Yagen no Yu is a simple sulfur spring — hypotonic, alkaline and hot at the wellhead. The water runs colorless and clear, yet carries a very faint rotten-egg note of H₂S and a weakly sulfurous taste, with 11.0 mg/kg of hydrosulfide ion behind it. At pH 9.0 it is solidly alkaline, and 55.5 mg/kg of metasilicic acid places it in Japan's beauty-water family — the slippery, skin-smoothing kind. Overall mineralization is light at 728 mg/kg, so the character comes from the sulfur rather than from body. The certificate's companion panel names it specifically for atopic dermatitis, psoriasis vulgaris, chronic eczema and pyoderma — the classic sulfur-spring indications.
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
From a certificate analyzed in 2022.
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Advanced malignant tumor, severe anemia or other marked physical debilitation
- Severe heart or lung disease causing breathlessness on slight exertion
- Severe kidney disease with edema
- Gastrointestinal bleeding
- Any visible bleeding
- Acute exacerbation of a chronic illness
- People with sensitive skin or mucous membranes
- Dry skin in the elderly
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⁺208mgP53 national85%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺26.2mgP43 national12%
- AmmoniumNH₄⁺3.3mgP86 national2%
- PotassiumK⁺2.7mgP25 national0.7%
- LithiumLi⁺0.20mgP37 national0.3%
- MagnesiumMg²⁺0.10mgP11 national0.1%
- AluminumAl³⁺0.0000mg0.0%
- CopperCu²⁺0.0000mg0.0%
- Iron (II)Fe²⁺0.0000mg0.0%
- Iron (III)Fe³⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- StrontiumSr²⁺0.0000mg0.0%
- ZincZn²⁺0.0000mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻314mgP67 national82%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻30.4mgP35 national6%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻31.2mgP85 national5%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻21.2mgP14 national3%
- Hydrogen sulfidesulfur character — antibacterialHS⁻11.0mgP94 national3%
- Fluoridetrace; drinking limits applyF⁻2.4mgP62 national1%
- BromideBr⁻1.2mgP64 national0.2%
- HydroxideOH⁻0.20mgP68 national0.1%
- IodideI⁻0.20mgP51 national0.0%
- Hydrogen phosphateHPO₄²⁻0.0000mg0.0%
- HSO4HSO40.0000mg0.0%
- NitriteNO₂⁻0.0000mg0.0%
- NitrateNO₃⁻0.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- AsO2AsO₂⁻0.0000mg
- BO2BO₂⁻0.0000mg
- HSiO3HSiO₃⁻0.0000mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃55.5mgP33 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂20.5mgP75 national
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAsNDmg
- CadmiumCdNDmg
- ChromiumCrNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
Measured at the source
On the certificate
From a certificate analyzed in 2022.
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Cold sensitivity
- Reduced gastrointestinal function (heaviness, bloating)
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Stress-related symptoms (sleep disorders, depressed mood)
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