A sodium-calcium chloride spring at isotonic strength — salty, heat-retaining, and richly mineralized at roughly 8.8 g/kg of dissolved solids that leaves a mineral film on the skin. The wellhead is a cool 27.5 °C, so the water is heated and served at around 42 °C with recirculation and chlorine disinfection.
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
Take care
- 病気の活動期(特に熱のあるとき)
- Active tuberculosis
- 進行した悪性腫瘍又は高度の貧血などの全身衰弱をきたしている場合
- 少し動くと息苦しくなるような重い心臓又は肺の病気
- むくみのあるような重い腎臓又は消化器病、出血している病気
- 慢性疾患の急性増悪期
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⁺2,687mgP93 national75%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺626mgP94 national20%
- Magnesiummild; a laxative if you drink itMg²⁺59.4mgP86 national3%
- StrontiumSr²⁺39.9mgP99 national0.6%
- PotassiumK⁺31.0mgP75 national0.5%
- AmmoniumNH₄⁺2.0mgP81 national0.1%
- LithiumLi⁺0.80mgP70 national0.1%
- BariumBa²⁺4.2mgP96 national0.0%
- Iron (II)Fe²⁺1.8mgP70 national0.0%
- ManganeseMn²⁺0.10mgP36 national0.0%
- Iron (III)Fe³⁺1.0mgP86 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5,267mgP93 national99%
- BicarbonateHCO₃⁻33.8mgP20 national0.4%
- BromideBr⁻18.4mgP93 national0.1%
- IodideI⁻1.7mgP86 national0.0%
- FluorideF⁻0.30mgP24 national0.0%
- Hydrogen phosphateHPO₄²⁻0.10mgP34 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- NitrateNO₃⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃22.1mgP7 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.0mgP19 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂3.1mgP24 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CadmiumCd< —mg
- ChromiumCr< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- 運動麻痺における筋のこわばり
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- 痔の症状
- Peripheral circulation disorders
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.