The essential difference between an onsen and a sento (communal bath house) is that the water in an onsen must be of volcanic origins, even if the water is reheated. Whereas a sento may use ordinary heated water. In Japan, Onsen is defined by the temperature of the water, usually with a temperature over 25 C and should contain 19 essential minerals designated by the Japanese Environmental Agency.
Kakenagashi is fresh and not recycled hot spring water Onsen. With Kakenagashi, the hot spring water keeps running in the bath tub and is kept fresh and clean. Usually a Geyser is the fountainhead, underneath the Onsen, where the hot spring water originates, so water in this type of Onsen is pure and fresh.
Health Benefits of Onsen
Onsen water is often thought to have healing powers according to its mineral properties. Senshitsu, or Onsen water quality differs from location to location of Onsen in each resort area.
To receive the full benefit of the healing powers of the hot spring water on you body, it is better not to wash your body by shower or hot water after getting out of the tub. But, remember, please check with the staff at the Onsen about the effects of the minerals on your body.
The water at some Onsen in Japan is possible to drink, however, some Onsen water has strong minerals that can be toxic if congested. So, you should ask the staff if the water is drinkable or not, before entering Onsen.
Here is a relaxation tip: rest your body completely for at least 30 minutes after taking Onsen. You will feel rejuvenated and ready for more travel in Japan.