A blended source pooling six wells from the Naruko district — the Onsen-jinja sulfur spring together with the Karasu-no-yu and Shimo-jigoku families. The water is strongly acidic at pH 2.5, sharp enough to prickle the skin and eyes, layered with the unmistakable sulfur note of H₂S and the metallic edge of iron(II) that lends a faint amber cast. At 181.8 mg/kg of metasilicic acid, the silica film softens the bite slightly, but the temperament stays in the family that Naruko is renowned for: long documented relief for stubborn skin conditions.
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
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
- 病気の活動期（特に熱のあるとき）
- Active tuberculosis
- 貧血など身体衰弱の著しい場合
- 少し動くと息切れがするような重い心臓・肺の病気
- 腎臓の病気
- 消化管出血
- 目に見える出血があるとき
- 皮膚又は粘膜の過敏な人
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⁺185mgP47 national45%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺41.3mgP54 national15%
- Magnesiummild; a laxative if you drink itMg²⁺—mg8%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺—mg7%
- Manganesetrace; companion to iron springsMn²⁺—mg7%
- Aluminumshows up in acidic volcanic waterAl³⁺24.8mgP92 national5%
- LithiumLi⁺1.4mgP86 national
- AmmoniumNH₄⁺0.20mgP30 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻599mgP89 national79%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻127mgP45 national19%
- HSO4HSO420.7mgP60 national2%
- Fluoridetrace; drinking limits applyF⁻0.40mgP31 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃182mgP81 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.10mgP5 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂165mgP86 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S16.3mgP96 national
- ArsenicAs0.09mgP62 national
- CadmiumCd< —mg
- CopperCu< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- 筋肉若しくは関節の慢性的な痛み又はこわばり（関節リウマチ等）
- 打撲・捻挫などの慢性期
- 冷え症
- 胃腸機能の低下
- 耐糖能異常（糖尿病）
- Mild hypercholesterolemia
- 切り傷
- ストレスによる諸症状
- Cuts
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