A sodium-led chloride spring with a bicarbonate sub-type — a salty, heat-retaining water softened by a bicarbonate layer. Weakly alkaline and gentle on the skin, with 7.1 g/kg of dissolved solids that leave a noticeable mineral film. The source is a low-temperature spring (under 34 °C), so it is reheated before bathing.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺2,094mgP92 national91%
- Magnesiummild; a laxative if you drink itMg²⁺41.1mgP84 national3%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺46.8mgP57 national2%
- Aluminumshows up in acidic volcanic waterAl³⁺—mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺—mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,971mgP87 national54%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,379mgP98 national38%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻79.2mgP51 national2%
- FluorideF⁻4.3mgP74 national0.2%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃63.1mgP39 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂—mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.2mgP19 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.