A sodium-led bicarbonate-chloride hot spring at 51.6 °C, with a chloride sub-type adding a salty edge to the bicarbonate softness. Weakly alkaline at pH 8.0 and soft on the skin, it sits in the Japanese beauty-water family. A generous 181 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother. The water carries a faintly yellow-brown tint and a mild salty note on the lips.
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
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⁺307mgP62 national82%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺27.8mgP44 national9%
- Potassiumminor; matters for drinking limitsK⁺45.8mgP82 national7%
- MagnesiumMg²⁺1.7mgP31 national0.9%
- LithiumLi⁺0.80mgP70 national0.7%
- AmmoniumNH₄⁺0.70mgP56 national0.2%
- BariumBa²⁺—mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺—mg
- StrontiumSr²⁺—mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻644mgP82 national67%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻146mgP47 national26%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻49.7mgP39 national7%
- FluorideF⁻1.7mgP55 national0.6%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃181mgP81 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.5mgP60 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂18.5mgP51 national
- CadmiumCd< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—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.