A sodium chloride spring — salty and heat-retaining, with the kind of body that lingers warmth on the skin. The water is weakly alkaline and soft on the skin, and at 80.3 °C at the source it ranks among Japan's hottest natural springs (cooled before bathing). It also carries a generous 200 mg/kg of metasilicic acid, which leaves a thin silica film for noticeably smoother skin.
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
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
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⁺432mgP71 national88%
- Potassiumminor; matters for drinking limitsK⁺71.2mgP88 national9%
- CalciumCa²⁺4.0mgP15 national0.9%
- MagnesiumMg²⁺1.0mgP28 national0.4%
- LithiumLi⁺2.5mgP93 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻645mgP76 national85%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻142mgP63 national14%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻14.0mgP9 national1%
- FluorideF⁻1.4mgP54 national0.3%
- ThiosulfateS₂O₃²⁻1.9mgP82 national0.1%
- NitrateNO₃⁻0.90mgP80 national0.1%
- Bromidetrace, sea-originBr⁻1.9mgP69 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃200mgP87 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂35.3mgP81 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂5.7mgP30 national
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.