A hot 94.3 °C sodium chloride-sulfate spring where chloride and sulfate sit nearly side by side, giving moderately mineralized, heat-retaining body. Weakly alkaline and soft on the skin, it carries a faintly yellow tint at the source along with a clear hydrogen sulfide note and a touch of salt on the tongue. A generous 322 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother 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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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⁺386mgP69 national63%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺75.6mgP66 national14%
- Potassiumminor; matters for drinking limitsK⁺138mgP94 national13%
- Magnesiummild; a laxative if you drink itMg²⁺27.0mgP74 national8%
- LithiumLi⁺1.2mgP79 national0.6%
- AmmoniumNH₄⁺1.6mgP76 national0.3%
- Iron (II)Fe²⁺0.80mgP59 national0.1%
- Iron (III)Fe³⁺0.40mgP76 national0.1%
- ManganeseMn²⁺0.60mgP68 national0.1%
- StrontiumSr²⁺0.40mgP62 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻452mgP73 national47%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻444mgP85 national34%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻310mgP66 national19%
- BromideBr⁻1.7mgP69 national0.1%
- Hydrogen sulfideHS⁻0.30mgP47 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃322mgP97 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂32.6mgP81 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂23.4mgP55 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.
- 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.