A sodium-led bicarbonate spring layered with chloride and sulfate components, served hot at 61.5 °C. Neutral pH gives a settled, even feel on the skin — it sits in the broader 'beauty-water' family thanks to the bicarbonate base and a remarkable 297 mg/kg of metasilicic acid that leaves a thin silica film behind. The water itself carries a faintly astringent edge and a soft trace of hydrogen sulfide.
Good for
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
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
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
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⁺265mgP58 national61%
- Magnesiummild; a laxative if you drink itMg²⁺37.1mgP81 national16%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺53.8mgP58 national14%
- Potassiumminor; matters for drinking limitsK⁺59.3mgP86 national8%
- AmmoniumNH₄⁺3.7mgP88 national1%
- LithiumLi⁺0.80mgP70 national0.6%
- ManganeseMn²⁺0.50mgP64 national0.1%
- Iron (III)Fe³⁺0.30mgP72 national0.1%
- Iron (II)Fe²⁺0.30mgP43 national0.1%
- StrontiumSr²⁺0.20mgP44 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- BariumBa²⁺NDmg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻439mgP74 national39%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻225mgP57 national35%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻228mgP68 national26%
- FluorideF⁻0.30mgP24 national0.1%
- NitrateNO₃⁻0.60mgP63 national0.1%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Hydrogen phosphateHPO₄²⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- HydroxideOH⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃297mgP97 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂14.9mgP68 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂151mgP85 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.50mgP73 national
- ArsenicAs0.06mgP55 national
- ChromiumCrNDmg
- CopperCuNDmg
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.