The Japanese word Onsen refers to “hot water,” “mineral water,” or “water vapor” that comes from underground. As a general rule one could say that Onsens are indoors or outdoors hot springs that include bathing facilities and traditional Japanese inns (Ryokan), guest houses, or hotels accompanying them.
Onsens are numerous and popular facilities in Japan, and you can find more than 25,000 natural hot spring sources across the country. In the Japanese law, it’s stated that an Onsen consists of mineralized hot spring water of at least 24°C and should originate from a depth of at least 1.5 kilometers.
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Moreover, depending on the varieties and amounts of minerals found in the water, such as sulfur, sodium, iron, salts, or magnesium, and so on, one distinguishes between certain types of Onsens.
Subject to the regulations of the specific Onsen facility, and the provincial, municipal and state laws, men and women may bathe together or otherwise separately, either in a bathing suit or alternatively in the nude.
An Onsen is meant to relax, to meditate, to warm or detoxify the body, or as medical therapy, and not a means to clean oneself; as such it’s required to enter the hot water source with a clean body. Entering the Onsen dirty is considered socially unacceptable. Mind also that one doesn’t go with the head or hair under the water (which in Japan is considered unhygienic).
Another peculiarity is that many Onsens don’t allow people with tattoos to use the Onsen. Traditionally, this was to avoid members of the Japanese mafia, the Yakuza, to mingle with the guests. In Japan, tattoos were (and often still are) strongly associated with the Yakuza. However, things have changed somewhat over time, and a number of Onsens nowadays allow people with small tattoos (especially tourists) to bathe in Onsens.
And a last remark: although Onsen bathing is generally safe, there may be some side effects in using Onsens. For instance, the heat may possibly aggravate high blood pressure and heart issues, poor hygiene measures may increase the chances to contracting Legionella disease or other infectious diseases, or one can burn oneself in too hot water.
