In winter, does raising indoor humidity help you avoid colds? What percentage should I aim for?
This answer is a draft. It stays unpublished until legal review and source verification are complete.
Come winter, you will hear "a humidifier keeps viruses away". Japanese rooms in winter, with the heating on (usually the air conditioner's heat mode), drop to 20–30% humidity: the season of sore throats, dry skin and static shocks. Here is what is known about humidity and infections, and the downsides of overdoing it.
Humidity guides and the basis
| Humidity | What happens |
|---|---|
| Below 30% | The membranes of the nose and throat dry out, and the cilia (which sweep foreign matter out) slow down. Dry skin, static. Droplets containing viruses dry out, shrink, and float in the air for longer |
| 40–60% | Recommended range. Virus survival tends to fall, the membranes' defences work better, and mould and dust mites do not spread easily |
| Above 60% | Condensation forms readily; mould (from 60%) and dust mites (active at 60–80%) increase. A factor in worsening asthma and allergies |
- Influenza virus survival: in a classic experiment (Harper, 1961), at 20–24°C (68–75°F) and 50% humidity or more, survival after 6 hours fell to a few per cent, while at around 20% humidity about 60% survived. The government also explains cold, dry conditions as the setting for epidemics.
- MHLW winter ventilation guide (2020): keep ventilating, with room temperature 18°C (64°F) or above and humidity 40% or above. The idea is "make up with a humidifier for the humidity that ventilation takes out".
- Building Sanitation Act standard: indoor relative humidity of 40–70% is the management standard for offices, commercial facilities and the like. If your office air conditioning is drying, you can cite this to ask for an improvement.
- Ways to humidify, and cautions:
- Humidifier: change the tank water every day and wash the tank and filter once a week. In particular, ultrasonic humidifiers (超音波式) turn bacteria in the water straight into mist, and neglected units have caused Legionella pneumonia (with deaths in Japan). Steam humidifiers (加熱式/スチーム式) have fewer bacteria problems, but watch the electricity cost and the burn risk. - Without a humidifier: a wet towel, drying laundry indoors, leaving the bathroom door open, houseplants, hot-pot (nabe) cooking.
- Hygrometer: place it out of the air conditioner's airflow, 1–1.5 m above the floor. Near the humidifier's outlet, it reads higher than the room really is.
- For a dry throat: besides humidity, frequent sips of water and a mask (while sleeping) also help.
[Verify] Check the Harper (1961) survival figures against the original (J Hyg (Lond) 1961;59:479-486). The MHLW "humidity 40% or above" wording is based on the winter 2020 ventilation leaflet.
Misunderstanding 1｜The higher the humidity, the fewer colds
Above 60%, mould, dust mites and condensation increase and bring other health problems. The goal is to stay within 40–60%.
Misunderstanding 2｜Leave the humidifier on and you are protected from infection
Humidifying only "sets the conditions"; it is a supplement. It does not replace hand-washing, masks, ventilation or vaccination.
Misunderstanding 3｜Just top up the humidifier water as it goes down
Bacteria multiply in the tank water within a day. Replace it daily, wash weekly. A neglected humidifier can cause pneumonia.
Misunderstanding 4｜Heating dries the air, and nothing can be done
Heating lowers "relative humidity" because warm air can hold more water, and a humidifier makes up for this. Room temperature 18°C and humidity 40% or above is the winter guide.
Misunderstanding 5｜If the air conditioner's humidity display says 50%, all is well
The readings on air conditioners and humidifiers are for the air right around the unit. Check with a hygrometer in the middle of the room, at bed height.
---
Sources
- Ministry of Health, Labour and Welfare, *Ventilation to improve "poorly ventilated enclosed spaces" in winter* [冬場における「換気の悪い密閉空間」を改善するための換気について] (2020; room temperature 18°C or above, humidity 40% or above)
https://www.mhlw.go.jp/content/10900000/000698848.pdf
- Ministry of Health, Labour and Welfare, *Building environmental sanitation management standards* [建築物環境衛生管理基準] (relative humidity 40–70%)
https://www.mhlw.go.jp/bunya/kenkou/seikatsu-eisei10/
- Ministry of Health, Labour and Welfare, *Influenza Q&A* [インフルエンザQ&A]
https://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou01/qa.html
⚠️ Editorial note: the source URLs above were identified by search. Our working environment cannot reach go.jp domains, so the contents have not been checked against the text. Verify every source before publication and record who checked it and when.
About this article
What we share here are ideas and everyday practices passed down among people living in Japan, together with general information drawn from public sources. They are not official recommendations, and they are not professional instruction. Practices vary between households and regions.
This is general information. Results vary from person to person and depend on your own conditions and surroundings.
Please decide for yourself, using your own judgement, whether any of this suits your situation. We cannot accept responsibility for any damage, loss or injury arising from following this article.
This article is not meant to diagnose or manage any specific medical condition. If you are worried about your health, see a doctor rather than relying on your own judgement.
Sources
Tell us if this has changed
Approved contributions keep the contributor's name, shown in five languages.
Related questions
Other languages：日本語 ／ 简体中文 ／ 繁體中文 ／ 한국어
Value Village is not a government body. Always confirm procedures on the official site.
Written by Value Village from public sources. Nothing here is reproduced from another site.