Not every onsen is the same onsen. Depending on which minerals are dissolved in the water rising from underground — its senshitsu (泉質), or water quality/type — the color, smell, texture, and benefits can differ completely. Once you understand water types, the question stops being "which onsen is better" and becomes "which onsen is right for me."
Onsen are classified by mineral content. A milky, sulfur-scented bath and a clear, odorless, silky one are essentially different categories of water.
What Counts as an Onsen? Understanding Gensen-kakenagashi
Legally, an onsen is groundwater heated by geothermal activity and containing specific minerals. Two terms you'll see often:
- Gensen-kakenagashi (源泉かけ流し): Fresh spring water that flows continuously through the bath without being recirculated or reused — considered the premium way to experience an onsen.
- Recirculated systems: Water that's heated, filtered, and recirculated — easier to manage for hygiene and consistent temperature.
Major Water Types and Their Benefits
Simple springs (単純温泉)
Low mineral content and gentle on the skin — a great choice for first-timers or sensitive skin.
Chloride springs (塩化物泉)
The salt content coats the skin and delivers strong warmth retention. You'll stay warm long after getting out, which is why these are sometimes called "the warming bath."
Bicarbonate springs (炭酸水素塩泉)
Known as the "bath of beauty" (bijin no yu) for how it softens dead skin cells, leaving skin noticeably smooth.
Sulfur springs (硫黄泉)
Recognizable by its distinctive rotten-egg smell and cloudy white or pale blue color. It has a strong personality and is said to be good for the skin and exfoliation (though it can be irritating, so sensitive skin should rinse lightly afterward).
Acidic springs (酸性泉)
A sharp, tingling water with strong antibacterial properties — Kusatsu is the classic example. If you have sensitive skin, avoid long soaks.
Iron springs (含鉄泉)
Iron reacting with air gives the water a reddish-brown tint. Said to help with warmth retention and circulation.
Carbonated springs (二酸化炭素泉)
Fine carbonation bubbles cling to your skin in the bath — known for promoting circulation.
Half the Fun Is the Color and Smell
Trying a milky-white sulfur spring, a reddish-brown iron spring, and a clear simple spring all on the same trip is part of the fun of onsen travel. Since each hot spring town tends to have its own signature water type, it can even be a useful way to decide where to go next.
💡 A caution on soaking: with stronger water types (sulfur, acidic), don't stay in too long, and if your skin still feels irritated afterward, rinse lightly with lukewarm water. Avoid bathing after drinking alcohol, on an empty stomach, or with high blood pressure.
In short, an onsen isn't just "water" — it's chemistry. Once you know your water types, you'll never pick your next onsen trip the same way again.
Where can you experience this?
Book directly at Japan's best onsen towns with Ryokanear.