In a closed bedroom, the sleeper breathes the air they have just exhaled, and CO2 shows how quickly that air is renewed.

Why it matters

Outdoor air contains about 420 ppm of CO2. An occupied room, with door and window closed, can exceed 2,000 ppm by early morning. The advanced guest, who measures at home, sees it on their sensor.

What science establishes

CO2 is an indicator of ventilation. ASHRAE, the reference body for ventilation, points this out: indoor CO2 does not on its own measure air quality, but it shows whether the air is being renewed. Its standards set ventilation rates, not a CO2 limit. The figure of 1,000 ppm often cited as a standard is not one. ASHRAE, 2022 and 2023. Established.

Better ventilating a bedroom improves measured sleep. In two experiments in student accommodation, night-time CO2 fell from about 2,400 to 2,600 ppm to 660 to 835 ppm, by opening a window or running an inaudible fan. Sleep measured at the wrist improved, as did the perceived freshness of the air, next-day sleepiness and concentration, and a test of logical reasoning. Strøm-Tejsen et al., 2016. Established, small samples, young adults.

What is emerging

Where the 800 ppm value comes from. A synthesis carried out for ASHRAE in 2025 concludes that the CO2 produced by sleepers should stay below 1,000 ppm, and preferably below 800 ppm. It recommends at least doubling the airflow rates set by current standards for bedrooms. The authors state that these results still need to be confirmed. Akimoto et al., 2025. This is the target value adopted by IO8.

CO2 itself, or what comes with it? In children, adding pure CO2 to a well-ventilated bedroom did not change next-day performance. Klausen et al., 2023. What matters seems to be the whole of the unrenewed air, of which CO2 is only the marker.

What we refuse to say

We do not say that the CO2 in a bedroom is toxic. At the levels measured in a bedroom, it is not.

We do not say that an air purifier lowers CO2: it does not remove it. Only outdoor air dilutes it.

We do not say that a night below 800 ppm improves health. We say that it indicates renewed air, and that measured sleep follows in the available studies.

What a hotel can do

Why the room builds up. Hotel windows open little, for reasons of safety, noise or air conditioning. Ventilation, where it exists, is often sized at the regulatory minimum, reduced at night, or obstructed.

Minimum baseline (around 100 to 250 € per room). An infrared CO2 sensor, readable by the guest; a check of the actual ventilation airflow in each room; unobstructed air inlets; a clear instruction on leaving the window ajar when noise and safety allow.

Optimised baseline (around a few thousand euros per room in a renovation, depending on the building). Mechanical ventilation with heat recovery, controlled by CO2 measurement, which increases the airflow when the room is occupied without letting in street noise.

The target. Night-time CO2 below 800 ppm, measured and published. Silence and renewed air are negotiated together: opening the window lets in noise, hence the value of mechanical ventilation.

What the Living Lab measures

Measurement planned. The Living Lab, near Lisbon, is being equipped in waves. CO2 in the bedroom will be recorded throughout the night, with the window closed and then with ventilation, and the curve published here with its date.

The audit applies the same reading: a sensor near the bed, a whole night, the value on waking and the time spent above 800 ppm.

Sources

• Strøm-Tejsen P et al. The effects of bedroom air quality on sleep and next-day performance. Indoor Air, 2016. doi:10.1111/ina.12254
• Akimoto M et al. New research on bedroom ventilation and sleep quality suggests that building standards should be revisited (ASHRAE 1837-RP). Science and Technology for the Built Environment, 2025. doi:10.1080/23744731.2025.2531317
• Klausen FB et al. The effect of air quality on sleep and cognitive performance in school children aged 10-12 years: a double-blinded, placebo-controlled, crossover trial. Int J Occup Med Environ Health, 2023. doi:10.13075/ijomeh.1896.02032
• ASHRAE. Position Document on Indoor Carbon Dioxide, 2022. ashrae.org
• ASHRAE. Indoor Carbon Dioxide, Ventilation and Indoor Air Quality, 2023. ashrae.org

Also from the founder

Question 15 · What is measured in the place itself: air, light, water?

The longevity audit for venues  ·  The Research Journal

IO8 Living Lab, near Lisbon. Measured, never promised.

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