黒川温泉（のし湯）Kurokawa Onsen (Noshiyu)
A sodium-led bicarbonate spring layered with sulfate and chloride — the juso-sen style, soft on the skin and squarely in Japan's beauty-water family. Moderately mineralized at 1,956 mg/kg and neutral in pH, the water is almost colorless and clear, carrying a faint note of hydrogen sulfide at the source. A generous 303.8 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards, and the wellhead itself ranks among Japan's hottest natural sources.
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, netto 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⁺321mgP65 national61%
- Magnesiummild; a laxative if you drink itMg²⁺45.5mgP85 national16%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺62.2mgP62 national13%
- Potassiumminor; matters for drinking limitsK⁺76.1mgP90 national8%
- LithiumLi⁺1.2mgP79 national0.8%
- AmmoniumNH₄⁺0.60mgP52 national0.1%
- Iron (II)Fe²⁺0.50mgP53 national0.1%
- ManganeseMn²⁺0.40mgP61 national0.1%
- StrontiumSr²⁺0.40mgP61 national0.0%
- ZincZn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻551mgP79 national40%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻352mgP78 national33%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻218mgP58 national27%
- BromideBr⁻0.70mgP52 national0.0%
- Hydrogen sulfideHS⁻0.20mgP36 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃304mgP97 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂21.8mgP75 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂130mgP82 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.50mgP71 national
- ArsenicAs< —mg
- CopperCu< —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.