Bedroom air

Why does the air in a shut bedroom go stale overnight?

Every sleeper breathes out carbon dioxide and moisture. With the door and window shut, little of it leaves, so levels climb through the night.

Breathing adds carbon dioxide all night. A 2025 ASHRAE-funded review estimates about 11 liters an hour for a person who sleeps through the night (Akimoto et al., 2025). In 93 Shanghai homes with doors and windows shut, average bedroom carbon dioxide was 1448 ppm. Of those bedrooms, 89.2 percent were over 1000 ppm (Springer conference paper).

Carbon dioxide also works as a rough gauge of air change. A Danish field study of 40 bedrooms found that carbon dioxide, volatile organic compounds and coarse particles all fell when a window was opened (Fan et al., 2022). The same still air holds them all in.

Opening up trades one set of problems for another. Earlier work summarized in one study found that an open window or door lowers carbon dioxide, temperature and humidity, while fine particles and noise rise (PMC observational study).

How do you ventilate a bedroom in winter without losing the heat?

A 2025 review set 800 ppm of carbon dioxide as the sleep target, about 17 cfm of outdoor air per sleeper. Heat recovery keeps most of the warmth.

Researchers at Waseda University and the Technical University of Denmark pooled 17 studies for an ASHRAE-sponsored review. Sleep was first disturbed at the ventilation rate that let carbon dioxide reach 1,000 ppm, so they proposed 800 ppm as the design target (EurekAlert, 2025). Holding it takes about 8 liters per second, or 17 cfm, of outdoor air per sleeper.

For two sleepers on a 25°F night, Night Air's arithmetic puts the heat that air carries off at roughly 1,470 Btu an hour. A cracked window and an open door both lower carbon dioxide. A heat recovery ventilator, which passes the outgoing air past the incoming air, keeps much of the warmth (HVI). The sizing and the costs are worked through in How do you ventilate a bedroom in winter without losing the heat?

How do you read a VOC reading, and what does a new mattress emit?

A home VOC sensor shows a trend rather than a dose. New foam gives off most in its first days, and warmth raises the rate, so air the room early.

Most home monitors fold many gases into one number. Many use Sensirion's VOC Index, which maps the average indoor air of roughly the past 24 hours to 100 (VisiblAir). A steady source can slowly become part of that learned background, so the number reports change better than it reports a dose.

Lab work fills in the mattress side. In a study of two memory foam mattresses, emissions peaked on day one (Beckett et al., 2022). Warming crib mattress foam from 73°F to 97°F raised its average emission rate from 56 to 139 µg/m²·h (Boor et al., 2014). The reading guide is How do you read a VOC reading, and what does a new mattress emit?

What are volatile organic compounds, and where do they come from in a bedroom?

VOCs are carbon-based chemicals that turn to gas at room temperature. New furnishings, paint and cleaners all release them indoors.

The EPA reports that levels of several organic compounds average 2 to 5 times higher indoors than outdoors (EPA). Its list of household sources includes paints, cleaners, cosmetics and fuels. It also notes that during and for hours after jobs such as paint stripping, levels may reach 1,000 times background outdoor levels.

A bedroom gathers several sources in one closed space. A new mattress, a fresh coat of paint and a bottle under the sink can all add to the same reading. German indoor air guidance treats a total under 300 µg/m³ as the target (PubMed), and that scale was written for lab-grade measurement.

Time and air change bring most new sources down. That is why a reading taken in the first week of a new mattress says more about the first week than about the years after it.

What does a heat recovery ventilator do, and what do sellers claim for it?

A recovery unit passes outgoing warm air past incoming cold air so the fresh air arrives warmed. Seller figures run from 66 to 97 percent.

The Home Ventilating Institute lists four kinds of mechanical ventilation for a house: exhaust, supply, balanced and energy recovery (HVI). A heat recovery ventilator moves only heat between the two airstreams. An energy recovery ventilator also moves some water vapor, which helps keep a dry winter room steadier. AHRI, the industry trade group, says energy recovery units typically recapture 40 to 80 percent of the energy in the outgoing air (AHRI).

Seller figures run higher, and they rarely say under what conditions. Of 10 seller pages for single-room units Night Air read on September 27, 2026, two tied the figure to an airflow, a temperature or a test standard. Five gave a percentage with no condition, and three gave none. One maker's own spec page notes that efficiency falls as airflow rises (Lunos Canada), so the top figure is a best case.

What do mattress emission labels cover, and what do they leave out?

One common label tests a foam sample and another tests the finished mattress. Neither total covers the heavier chemicals such as flame retardants.

CertiPUR-US tests foam samples in a small chamber after 72 hours of conditioning, and formaldehyde must test below 0.1 parts per million (CertiPUR-US). The program covers the foam only, not the glue, the cover or the fire barrier in the finished bed.

GREENGUARD Gold, run by UL, sets limits for the finished mattress. Its maximum predicted concentration is 0.22 mg/m³ total VOCs and 9 µg/m³ formaldehyde (UL certification criteria). The two labels come from different tests, so their numbers cannot be set side by side, and a low total covers only the lighter compounds.

How do fresh air, heat and moisture trade against each other at night?

Every cubic foot of outdoor air brings oxygen in and carries carbon dioxide, moisture and heat out. Winter makes the heat the expensive part.

Fresh air solves several problems at once. It lowers carbon dioxide, it carries off the moisture sleepers breathe out, and it dilutes whatever the furnishings release. In winter it also carries off heat, which is why most people shut the window when the weather turns.

The moisture side helps in a cold month. The University of Minnesota Extension says that in winter a window opened a little lets warm, moist air out (UMN Extension). That can keep the glass dry. Night Air's bedroom moisture topic sets out the humidity bands.

The heat side is the cost. A room ventilated to the 800 ppm target loses more warmth than a shut one, and the outdoor low that arrives near sunrise makes the early hours the costliest. The bedroom temperature topic covers that overnight curve.

What can a reader check about their own bedroom air?

A carbon dioxide monitor by the bed shows how fast a shut room goes stale. Try one night door shut and one door ajar, then compare readings.

A carbon dioxide monitor is the most useful single gauge for a shut bedroom. Set it near the bed, away from the sleeper's breath, and read the peak in the morning. One night with the door shut and one with it ajar show how much the door alone changes. Recent research puts the start of disturbed sleep at about 1,000 ppm (EurekAlert, 2025).

A VOC monitor answers a narrower question. It shows whether something new in the room is still giving off gases, and whether airing the room brings the number down over the following days.

Night Air's room check starts from what a sleeper notices on waking, such as a heavy head or a smell, and points to the reading worth taking first.

What has Night Air published on this?

• How do you read a VOC reading, and what does a new mattress emit?
• How do you ventilate a bedroom in winter without losing the heat?

Common questions

Why does a bedroom feel stuffy by morning?
Sleepers breathe out carbon dioxide all night, and a shut door and window leave it nowhere to go. In closed Shanghai bedrooms it averaged 1448 ppm overnight.

What carbon dioxide level disturbs sleep?
A 2025 ASHRAE-sponsored review found sleep first disturbed at about 1,000 ppm and proposed 800 ppm as the target for bedroom ventilation.

Does a cracked window waste a lot of heat?
Some. By Night Air's arithmetic, ventilating for two sleepers on a 25°F night carries off roughly 1,470 Btu an hour. A heat recovery unit keeps much of that warmth.

Is a home VOC reading accurate?
It is better read as a trend than a dose. Many low-cost sensors cannot report a true concentration, and a steady source can become part of the learned background.

How long does a new mattress give off VOCs?
Emissions are highest at first. In one study of two memory foam mattresses they peaked on day one and fell over the following weeks.

Sources

Night Air's reading for this piece, in the order it is cited, each with the question it helped answer.

1. Akimoto et al., 2025 (tandfonline.com), helped answer “Why does the air in a shut bedroom go stale overnight?”.
2. Springer conference paper (link.springer.com), helped answer “Why does the air in a shut bedroom go stale overnight?”.
3. Fan et al., 2022 (sciencedirect.com), helped answer “Why does the air in a shut bedroom go stale overnight?”.
4. PMC observational study (pmc.ncbi.nlm.nih.gov), helped answer “Why does the air in a shut bedroom go stale overnight?”.
5. EurekAlert, 2025 (eurekalert.org), helped answer “How do you ventilate a bedroom in winter without losing the heat?”; “What can a reader check about their own bedroom air?”.
6. HVI (hvi.org), helped answer “How do you ventilate a bedroom in winter without losing the heat?”; “What does a heat recovery ventilator do, and what do sellers claim for it?”.
7. VisiblAir (visiblair.com), helped answer “How do you read a VOC reading, and what does a new mattress emit?”.
8. Beckett et al., 2022 (pubmed.ncbi.nlm.nih.gov), helped answer “How do you read a VOC reading, and what does a new mattress emit?”.
9. Boor et al., 2014 (pubs.acs.org), helped answer “How do you read a VOC reading, and what does a new mattress emit?”.
10. EPA (epa.gov), helped answer “What are volatile organic compounds, and where do they come from in a bedroom?”.
11. PubMed (pubmed.ncbi.nlm.nih.gov), helped answer “What are volatile organic compounds, and where do they come from in a bedroom?”.
12. AHRI (ahrinet.org), helped answer “What does a heat recovery ventilator do, and what do sellers claim for it?”.
13. Lunos Canada (lunoscanada.com), helped answer “What does a heat recovery ventilator do, and what do sellers claim for it?”.
14. CertiPUR-US (certipur.us), helped answer “What do mattress emission labels cover, and what do they leave out?”.
15. UL certification criteria (snowsoundusa.com), helped answer “What do mattress emission labels cover, and what do they leave out?”.
16. UMN Extension (extension.umn.edu), helped answer “How do fresh air, heat and moisture trade against each other at night?”.