Conventionally, many onsen providers heat up the water or add cold water to adjust the temperature to approx. 41°C (105.8°F) which is the average temperature of Japanese bath.
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This social aspect, however, led many onsen and sento to prohibit tattooed guests. The Japanese taboo toward tattoos stems from their association with members of Japanese organized crime. Gangsters in Japan are typically heavily tattooed, and body art in Japan came to be associated with unsavory characters.
Avoid bathing in an onsen in the following cases. If you have chronic bronchitis and fever or difficulty in breathing. If you have stable angina and chest pain becomes frequent. Bathing may cause serious problems with dehydration or thrombosis.
Of course, you should stay cognizant of how long you've been in the water. The intense heat can cause dehydration, fainting, and even burns depending on the temperature, so stick to 10- or 15-minute soaks with breaks in between to allow your body temperature to return to normal.