Nanakuri-no-yu rises in the Yokoyu quarter of Yamanouchi, Nagano, as a sodium-calcium sulfate-chloride spring — sulfate leads the anions with a chloride edge behind it. The water is neutral and hypotonic, moderately mineralized at about 1.4 g/kg of dissolved solids: almost colorless and clear, odorless, with a faint salt tang on the lips. What lifts it above an ordinary sulfate bath is metasilicic acid at 153.8 mg/kg, enough silica to leave a thin moisturizing film and place the water squarely in Japan's beauty-water family. Worth knowing that the certificate was sampled at the bath inlet of the No.7 public bath rather than at the wellhead, so these figures describe the water as it actually reaches the tub.
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⁺253mgP59 national58%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺139mgP80 national37%
- Potassiumminor; matters for drinking limitsK⁺21.1mgP65 national3%
- Magnesiummild; a laxative if you drink itMg²⁺4.6mgP49 national2%
- LithiumLi⁺0.60mgP59 national0.5%
- StrontiumSr²⁺0.60mgP73 national0.1%
- AluminumAl³⁺0.05mgP30 national0.0%
- ManganeseMn²⁺0.10mgP36 national0.0%
- Iron (II)Fe²⁺0.07mgP17 national0.0%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻449mgP85 national50%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻313mgP67 national47%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻29.8mgP18 national3%
- CarbonateCO₃²⁻3.5mgP27 national0.6%
- FluorideF⁻1.2mgP49 national0.3%
- BromideBr⁻1.6mgP69 national0.1%
- Hydrogen phosphateHPO₄²⁻0.70mgP76 national0.1%
- IodideI⁻0.40mgP66 national0.0%
- NitrateNO₃⁻0.08mgP20 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃154mgP70 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂44.8mgP86 national
- Metaarsenious acidHAsO₂0.80mgP87 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.1mgP35 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.