholds come from and what to try when the line keeps climbing. If you would rather see the readings in your own rooms than read about them, that is what a kit for homeowners does.

Key Takeaways

• Under 800 ppm is the figure the CDC names as a target for a room getting enough outdoor air. Past roughly 1,000 ppm of CO₂, most people start calling a bedroom stuffy.
• The air outside is about 428 ppm, which is why ASHRAE frames the indoor number as a difference: about 700 ppm above outdoors.
• In a study of single-occupancy bedrooms, rooms averaged 2,395 ppm of CO₂ overnight with the intake fan off and 835 ppm with it running.
• An open bedroom door is the cheapest test available, and one night of readings settles whether it works in your house.
• CO₂ is a ventilation reading, not an air quality score. It says nothing about PM2.5, radon or VOCs.

What counts as a high CO₂ level in a bedroom

Four readings cover almost everything you will see on the bedroom line:

• About 428 ppm is the air outside. NOAA's Global Monitoring Laboratory put the global monthly average at 427.62 ppm in June 2026, and a room with a window open lands close to it.
• Under 800 ppm is the figure the CDC names as one target for a space getting enough outdoor air, with the caveat that a single number does not fit every room.
• 1,000 ppm and up is the long-standing line for a room worth looking at. Past roughly 1,000 ppm most people start describing a bedroom as stuffy.
• 2,000 ppm and up is common in a bedroom with the door and window shut and one or two people in it all night.

ASHRAE's position document on indoor carbon dioxide explains where the 1,000 ppm figure came from, and it was never set as a hazard level. It stood in for odors and other pollutants that were hard to measure, and ASHRAE Standard 62.1 now frames the target as a difference from outdoors: a steady-state concentration no more than about 700 ppm above the outdoor air. With outdoor air at 428 ppm, that lands around 1,130 ppm indoors. Either way, a high reading in a bedroom means the room is not getting much fresh air, and nothing more than that.

Why bedroom CO₂ climbs overnight

A bedroom is a small volume with a shut door for eight hours and someone in it breathing the whole time. The CO₂ has nowhere to go, so the line rises through the night, flattens out when what you add matches the little that leaks past the door and window frames, and drops when someone opens the door in the morning.

The effect has been measured directly. In two field experiments in single-occupancy student bedrooms, published in Indoor Air, overnight CO₂ averaged 2,395 ppm when an intake fan was disabled and 835 ppm when it ran. In the pilot experiment, rooms averaged 2,585 ppm with the window shut against 660 ppm with it open. Measured sleep quality, the reported freshness of the air, next-day sleepiness and performance on a logic test all improved at the lower readings.

Two people in the room roughly doubles the source. A cat on the bed adds a little. An ensuite with its door left open adds volume and helps slightly.

Where to put the monitor in a bedroom

Put it at roughly bed height, on a nightstand or a shelf, and a few feet from your face. Breathe directly at a CO₂ monitor and it will read several thousand ppm, which tells you about your breath rather than about the room.

Two more placement rules:

• Keep it out of the window draft and off radiators and heat vents. Both give you the air moving past the monitor instead of the air in the room.
• Keep it out of closets and out from behind the door. You want the space you sleep in.

What you are after is the shape of the line rather than one reading. A normal bedroom trace is flat through the day, starts climbing within an hour of the door shutting, plateaus somewhere between 2am and 5am, and falls off a cliff when the door opens. Measured Air keeps every reading, so you can hold last night up against last month and see whether a change stuck. That history is the part a handheld meter cannot give you, and it is the whole idea behind the line on the home page: the air outside is measured and the air you breathe isn't.

What to try when the bedroom CO₂ level is too high

Work cheapest first, and give each change one full night before you judge it.

1. Open the bedroom door. It costs nothing and needs no equipment, and it moves the overnight line more than anything else on this list. The bedroom stops being a sealed box and starts sharing air with the rest of the floor.
2. Crack a window an inch. Check the outdoor reading before you do. On a smoke day or in pollen season, trading 1,500 ppm of CO₂ for a jump in PM2.5 is a poor swap, which is why an indoor number is only useful next to the air outside your own address.
3. Run an exhaust fan. A bathroom or hallway fan on a timer pulls stale air out and pulls outdoor air in through wherever the house leaks. It is noisy, which is why most people give up on it.
4. Check the ventilation you already have. If the house has an HRV, an ERV, or a fresh air intake on the furnace, find out whether it runs at night. Plenty are wired to the furnace fan and go quiet as soon as the thermostat is satisfied.
5. Skip the purifier for this one. A HEPA room purifier filters particles out of the air already in the room. CO₂ passes straight through it, and the CO₂ line will not move when you switch it on. If your PM2.5 is also high, a purifier is the right tool for that reading and the wrong one for this.

Every fix on that list swaps something for something. Outdoor air brings CO₂ down and brings whatever is outside in with it. Ventilating harder in January costs heat. A reading tells you which swap you are making.

What a CO₂ reading does not tell you

Indoor CO₂ readings do not give an overall indication of air quality, which ASHRAE says plainly in the position document above. A bedroom sitting at 700 ppm can still have PM2.5 at 30 µg/m³ from a candle, VOCs off-gassing from new furniture, or a radon problem that needs months of data before it means anything. CO₂ answers the ventilation question and leaves the rest open.

The second limit is more basic. A monitor reports what the air did. It does not move any air. Nothing in the bedroom changes until you open the door, open the window, or run the fan, and the reading's job is to tell you which of those three worked.

A reading is also not a diagnosis. The thresholds here come from the CDC, ASHRAE and published research, and none of them describe what a given number will do to a particular person.

Frequently asked questions

What is a good CO₂ level for sleeping?

Under 800 ppm. That is the figure the CDC names as a target for a space getting enough outdoor air, and in the bedroom research above, rooms averaging 835 ppm produced better measured sleep and better next-day test results than the same rooms at 2,395 ppm. Anywhere under 1,000 ppm is a reasonable place to stop working on it.

Is 1,500 ppm of CO₂ in a bedroom dangerous?

It is not an exposure hazard. NIOSH and OSHA both set the workplace limit for CO₂ at 5,000 ppm as an eight hour average, more than three times a stuffy bedroom. What 1,500 ppm tells you is about ventilation: very little outdoor air is reaching the room.

Does closing the bedroom door raise CO₂?

Yes, and in most houses it is the largest single factor. A shut door leaves a small volume with almost no air moving through it, so exhaled CO₂ builds up instead of spreading through the house. The same pattern shows up with windows: in the pilot part of the bedroom study, rooms averaged 2,585 ppm with the window shut and 660 ppm with it open.

Where should I put an air quality monitor in a bedroom?

On a nightstand or shelf at about bed height, a few feet from where your head goes. Keep it out of window drafts and off heat sources. If you are choosing which rooms to cover first, the bedroom and the room where people spend the evening are the two that settle the most decisions.

How long does it take for bedroom CO₂ to come down?

The line usually starts falling within minutes of a door or window opening, and keeps falling toward the outdoor number while air is moving. If it barely moves after half an hour with the door open, the rest of the house is stale too, and the fix is outdoor air rather than internal doors.

Does a bedroom CO₂ monitor need to be wired in?

No. Measured Air monitors arrive preconfigured and paired, so setup is power and a Wi-Fi password, and readings start appearing within a few minutes of the first monitor coming online. Nothing is mounted or wired in, and nothing needs undoing when a renter moves out.

The short version

Three readings tell you most of what you need about CO₂ levels in a bedroom: about 428 ppm outdoors, under 800 ppm for a room getting enough fresh air, and 1,000 ppm as the point where most people start calling a bedroom stuffy. A closed bedroom with someone asleep in it runs well past 1,000 ppm by morning, and the studies of real bedrooms put the overnight average above 2,300 ppm with the intake fan off.

The first test is free. Leave the door open for one night, watch the line, and compare it against the night before. If the number drops, you have your answer and it cost nothing. If it doesn't, work down the list: a cracked window, an exhaust fan on a timer, then the ventilation equipment the house already has. A purifier will not touch this reading.

Guessing does not get you there, because a stuffy room at 900 ppm and a stuffy room at 2,400 ppm feel about the same and call for different work. Order a kit sized to your rooms, put a monitor in the bedroom, and stop guessing whether the air in your house is fine.