Going to onsen, or hot spring spa, is a favorite pastime for Japanese. Overnighting at a posh ryokan, offering sumptuous meals and attractive views, is a luxury. Being a volcanic archipelago, however, Japan can provide hot spring anywhere, if deep enough holes are dug up. Thus, onsen have sprung up even in crowded cities like Tokyo, and have become readily accessible. Today we can stop at a spa on the way home after work. In the past, only deep hot bath was available, but it is not rare today to find jacuzzi, sauna and hammam, or steam bath, as well (See a photo of the hot spring spa in Tokyo I enjoy going).
What I like the most at the onsen I frequent in Tokyo is hammam as I enjoyed it in Western Europe while I was based in Geneva. A big difference between the one here and those I experienced over there is a large vaseful of salt placed at the entrance. We scoop a handful of it and rub it all over the body while sitting and sweating there.
I thought salt was being used for the same purpose as in preparing shimesaba, or salted, vinegar-marinated mackerel, as a topping for sushi (see the photo from https://snapdish.co/d/). Salt rubbed around the fish would extract water from the flesh, making it firm. I thought it was a Japanese way to perspire more to turn flabby muscles into a firm physique while in a steam bath.
What I was told later was that by scrubbing the body with salt, we let tiny grains of salt do a good job of even cleaning pores of the skin to help us better perspire. By all means, we won’t come out like a Greek statue only after 30 minutes in it. Whatever its effect may be, I prefer to sit in hammam with salt.