The names I’ve given to these types of baths are a combination of translating the Japanese name with my own nomenclature. Even in Japanese the names of the types of baths changes among different places and no nomenclature seems set in stone.
Normal bath - 風呂、浴場
A basic basin filled with hot water, typically 41℃. These baths are also classified in Japanese as fukai 深い deep or asari 浅いshallow. In general, you can lay down flat in a shallow bath and still keep your head above water, while a deep bath would rise up to your waist or higher if you stood in it.
Bubble bath - 泡風呂
These are not soap bubbles, but air bubbles coming up from the bottom of a tub. Quite common and typically put into part of a normal bath.
Electric bath - 電気風呂、ヘルスバス
Running electric current through the water must be the stupidest idea ever, and trust me I thought the same the first time I saw one of these, but it isn’t. An electric baths are typically built into a corner of a normal bath or in their own separate basin. These baths have two white panels with holes in them, typically placed across from each other. The current, less than what is contained in a AA battery, emits from the panels. The closer you get the stronger the effect. The sensation is a vibrating tingling in your muscles. You’ll notice that it is typically one of the most popular places in the bath and you sometimes have to way to use it. Personally I use it on my lower back and calfs, but people do all sorts of things with it.Of course there is a health risk in using electric baths. There is always a warning sign near the electric bath that lists off the various aliments that would prohibit its use. The main are high blood pressure and heart ailments. I only use it on my body from my lower back and below. Lowering your body so the electricity is gets to your heart seems like a bad idea.
Jet baths - ジェットバス、エステバス
This is my catchall name for any type of pressurized jet that you would see in most hot tubs. There are a myriad of types used in Japan that are aimed at relieving all sorts of muscle pain. The most basic of them consists of a single jet, but some have jets in pairs, in rows, in a large sunflower pattern, jets that spin in circles, and so on. You either sit, stand or lay down in these baths. Besides the lower back, the sides, legs, feet and shoulders have special jet baths. Some of these baths even have stainless steel headrests that are cooled by water on the inside.All of these jets are given names like “rolling bath” or “este bath” for example. Most sento will have a plaque, typically provided by the maker of the bath, on the wall naming and explaining the benefits of type of jet configuration. These plaques also have some of the best drawings on them. Invariably there will be a woman with a bit of her breasts showing and strategically placed air bubbles covering the rest of her body. I love these drawings. They, like most sentos themselves, are artifacts of a era which ended long ago. I’ve noticed that many recently built supersento will have a single labyrinthine bath filled with different jets. They look like a level out of the original Legend of Zelda game for the NES when seen from above.
Mircobubble / Nanobubble - ムイクロ・ナノバブル
These baths are infused with tiny bubbles of gas to the degree that the water appears milky or opaque. They can tickle sometimes and are most common cooler than the other baths.
Carbonated - 炭酸風呂
The bath is carbonated artificially and produces bubbles on your skin. Imagine bathing in soda water. These baths are typically are around 38~39 degrees as if they get any hotter the bubbles will leave the water.
Sleeping baths - 寝湯
These baths are stone slabs typically at a slight incline with a place for one’s head. The bath is filled with a few centimeters of water, but not enough to cover a person. In other designs the water only flows down the stone.
Tub baths - 壷湯
These are big round cauldrons that typically fit one or two people. Often they are filled with hot spring water, herbal additives, or other treatments. Typically they are quite popular.
Showers - シャワー
This means a standing shower using the same spray as in the washing area or a large sunflower type head.
This is a type of shower where the water comes from multiple jets on mounted on the wall. Because of the spray, these are set in semi-enclosed areas. The temperature of the water cannot be changed and can be quite cold. I have found that these are often broken.
Waterfall - 打たせ湯
Water is poured down from above either by a forceful jet, or just via the pull of gravity. The power of the water is used to massage typically the shoulders, neck and head. These are either in a semi-enclosed space, outdoors or both due to the large castoff that happens when you stand underneath. Putting your head into the stream is fine, although some of these are very strong and it might be painful.
Foot bath - 足湯
These are low, shallow baths that are only meant for your feet and lower leg. When found in the bathing areas, they are typically attached to a stone seat that has warm water flowing over it. Often times these are found outside and offered for free. Many onsen villages will have a central foot bath filled with that towns spring water. Sometimes the foot bath is attached to an souvenir store or near the train station.A derivation of the foot bath is to have the bottom is filled with smooth rock to massage your feet as you step around in the bath. This is something that I’ve only seen west Kobe in Kakogawa and Himeji.
Herbal bath / Medicated bath / Rotating bath - 薬湯、替わり湯
Many sento that do not have a natural hot spring will have some sort of herbal bath. These use artificial fragrances, minerals, plant extracts, and herbs to change the way the water feels and smells. Either the type of additive will change daily, weekly, or they don’t change at all. If the additive stays the same it is most likely a type of Chinese medicinal herbs called kanpo 漢方. Often the herbs are in a fabric sack that sits in the water like a big tea bag. Typically herbal baths make the bath water look like tea, while the more exotic, and perhaps artificial, additives can make the water green, blue or pink. Herbal additives typically have signs that claim various health benefits.
Meguri bath - 巡り
Meguri means tour or pilgrimage. These baths let you tour the various hot springs of Japan in one place--at least that’s what they say. The water in a meguri bath is treated with minerals and salts in order to make it feel like a famous onsen. On occasion I’ve been quite surprised by these baths, other times they didn’t feel like anything different. I’m also not sure how they do this to the water. Is it actually tanked down and recycled, which seems unlikely, or is it just a powder they put into the water?Typically there will be a sign stating the name and location of the hot spring the bath is mimicking.
Salt water bath - 潮風呂
These occur in two ways. Either actual ocean water is treated, heated and pumped into the baths, or salt is added to the water. Some places add so much salt making it more saline than sea water. This gives the same effect as Dead Sea water making floating very easy.Be careful if you have any cuts or sores on your body as they will sting.
Radon bath - ラドン風呂
The water is treated with radon gas and sometimes also sprayed as a mist into the room. The radioactivity is absorbed through the skin and breathed in. These baths are often enclosed as not to contaminate the other bathers. Some think radon is good for the health while others think knowingly exposing your body to radioactivity is just asking for cancer. Either way it’s not recommended to stay for long periods of time. I would also think children and pregnant women should also stay away from these baths.
Water Temprature Types
Cold bath - 水風呂
Typically chilled to 17-19℃ cold water baths are typically used for cooling down after using the sauna. You’ll also find that they are very popular with the young children. One sign that is typically placed near the cold water bath is:
汗をかけて入ってください。(Wash off sweat before entering.)
After sitting in the sauna for a few minutes anyone will sweat. It is only polite to splash off that sweat before entering the bath. Sadly, you’ll notice that this rule isn’t followed very strictly. One rule which is typically ignored for the cold water bath is not putting your head under the water. Dunking your whole body into the cool water after baking in the sauna feels too good. I am guilty of doing this on occasion too.
Luke warm bath - ぬるい風呂
Any bath with a temperature from 38℃ ~ 40℃
Any bath with a temperature from 40℃ ~ 42℃
Extra hot bath - 高風呂
Any bath with a temperature over 42℃
Natural Spring Bath Types
Kakenagashi / Gensen - 掛け流し、源泉
The water, either as it bubbles up or is pumped from the ground, is poured directly as-is into the bath. All of the all overflow or drainage from the spring water is not recycled back into the tubs. Naturally occurring hot springs would also be classified as kakenagashi. Because the idea of a kakenagashi bath is to experience the hot spring in its most essential, natural form, they are most often not altered or treated with any chemical agent. Even though they are not treated, kakenagashi baths are thought to be the clean as new water is constantly refilling the tubs and there is no chance for any bacteria or other disease causing vector to form.
Heated (Kaon) - 加温
The spring water is heated to the desired temperature because the well temperature is too low. This is sometimes done in conjunction with recycling the spring water.
Water Added / Cooled (Kasui) - 加水
Normal tap water is added to the spring water to cool it and/or to dilute it. This is typically an accepted practice, but done in excess the quality of the spring water is lowered to the point of having no effect. During the onsen scandal of 2004 the overuse of kasui was addressed and laws were put in place to limit its use.
Recycled (Jyunkan) - 循環
The overflow is saved, treated, reheated (if applicable) and reintroduced into the baths. Recycling the water is accepted practice, but if done in an unethical manner it can lead to bacteria and fungus growing in the water which could cause disease. The use of recycled spring water that has not been treated is against the law.
Sauna - サウナ
A normal dry sauna. Typically signs ask that you towel off any excess water on your skin before entering.
Steam Sauna / Mist Sauna - ステームサウナ、ミストサウナ
A sauna where the room is heated by a faucet spraying out super heated water or by hot mist being put into the room. Sometimes herbs or a fragrance is added to water.
Salt Sauna - 塩サウナ
This is a sauna where a salt is available to rub on your skin. Rubbing salt on your skin is thought to increase sweating. Typically there are also pads to sit on in the sauna. Manners say that you should rinse off the mat after you use it. It is also very important that you rinse yourself off in the shower before entering any bath after leaving a salt sauna.
Radon sauna - ラドンサウナ
This is a steam sauna with radon mixed into the water. Like a radon bath it is thought to fight off cancer and be beneficial to health, but like the radon bath you should make your own decision about these baths. Children and pregnant women should avoid using the radon bath.
löyly - ロウリュ
This is a service offered at some supersento where staff will enter the dry sauna and fan the guests raising the heat. It increases the blood flow and sweating . Sometimes the staff will also add a fragrance to the room.