Key takeaways
- In small Danish trials, students slept a little more efficiently or fell asleep faster when their bedrooms got more fresh air, but next-day effects were mixed23.
- A 2025 review of 17 studies, sponsored by the engineering society ASHRAE, suggests keeping overnight bedroom carbon dioxide below 1,000 parts per million, ideally below 800, using it as a gauge of fresh air rather than a poison4.
- Carbon monoxide poisoning not linked to fires kills more than 400 Americans a year, and CDC says to put a battery or battery-backup alarm near every sleeping area5.
- EPA generally recommends indoor humidity of 30 to 50% (it gives 30 to 60% for controlling mold and mites), and says to refill humidifiers with fresh water daily8.
- Ozone-generating “purifiers” can harm your lungs, and houseplants clean air far too slowly to matter; a mechanical HEPA filter is the better buy91012.
In a set of dormitory rooms north of Copenhagen, 16 international students slept for two weeks with their windows shut2. For one week, nothing else changed. For the other, a quiet fan pulled in outdoor air whenever carbon dioxide rose past 900 parts per million (ppm)1.
With the fan off, carbon dioxide averaged 2,395 ppm overnight, and readings in some rooms reached 3,30012. With it on, levels averaged 8352.
On fan nights, wrist monitors showed the students spent more of their time in bed asleep, and they did somewhat better on a reasoning test the next morning2. They said the air felt fresher and they felt more rested1.
The catch: they woke with drier mouths and skin1.
That study, from the Technical University of Denmark, is one of the best-known tests of bedroom air and sleep. It’s also small and early. The steps with stronger backing are less glamorous.
How carbon dioxide piles up in a closed bedroom
You breathe out carbon dioxide all night. In a closed room, it builds up faster than it leaks away.
A 2017 ASHRAE Journal report on the Danish work noted that published measurements in closed bedrooms routinely top 2,500 to 3,000 ppm1. In a companion experiment by the same team, rooms averaged about 2,585 ppm overnight with the window closed and about 660 ppm with it open3.
At these levels, carbon dioxide is mostly a messenger. A 2025 review treats it as a gauge of how much fresh air reaches the room, not as a pollutant in itself4. A high reading means stale air.
That same review frames fresh-air needs per person4. So two people behind a closed door need twice the fresh air one sleeper does.
Fresh air improved sleep in small Danish trials
The fan study ran one week with the fan and one week without, in balanced order, with eight men and eight women2. Sleep efficiency, the share of time in bed spent asleep, was significantly higher with the fan2. Scores improved on a grammatical reasoning test but not on a numbers test2.
The window study, by the same team, used 14 rooms3. With the window open, students fell asleep faster by wrist-monitor measures and said they found it easier to drop off3. But their ratings on a standard sleep quality scale didn’t change, and neither did their next-day test scores3.
The limits:
- Small groups. Each tested 14 to 16 students23.
- Wrist monitors, not brain waves. Actigraphs estimate sleep from arm movement2.
- Tangled causes. Students may have noticed the fan, and more ventilation also dried the air, so the effects can’t be fully pulled apart2.
- One research group. Much of this work comes from the same Danish network. Two authors of the trials, Wargocki and Wyon, also wrote the 2025 review234.
The fan study was paid for by a Danish government science agency, not a product maker2.
A 2025 review, sponsored by the engineering society ASHRAE, pulled together 17 studies with 22 sets of experimental data4. The lowest ventilation rate that disturbed sleep matched a carbon dioxide level of about 1,000 ppm4. The highest level with no observed effect was 850 ppm, though the authors didn’t treat that as a proven safe line because sensors carry some error4.
Their advice: keep bedroom carbon dioxide below 1,000 ppm, preferably below 8004. That takes about 8 liters per second of outdoor air per person, at least twice what many home building standards require4.
| Overnight bedroom CO2 | What the research suggests |
|---|---|
| Below 800 ppm | The 2025 review’s preferred target4 |
| 800 to 1,000 ppm | Gray zone; 850 ppm was the highest level with no observed effect4 |
| About 1,000 ppm and up | Matched the lowest ventilation that disturbed sleep4 |
| 2,000 to 3,000+ ppm | Common in closed bedrooms in published measurements13 |
The verdict: promising but early. Opening a window is about as natural as a health habit gets, it costs nothing when the outdoor air is good, and the small studies partly back the instinct. But no study we found shows that a carbon dioxide reading predicts how well you’ll sleep tonight.
Carbon monoxide is the bedroom gas that kills
Carbon monoxide is a different story. You can’t see it or smell it, CDC warns, and it can kill with no warning at all: people who are asleep or drunk can die before they ever feel symptoms5.
Each year in the United States, carbon monoxide poisoning not linked to fires causes more than 400 deaths, more than 100,000 emergency room visits and more than 14,000 hospital stays, according to CDC5.
When symptoms do show up, CDC says they often feel like the flu: headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion5. The American Academy of Pediatrics tells doctors to consider carbon monoxide when several people in one home have the same vague symptoms6.
Infants, older adults and people with chronic heart disease, anemia or breathing problems are more likely to get sick5. Children are more susceptible because of their faster metabolism, and a baby in the womb is especially vulnerable, the AAP says6.
Where to put alarms. CDC says to install battery-powered or battery-backup alarms near every sleeping area5. The AAP adds one in the hallway near each separate sleeping area and on every floor, including the basement6. Check the batteries when you change your clocks each spring and fall, and replace alarms as the maker directs or every five years5.
What never belongs indoors. CDC’s list5:
- A generator, even in a garage with the door open. Run it outside, more than 20 feet from windows, doors and vents.
- A charcoal grill, a portable camp stove or a flameless chemical heater.
- A gas oven used to heat the home.
CDC also advises a yearly service for furnaces, water heaters and other fuel-burning appliances, plus a yearly chimney check5.
If the alarm sounds or you feel symptoms, get everyone outside to clean air right away, call 911, don’t drive and stay away until you know it’s safe, North Carolina health officials advise7.
Humidity, mold and dust mites: aim for the middle
Mold and dust mites can trigger asthma, and mites, a source of powerful allergens, thrive in damp, warm places, according to the U.S. Environmental Protection Agency8. “The key to mold control is moisture control,” EPA says8.
EPA generally recommends indoor relative humidity of 30 to 50% for homes8. Elsewhere on the same page, it says keeping humidity between 30% and 60% helps control mold, dust mites and cockroaches8. A cheap humidity meter, called a hygrometer, tells you where you stand. Dry out water-damaged carpet or building materials within 24 hours if you can8.
For dust mites, EPA suggests allergen-proof mattress covers and washing bedding in hot water, about 130°F8. If you’re allergic, leave the house while someone else vacuums, since vacuuming can stir mite allergens into the air8.
Too dry has costs, too, as the Danish students’ dry mouths and skin showed12.
If you run a humidifier, keep it clean. EPA warns that humidifiers can become breeding grounds for germs and molds, and says to clean cool-mist and ultrasonic models as the maker directs and refill them with fresh water every day8.
Our bedroom temperature article covers muggy nights.
Air purifiers help a little, but ozone “purifiers” and houseplants don’t
HEPA filters. A high-efficiency particulate air (HEPA) filter traps fine particles. In a 2023 trial in the Journal of Sleep Research, 30 healthy adults each slept two weeks with a HEPA purifier and two weeks with a purifier holding a fake filter. Neither they nor the researchers knew which was which13.
With the real filter, people slept about 12 minutes more a night and spent about 19 more minutes in bed13. But they also spent more time awake after first falling asleep, and many other sleep and mood measures didn’t change13. Dyson, which sells air purifiers, funded the study13. The verdict: promising but early.
If you buy one, the Public Health Institute’s asthma program suggests a mechanical HEPA unit sized for your room, and notes that effective models cost $250 or less10. A wildfire factsheet adapted from EPA advice says to pick one whose clean air delivery rate (CADR) for tobacco smoke is at least two-thirds of the room’s area in square feet11. For a 150-square-foot bedroom, that means at least 100.
Skip extras with names like ionizer, plasma, photocatalytic oxidation, hydroxyl generator or UV light, and turn the ionizer off if your HEPA unit has one10. Check the noise rating, too: the same guide calls units above 55 decibels quite noisy10.
Ozone generators. These machines make ozone on purpose and are often sold with phrases like “activated oxygen” or “super oxygenated” air9. The California Air Resources Board warns that ozone inflames the lungs, can worsen asthma and can cause permanent lung damage at high exposures9. It doesn’t remove particles, and at safe levels it doesn’t kill germs well9.
California has limited ozone from indoor air cleaners since 2007, and its air board strongly advises against ozone generators where people or animals are present9.
Houseplants. The idea that plants scrub indoor air traces to 1989, when NASA tested plants in sealed chambers while looking for ways to clean air on space stations12. In 2019, Drexel University engineer Michael Waring and his doctoral student Bryan Cummings reported on 12 studies from three decades of plant research12.
Ordinary air exchange cleared chemical vapors far faster than plants did12. Matching a couple of open windows would take 10 to 1,000 plants per square meter of floor12.
Plants bring real pleasures: color, care and a bit of nature indoors. Cleaning the air isn’t one of them. An open window, the oldest natural air cleaner, does that job better.
On wildfire smoke nights, seal the bedroom and filter the air
Smoke flips the usual advice: close the window and filter the air.
A factsheet from Washington’s Spokane Regional Health District, adapted from EPA guidance, explains how to make a “clean room”; a bedroom with its own bathroom is a good choice11. In short:
- Close windows and doors, and set any air conditioner to recirculate11.
- Run a properly sized portable HEPA cleaner nonstop on its highest speed11.
- If your heating and cooling system can handle it, use a filter rated MERV 13 or higher; check with a technician first11.
- Don’t smoke, burn candles, fry food or spray aerosols, and use only a HEPA-labeled vacuum11.
- Air the room out when outdoor air improves, even briefly11.
A homemade box-fan filter is a fallback: a portable air cleaner is likely better, and evidence on how well homemade versions work is limited11. For bedroom use, the key rule: choose a box fan made in 2012 or later with a UL or ETL safety mark, which signals a built-in thermal fuse11. Use an older fan only if you know it has a thermal fuse, and never leave it running unattended or while you sleep11.
Pair the fan with a MERV 13 filter, arrows pointing with the airflow, and replace it when it looks dirty or smells of smoke11.
If you can’t keep the room cool, the power fails or too much smoke still gets in, consider going somewhere with cleaner, cooler air11.
Your breathe-easier bedroom checklist
Every night
- Let fresh air in when outdoor air is clean: crack a window or the bedroom door, or run a fan that brings in outside air23.
- If you share the room, plan on more fresh air, since needs scale with each sleeper4.
- Keep generators, grills and camp stoves outdoors, and get fuel-burning appliances serviced yearly5.
Once a season
- Test carbon monoxide alarms and change batteries when the clocks change5.
- Check humidity and clean the humidifier8.
- Check purifier and furnace filters, and replace them when they look dirty11.
Before smoke season
- Pick your clean room and buy or build a filter ahead of time11.
| Gadget | Do you need it? | What to look for | Target |
|---|---|---|---|
| Carbon monoxide alarm | Yes | Battery or battery backup; a digital readout can show peak levels5 | Near every sleeping area and on every floor56 |
| Humidity meter | Helpful | Any simple digital hygrometer | 30 to 50%8 |
| CO2 monitor | Optional | One that reports CO2 in ppm; readings carry some error, so watch trends4 | Under 1,000 ppm overnight, ideally under 8004 |
| HEPA purifier | Optional; most useful on smoke days | Mechanical HEPA, no ozone, sized for the room1011 | Tobacco-smoke CADR at least two-thirds of the room’s square feet11 |
A carbon dioxide monitor is for learning, not worry. For a week, note the morning reading with the door closed, then with it cracked, and see whether you feel different.
What the evidence supports
Two “Strong” ratings below rest on agency guidance and poisoning counts, not on trials.
| Claim | Evidence level | What the research shows |
|---|---|---|
| More bedroom ventilation improves sleep | Small student trials and a 2025 review; mostly one research network234 | |
| Bedroom CO2 is itself toxic at typical levels | Researchers use it as a gauge of fresh air, not as a pollutant4 | |
| Carbon monoxide kills sleepers, and CDC and the AAP recommend alarms near sleeping areas | National CDC poisoning counts and agency guidance; we found no trials of alarms themselves56 | |
| EPA calls moisture control the key to mold control | Long-standing EPA guidance, not home trials8 | |
| A HEPA purifier improves sleep in healthy adults | One 30-person, Dyson-funded trial with mixed results13 | |
| Ozone generators clean air safely | Ozone harms lungs and doesn’t remove particles9 | |
| Houseplants clean bedroom air | 10 to 1,000 plants per square meter would be needed12 |
When bedroom air is a warning sign
Get everyone outside and call 911 if a carbon monoxide alarm sounds or you have symptoms such as headache, dizziness, nausea, shortness of breath or chest pain7. Be especially alert when several people in the home have the same vague symptoms at once6.
EPA lists sneezing, watery eyes, coughing and shortness of breath among the possible effects of mold, mites and other biological pollutants8. If symptoms like these flare in a damp or moldy home, tell your clinician about the damp.
If fresh air doesn’t help, the cause may not be the room. MedlinePlus lists signs that may point to a sleep disorder, including these14:
- A bed partner says you snore loudly, snort, gasp or make choking sounds14.
- You often feel sleepy during the day, take frequent naps or doze off at the wrong times14.
- You regularly take more than 30 minutes to fall asleep14.
Our upcoming articles on sleep apnea and on medical evaluation cover next steps.
Questions readers ask
Should I sleep with the window open?
When the outdoor air is clean and the night is quiet and safe, it’s a free experiment. In one Danish study, an open window helped students fall asleep faster, though their sleep ratings didn’t change3. Close it on smoke days, and weigh street noise; our noise article covers that trade-off.
Is the carbon dioxide in my bedroom dangerous?
At closed-bedroom levels, researchers treat carbon dioxide as a sign of stale air, not a poison4. The bedroom gas that kills is carbon monoxide, so an alarm matters more than a carbon dioxide monitor5.
Is a gas stove a problem for bedroom air?
Never use a gas stove or oven to heat your home56. The AAP also recommends an exhaust hood over every gas stove to cut indoor air pollution, so run it when you cook6.
The bottom line
Fresh air at night may help a little. Small trials and a 2025 review point the same way, so treat stale air as a reason to crack a door or window, not to buy gadgets.
The steps that matter most protect your safety: a carbon monoxide alarm near where you sleep, no generators or grills indoors, humidity in the middle range, a clean humidifier and a real filter on smoke days. Skip ozone machines, and enjoy houseplants for what they are.
Related reading
About this article
This article was researched using U.S. government and state health agency guidance, the American Academy of Pediatrics, and peer-reviewed studies, listed below, and was reviewed in October 2026. It is general education, not medical, legal or financial advice..
Sources
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- Public Health Institute, Regional Asthma Management and Prevention. “Factsheets & Resources: Choosing Safe, Effective & Affordable Air Cleaners.” July 11, 2023. phi.org
- Spokane Regional Health District. “Wildfire Smoke: Create a Clean Room.” Factsheet adapted from U.S. EPA document EPA-452/F-22-001, May 2022. assets.srhd.org
- Drexel University. “Study: Actually, potted plants don’t improve indoor air quality.” News release on Cummings BE, Waring MS, Journal of Exposure Science and Environmental Epidemiology, via Newswise, November 6, 2019. newswise.com
- Lamport DJ, Breese E, Gião MS, Chandra S, Orchard F. “Can air purification improve sleep quality? A 2-week randomised-controlled crossover pilot study in healthy adults.” Journal of Sleep Research, 2023;32(3):e13782 (University of East Anglia repository record). ueaeprints.uea.ac.uk
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