Hosshō-no-yu at Manza is a simple sulfur spring of the hydrogen sulfide type, carrying an unusually heavy 286 mg/kg of free H₂S — the water is colorless and clear, but the unmistakable rotten-egg note comes off it immediately. At pH 3.1 it is weakly acidic and faintly tangy on the skin, and the dissolved solids are light at 0.64 g/kg, so the character of this water comes from its gases rather than its salts. A generous 75.7 mg/kg of metasilicic acid still leaves the kind of thin silica film that makes skin feel noticeably smoother afterwards. The sample was drawn at the inflow pipe of the aeration tank rather than at the wellhead itself.
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
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺90.1mgP69 national50%
- Magnesiummild; a laxative if you drink itMg²⁺16.1mgP68 national15%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺29.2mgP6 national14%
- Aluminumshows up in acidic volcanic waterAl³⁺7.5mgP86 national9%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.80mgP63 national9%
- Potassiumminor; matters for drinking limitsK⁺5.2mgP34 national2%
- Iron (II)Fe²⁺1.4mgP67 national0.6%
- ManganeseMn²⁺0.79mgP73 national0.3%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻344mgP77 national81%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻55.4mgP28 national18%
- HSO4HSO49.2mgP57 national1%
- FluorideF⁻1.4mgP55 national0.8%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃75.7mgP45 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.90mgP12 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂440mgP95 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S286mgP100 national
- ArsenicAsNDmg
- CopperCuNDmg
- 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.
- 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