A magnesium-calcium-sodium sulfate spring (41.8 °C), neutral and moderately mineralized at 3,332 mg/kg — settled, mineral-rich body without a heavy edge. A touch of metasilicic acid layers in a soft, slightly silky feel on the skin.
Good for
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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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.
- Magnesiummild; a laxative if you drink itMg²⁺264mgP97 national43%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺304mgP89 national30%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺264mgP58 national23%
- Potassiumminor; matters for drinking limitsK⁺68.0mgP88 national3%
- Iron (II)Fe²⁺0.70mgP58 national0.1%
- ManganeseMn²⁺0.70mgP72 national0.1%
- ZincZn²⁺0.07mgP69 national
- CopperCu²⁺0.02mgP75 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.01mgP19 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,884mgP100 national77%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻284mgP62 national16%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻208mgP58 national7%
- BromideBr⁻0.80mgP54 national0.0%
- Hydrogen sulfideHS⁻0.20mgP40 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃50.2mgP29 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.7mgP28 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.8mgP38 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP61 national
- ArsenicAs0.03mgP40 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.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- 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.