寿の湯（法師温泉）Kotobuki-no-yu (Hoshi Onsen)
A calcium- and sodium-led sulfate spring with a settled, mineral-rich feel. Mildly alkaline and soft on the skin, it is moderately mineralized with noticeable body. The posting at the Choju-no-yu bath states that the water is served as untreated source flow, with no dilution, heating, recirculation or disinfection.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺279mgP88 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺93.9mgP26 national
- Potassiumminor; matters for drinking limitsK⁺3.4mgP28 national
- Magnesiummild; a laxative if you drink itMg²⁺0.22mgP15 national
- Manganesetrace; companion to iron springsMn²⁺0.10mgP37 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻709mgP93 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻114mgP45 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻16.5mgP11 national
- Fluoridetrace; drinking limits applyF⁻0.70mgP38 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃39.9mgP21 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.3mgP37 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.13mgP70 national
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.