Your bedroom could be the unhealthiest room in your home

by Lala Rukh, The Conversation

edited by Swati Mestri, reviewed by Andrew Zinin

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Close the door, shut the windows and turn up the heat, and you have all the ingredients for a sick bedroom in your home—and you spend about eight hours there every night.

Kitchens and bathrooms usually take the blame for stale air and dampness. But researchers spent a winter monitoring airtight, naturally ventilated homes across Scotland. In more than four out of five homes, carbon dioxide (CO₂) levels remained above what is considered a safe threshold for the entire night. Despite this, more than nine in 10 occupants thought the air in their bedroom was good.

The reason is not the room. It is you. Awake in the living room, you are a thermostat with hands. When the air turns stuffy, you crack a window. When you feel cold, you close it. This correction stops in bedrooms once you fall asleep, and two things build up: the CO₂ you breathe out and moisture with it.

High levels of CO₂ act as a proxy for stale air. CO₂ levels are measured in parts per million (ppm), a unit used to describe very small concentrations of a substance.

A study examining 40 Danish bedrooms revealed a troubling threshold: When CO₂ levels reach 1,800 ppm, participants' cognitive function the following day suffered. Sleep quality starts to suffer well below that.

Recognizing this risk, strict European standards allow living spaces to reach 1,350 ppm above outdoor levels while restricting bedrooms to just 950 ppm above outdoor levels. For reference, CO₂ levels in fresh outdoor air are typically between 420 and 450 ppm.

Between 2022 and 2025, I continuously monitored temperature, humidity and CO₂ data using sensors in around 200 rooms in highly energy-efficient but airtight Irish social housing. The bedrooms spent roughly two hours every night in winter above unhealthy levels, while the living rooms next door were fine. The same bedrooms were fine again during summer. The likely reason is that people keep their windows shut in winter to stay warm and open them in summer.

Serious health consequences

Two sleeping adults breathe out about a pint of water (20 fluid ounces) a night. I run a dehumidifier at home over the winter—it holds two liters (0.5 gallons), and from October onward, I empty it every other morning.

Moisture, when trapped indoors, settles on the coldest surface—usually the window or the wall behind a wardrobe. A cold, damp surface in a warm, moist room provides ideal conditions for mold to grow.

This is not just a cosmetic problem. Prolonged exposure to mold in an apartment in Rochdale, a town in Greater Manchester, killed two-year-old Awaab Ishak in 2020. Consequently, Awaab's Law now legally compels social landlords in England to address dampness and mold within strict, fixed deadlines.

Small children are the most exposed among us because their lungs and immune systems are still developing, and they sleep longer than adults.

Airing a room by opening the window clears stale air and moisture at once, but it lets the heat out. That is the trade-off between fresh and warm air.

The trickle vents (small slots built into window frames for background airflow) are often found shut after energy upgrades, and a 2024 Irish study of retrofitted homes found bedroom CO₂ higher after the work than before, with around half showing visible mold.

This feels like déjà vu. After the oil shocks of the 1970s, offices were sealed and ventilation cut to save fuel, and occupants got headaches, itchy eyes and a fog that lifted only when they went home. It was called sick building syndrome. It's the same—except nobody gets to go home from their bedroom.

Energy-efficient but sick buildings often cost society far more than they save. Renters sit in the middle of it, unable to carry out expensive building changes.

What to do

If an open window loses heat, at least keep trickle vents open. Leave the bedroom door ajar, which gives the moisture somewhere to go.

The German daily ritual of Lüften, a generational practice, is now popularized on TikTok as "house burping." By throwing the windows wide open for 10–15 minutes immediately after getting up, you rapidly purge stale air and moisture while minimizing heat loss.

When it is extremely cold—even for Lüften—people often reach for a dehumidifier. These devices cost roughly £90–£300, and a typical dehumidifier draws around 250 watts and costs around 6p–9p an hour to run.

Most dehumidifiers have a humidistat (a small electronic device used to measure and control the humidity) that you can set to a minimum target. A sensible target for a bedroom is 60%—the machine runs until the room reaches it, shuts itself off and wakes again when the humidity creeps back up. Others run for a specified period, set as desired, or continuously.

Dehumidifiers are a genuine help for a cold, damp bedroom when there are few other options.

While none of this replaces a proper ventilation system, it changes what you breathe at night. For definitive proof of bad air quality in your bedroom, use an indoor air quality monitor. Seeing the hard data is often the best prompt to open a window, clear a vent or switch on a dehumidifier.

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Sleep & RecoverySleep medicineHealthy livingCommon illnesses & Prevention Provided by The Conversation Who's behind this story?

Swati Mestri

Swati Mestri holds a bachelor's degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space. Full profile →

Andrew Zinin

Master's in physics with research experience. Long-time science news enthusiast. Plays key role in Science X's editorial success. Full profile →

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