A sodium-led bicarbonate-chloride spring — the classic jūsō-sen (soda-water) style with a salty edge layered in. Weakly alkaline and soft on the skin, it sits in the Japanese beauty-water family valued for leaving skin smooth. The water carries a faint yellow tint and a slight iron taste, with moderate mineralization (about 2,205 mg/kg dissolved solids) giving it noticeable body.
Good for
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
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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
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
- acute phase of illness (especially when feverish)
- active tuberculosis
- advanced malignant tumors or severe anemia with marked physical debilitation
- severe cardiopulmonary insufficiency causing breathlessness on slight exertion
- severe renal failure with edema
- gastrointestinal bleeding
- any visible bleeding
- acute exacerbation of a chronic illness
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⁺650mgP78 national97%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺6.1mgP20 national1%
- PotassiumK⁺11.0mgP48 national1.0%
- MagnesiumMg²⁺1.6mgP31 national0.5%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.70mgP60 national
- StrontiumSr²⁺0.40mgP59 national
- LithiumLi⁺0.09mgP17 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- CopperCu²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- ZincZn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,200mgP91 national71%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻260mgP61 national27%
- Fluoridetrace; drinking limits applyF⁻7.7mgP88 national1%
- CarbonateCO₃²⁻6.7mgP39 national0.8%
- BromideBr⁻0.10mgP16 national0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃35.0mgP18 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂24.0mgP76 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂23.0mgP55 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.01mgP29 national
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
Measured at the source
On the certificate
- chronic muscle or joint pain and stiffness
- peripheral circulatory disorders
- diminished gastrointestinal function
- mild hypertension
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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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.