中の湯（旧本館泉）Nakanoyu (Former Main Building Spring)
A simple sulfur spring of the hydrogen-sulfide type — colorless and clear in the cup, yet carrying the unmistakable faint rotten-egg note of H₂S, with free hydrogen sulfide, hydrosulfide and thiosulfate all present in the water. Dissolved solids come to just 536 mg/kg, so the water is lightly mineralized and gentle on the body, and the pH sits essentially neutral at the wellhead. What stands out chemically is metasilicic acid at 147.3 mg/kg — enough silica to leave the thin film that makes skin feel noticeably smoother afterwards.
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
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⁺82.8mgP24 national70%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺21.3mgP39 national21%
- Potassiumminor; matters for drinking limitsK⁺10.4mgP46 national5%
- Magnesiummild; a laxative if you drink itMg²⁺1.8mgP34 national3%
- LithiumLi⁺0.40mgP47 national1%
- AmmoniumNH₄⁺0.60mgP52 national0.6%
- StrontiumSr²⁺0.30mgP52 national
- Manganesetrace; companion to iron springsMn²⁺0.20mgP46 national
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- BariumBa²⁺NDmg
- CopperCu²⁺NDmg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺NDmg
- Iron (III)Fe³⁺NDmg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺NDmg
- ZincZn²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻157mgP53 national48%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻78.7mgP33 national41%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻23.3mgP34 national9%
- Fluoridetrace; drinking limits applyF⁻1.7mgP55 national2%
- ThiosulfateS₂O₃²⁻2.1mgP84 national0.7%
- Hydrogen sulfideHS⁻0.40mgP50 national0.2%
- Bromidetrace, sea-originBr⁻0.20mgP28 national
- NitrateNO₃⁻0.20mgP42 national
- AsO2AsO₂⁻NDmg
- BO2BO₂⁻NDmg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Hydrogen phosphateHPO₄²⁻NDmg
- HSiO3HSiO₃⁻NDmg
- HSO4HSO4NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- NitriteNO₂⁻NDmg
- HydroxideOH⁻NDmg
- PhosphatePO₄NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃147mgP68 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.8mgP43 national
- H2SO4FreeH2SO4FreeNDmg
- Phosphoric acidH₃PO₄NDmg
- Metaarsenious acidHAsO₂NDmg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂73.4mgP74 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S2.4mgP81 national
- ArsenicAs0.01mgP25 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.
- 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