A sodium-led sulfate-chloride spring emerging hot at 42.3 °C. At pH 9.7 it is alkaline, giving the slippery, skin-smoothing feel of the Japanese beauty-water family, with sulfate adding quiet body and a salty chloride edge. Moderately mineralized, it bathes with noticeable substance 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
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, netto 熱の湯).
cold sensitivity
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⁺351mgP67 national83%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺55.9mgP60 national15%
- AmmoniumNH₄⁺2.6mgP84 national0.8%
- PotassiumK⁺4.3mgP32 national0.6%
- AluminumAl³⁺0.10mgP56 national0.1%
- HydrogenH⁺0.0000mg0.0%
- BariumBa²⁺0.10mgP49 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.10mgP29 national
- Manganesetrace; companion to iron springsMn²⁺0.10mgP37 national
- StrontiumSr²⁺0.10mgP24 national
- LithiumLi⁺—mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻383mgP81 national46%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻236mgP60 national38%
- BO2BO₂⁻42.0mgP99 national6%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻70.4mgP96 national5%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻16.2mgP11 national3%
- Fluoridetrace; drinking limits applyF⁻4.6mgP76 national1%
- Hydrogen sulfideHS⁻1.5mgP74 national0.3%
- HydroxideOH⁻0.80mgP84 national0.3%
- BromideBr⁻0.10mgP14 national0.1%
- NitrateNO₃⁻0.30mgP49 national
- Iodideyellows in air; helps blood lipidsI⁻0.20mgP51 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- CadmiumCd< —mg
- ChromiumCr< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —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.