Value Village ／ Health
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 It stays unpublished until legal review and source verification are complete.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 | 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 |
- 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.
[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.
Above 60%, mould, dust mites and condensation increase and bring other health problems. The goal is to stay within 40–60%.
Humidifying only "sets the conditions"; it is a supplement. It does not replace hand-washing, masks, ventilation or vaccination.
Bacteria multiply in the tank water within a day. Replace it daily, wash weekly. A neglected humidifier can cause pneumonia.
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.
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.
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https://www.mhlw.go.jp/content/10900000/000698848.pdf
https://www.mhlw.go.jp/bunya/kenkou/seikatsu-eisei10/
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.
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.
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