A blended source that combines the Yokoyu No.1 and No.2 wells with Netsu-no-yu, in Yamanouchi. Sodium and calcium lead a chloride water, with sulfate adding quiet body; it is weakly alkaline and soft on the skin, almost colorless and clear, with a faint salty, sulfurous note on the tongue. Metasilicic acid runs to 199 mg/kg — enough silica film to place this squarely in Japan's beauty-water family. The source is among the country's hottest at 86.5 °C, and worth noting: this certificate was sampled at the bath inlet of Matsu-no-yu rather than at the wellhead.
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, netto 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⁺335mgP66 national69%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺112mgP76 national26%
- Potassiumminor; matters for drinking limitsK⁺30.9mgP76 national4%
- LithiumLi⁺0.90mgP71 national0.6%
- MagnesiumMg²⁺0.50mgP20 national0.2%
- StrontiumSr²⁺0.50mgP68 national0.1%
- AluminumAl³⁺0.04mgP28 national0.0%
- ManganeseMn²⁺0.04mgP20 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻448mgP74 national60%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻364mgP79 national36%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻47.7mgP29 national4%
- FluorideF⁻1.4mgP53 national0.3%
- BromideBr⁻2.2mgP73 national0.1%
- ThiosulfateS₂O₃²⁻1.2mgP72 national0.1%
- Hydrogen sulfideHS⁻0.70mgP62 national0.1%
- IodideI⁻0.60mgP74 national0.0%
- Hydrogen phosphateHPO₄²⁻0.20mgP52 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃199mgP85 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂65.0mgP92 national
- Metaarsenious acidHAsO₂1.4mgP91 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.3mgP22 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.07mgP51 national
- CadmiumCdNDmg
- ChromiumCrNDmg
- 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.
- 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.