A sodium-calcium chloride-sulfate spring created by blending steam-generated water from Owakudani and Hakone Tozan Railway wells. At pH 2.5 it is strongly acidic, sharp enough to sting slightly on contact and traditionally cited for stubborn skin conditions. The salt-and-sulfate body gives it noticeable mineral weight; the source emerges at a hot 56 °C and is cooled to about 41 °C at the bath.
Good for
Skin conditions
Sulfur and acidic waters are antibacterial and keratolytic, studied for atopic dermatitis and psoriasis.
atopic skin, eczema
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
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
Take care
Sensitive-skin caution
Strongly acidic or sulfur-rich water can sting broken, atopic-flare, or dry elderly skin. Ease in slowly and rinse off if it stings.
broken or dry skin
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⁺155mgP41 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺81.1mgP67 national
- Magnesiummild; a laxative if you drink itMg²⁺29.8mgP76 national
- Potassiumminor; matters for drinking limitsK⁺20.0mgP62 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺19.8mgP95 national
- Aluminumshows up in acidic volcanic waterAl³⁺6.3mgP85 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺3.2mgP74 national
- Manganesetrace; companion to iron springsMn²⁺1.4mgP85 national
- StrontiumSr²⁺0.24mgP45 national
- LithiumLi⁺0.17mgP31 national
- ZincZn²⁺0.03mgP57 national
- CopperCu²⁺NDmg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻402mgP71 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻390mgP81 national
- HSO4HSO441.3mgP64 national
- Bromidetrace, sea-originBr⁻0.48mgP40 national
- NitrateNO₃⁻0.42mgP51 national
- Fluoridetrace; drinking limits applyF⁻0.12mgP10 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃120mgP61 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂14.1mgP65 national
- H2SO4FreeH2SO4Free0.43mgP70 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP61 national
- ArsenicAs0.20mgP80 national
- LeadPb0.01mgP46 national
- CadmiumCdNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
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.
- Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
- 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.
- Ministry of the Environment, Japan. 禁忌症及び入浴又は飲用上の注意事項 (2014 notice).official