How to Air Out Your Home: How Long, How Often and When Not To

The address of this page says ten minutes a day. Treat that as a rough guide, not a rule: how fast a room's air is replaced depends on the wind, the temperature difference between inside and out, how wide the window opens and whether air can flow straight through. The German Environment Agency suggests about five minutes with windows wide open two or three times a day in winter and 10–20 minutes in summer (UBA, Germany). The principle matters more than the minutes: wide and brief, and more often when you cook, shower, dry laundry or have guests. How airing compares with a purifier or humidifier: the overview.

What is happening

What builds up. People breathe out CO₂ together with moisture and other emissions. Showers, cooking and laundry dried indoors add water. Furniture and building materials give off chemicals all the time (EPA). Cooking, especially on gas, candles, gas heaters and cleaning sprays add particles and gases (NICE, UK).

Exchange is gradual. Outdoor air comes in through gaps, open windows and fans, pushed by wind and by the temperature difference (EPA). In tracer-gas measurements in two US homes, opening a single window multiplied the air change rate several times over, roughly in proportion to how wide it was opened (Howard-Reed et al.). Still, airing works by dilution, not like a switch: even with six complete air changes an hour and perfect mixing, removing 99% of a contaminant takes about 46 minutes (CDC). “A complete air exchange in ten minutes” is not supported; each airing replaces part of the air.

Why winter airing dries a home. Cold outdoor air holds little water, so once it is warmed indoors its relative humidity drops sharply. The WHO's example: air at −8°C and 100% humidity, warmed to 20°C, ends up at about 15% (WHO). That is why brief winter airing removes moisture so well, and why heated homes feel dry.

Wide and brief versus a window left ajar. A narrow opening does move air. The case against a window tilted open for hours in winter is different: it wastes much more heat and chills the wall around the window, where condensation and mould then form (UBA). In summer that argument does not apply.

What a CO₂ monitor tells you. CO₂ shows how much outdoor air each person is getting, not overall air quality (ASHRAE); outdoors it is about 420–430 ppm (NOAA). The familiar 1,000 ppm line dates from the 19th-century hygienist Max von Pettenkofer as a marker of stale air; ASHRAE removed it from its standard as widely misread and sets no universal CO₂ limit for air quality or safety. Studies disagree on whether CO₂ at typical indoor levels affects thinking or sleep by itself; stuffiness may come from other pollutants that build up alongside (ASHRAE). The UK Health and Safety Executive treats readings consistently above 1,500 ppm in an occupied room as poor ventilation that needs action, and calls CO₂ a rough guide, not a “safe level” (HSE). For scale, the US workplace limit is 5,000 ppm averaged over a shift (NIOSH, OSHA). Choose an NDIR monitor rather than an “eCO2” one and keep it away from windows, doors and vents and more than 50 cm from faces (HSE).

What to do

• Open wide, briefly, several times a day. Windows on opposite sides open at once give the fastest exchange (UBA).
• Minutes as a guide from Germany (UBA): about 5 per room in winter, 10–20 in summer, 5–10 for the bedroom in the morning. Allow longer on still days or when it is as warm outside as in.
• Air when moisture is made. Run an extractor that vents outside while cooking and showering, keep the kitchen or bathroom door shut, use pan lids, and dry laundry outdoors when you can (EPA; UK government guidance).
• Keep background ventilation working: trickle vents on windows and kitchen and bathroom extractors. Renters in the UK should report ventilation that does not work to the landlord (NICE).
• Bedroom: small studies found much lower night-time CO₂ and somewhat better sleep with a window open (Strøm-Tejsen et al.). Ventilate through a window or vent, not by leaving the door open: sleep with the bedroom door closed.
• Sealing draughts and airing work together. Weatherstripping and sealing small gaps stop heat leaking all day; airing brings fresh air in when you choose. If windows mist up after sealing, air more often.

When this is not the case

• Smoke or smog outside. Keep windows shut; the EPA (US) advises one clean room with a purifier.
• High pollen. The NHS (UK) advises keeping windows and doors closed as much as possible.
• Summer and cold rooms. Warm, humid outdoor air let into a cool basement or storeroom can condense on its walls, so summer airing does not dry every room (UBA).
• Small children. Wide-open windows only while an adult is in the room; an insect screen will not hold a child.
• Noise or security. NICE says open windows “where possible and safe”; otherwise vents and extractors do the work.
• Damp or mould that keeps coming back. Airing helps but does not fix a leak or a cold wall: see checking for mould and measure with a hygrometer.
• Someone at home is ill: see when someone is sick at home.

Sources

• Umweltbundesamt — Wie lüfte ich richtig?
• EPA — How much ventilation do I need in my home?
• EPA — Improving indoor air quality
• EPA — Cleaner air room at home
• Howard-Reed, Wallace, Ott — opening windows and air change rates in two homes
• CDC — Environmental infection control, Appendix B: air
• WHO — Guidelines for indoor air quality: dampness and mould
• NICE NG149 — Indoor air quality at home
• UK government — Damp and mould: understanding and addressing the health risks
• ASHRAE — Position document on indoor carbon dioxide
• HSE — Using CO2 monitors
• NOAA — Trends in atmospheric carbon dioxide
• NIOSH — Carbon dioxide
• Strøm-Tejsen et al., Indoor Air, 2016
• NHS — Hay fever

Checked: 24 September 2026