A blended source: Akandana-no-Yu and Yunoue-no-Yu are combined with local valley water, and the certificate samples the mix at the hotel's mixing-tank outlet rather than at a wellhead. The result is a sodium-calcium bicarbonate-chloride water — the juso-sen style, soft on the skin — with a salt-tinged finish and a neutral pH that keeps it undemanding. It runs colorless and clear but carries a faint carbonic tang and a whisper of hydrogen sulfide, the fingerprint of 187 mg/kg of free CO₂ still dissolved in it. Metasilicic acid at 177.8 mg/kg puts it squarely in Japan's beauty-water family, leaving the kind of silica film that makes skin feel noticeably smoother afterwards.
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, netto 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
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⁺180mgP47 national63%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺55.3mgP60 national22%
- Magnesiummild; a laxative if you drink itMg²⁺13.6mgP66 national9%
- Potassiumminor; matters for drinking limitsK⁺22.3mgP67 national5%
- LithiumLi⁺1.0mgP74 national1%
- StrontiumSr²⁺0.90mgP79 national0.2%
- BariumBa²⁺0.20mgP72 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻406mgP72 national56%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻158mgP51 national37%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻35.8mgP37 national6%
- FluorideF⁻0.50mgP30 national0.2%
- NitrateNO₃⁻0.40mgP55 national0.1%
- BromideBr⁻0.40mgP41 national0.1%
- Hydrogen sulfideHS⁻0.03mgP16 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃178mgP79 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂15.1mgP68 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂187mgP87 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.07mgP51 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —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.
- 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.