A hot (48.7 °C) sodium chloride spring with notably high mineralization at 11.27 g/kg, falling on the hypertonic side. At the wellhead the water shows a faint yellow tint, a mild salty edge and a whisper of hydrogen sulfide — the cert even notes visible gas escape from this drilled self-flowing source. Weakly alkaline pH and the chloride character together give the bath strong heat retention and the characteristic mineral film on skin afterwards.
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⁺4,031mgP96 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺132mgP78 national4%
- PotassiumK⁺66.8mgP88 national0.9%
- MagnesiumMg²⁺11.7mgP63 national0.5%
- Iron (II)Fe²⁺0.40mgP48 national0.0%
- ManganeseMn²⁺0.20mgP45 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻6,462mgP94 national97%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻293mgP65 national3%
- BromideBr⁻8.0mgP87 national0.1%
- Hydrogen sulfidesulfur character — antibacterialHS⁻2.3mgP80 national
- Iodideyellows in air; helps blood lipidsI⁻0.04mgP24 national
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂228mgP98 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃31.8mgP14 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂9.6mgP40 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —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.