Value Village ／ Health
This answer is a draft. It stays unpublished until legal review and source verification are complete.
Dry indoor air in winter is a shared complaint wherever people heat their homes. In Japan, "40–60% humidity" is a widely known official benchmark, and as humidifiers became common a set of home-grown techniques grew up around them. If you have just spent your first winter in a Japanese flat with a heat-pump air conditioner, this will be familiar.
| Item | Figure | Source |
|---|---|---|
| Relative humidity under the Building Environmental Hygiene Management Standards | 40–70% | MHLW (Building Management Act) |
| Indoor humidity generally described as comfortable | 40–60% | Various public sources |
| Tokyo, January monthly mean relative humidity | About 52% (outdoors) | Japan Meteorological Agency, climatological normals |
| Heated room without humidification | Can fall to the 20s–30s% | Typical measurements |
Air holds more water vapour as it gets warmer. So the same amount of moisture gives a lower relative humidity once the heating warms the room. That is the mechanism behind "dry rooms in winter".
[Verify] Check the standard against the MHLW Building Environmental Hygiene Management Standards, and the humidity normals in the Japan Meteorological Agency "Past weather data search".
Drying laundry indoors, leaving the bathroom door open, cooking a hot-pot (nabe), keeping houseplants, hanging up a wet towel — many methods are shared among people living in Japan. The effect is gentler than a humidifier, but they cost nothing in electricity.
Bacteria multiply easily in the water tank, and ultrasonic models in particular spray the water out as fine droplets. Change the water daily and clean the unit regularly as the product label instructs. Cases of "humidifier lung" (加湿器肺, an inflammatory lung reaction) from neglected humidifiers have been reported.
Above 60%, condensation forms on windows and walls and causes mould and dust mites. Keep within 40–60%.
An air conditioner does not remove moisture; relative humidity falls simply because the air is warmer. Kerosene and gas heaters give off water vapour as they burn, but ventilation is essential with them. Whatever the heating, the key is combining humidification with ventilation.
Wearing a mask in bed is widely used among people living in Japan because the moisture in your own breath eases dryness in the mouth and nose. The precondition is not to force it if you find it hard to breathe.
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https://www.mhlw.go.jp/bunya/kenkou/seikatsu-eisei10/
https://www.data.jma.go.jp/stats/etrn/view/nml_sfc_ym.php?prec_no=44&block_no=47662
https://www.caa.go.jp/policies/policy/consumer_safety/caution/
⚠️ 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.
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.
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