A blend of the Oshirakawa No. 2 and No. 3 sources, sampled from the holding tank where the two meet: a sodium chloride spring, salty and heat-retaining. At 68.8 °C it ranks among Japan's hottest natural sources, and it is weakly alkaline and soft on the skin. A generous 174 mg/kg of metasilicic acid puts it in Japan's beauty-water family, leaving the thin silica film that makes skin feel noticeably smoother afterwards. The water is colorless and clear but carries a distinct hydrogen sulfide note, over a moderate 1,170 mg/kg of dissolved solids.
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, netto 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⁺297mgP63 national82%
- Potassiumminor; matters for drinking limitsK⁺55.1mgP86 national9%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺24.0mgP41 national8%
- MagnesiumMg²⁺1.7mgP32 national0.9%
- Aluminumshows up in acidic volcanic waterAl³⁺0.07mgP35 national
- Manganesetrace; companion to iron springsMn²⁺0.05mgP22 national
- FeTotalFe< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻410mgP73 national77%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻146mgP52 national16%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻49.8mgP41 national7%
- FluorideF⁻0.90mgP43 national0.3%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- NitrateNO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃174mgP78 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.6mgP60 national
- Metaarsenious acidHAsO₂0.90mgP88 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂19.5mgP50 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.