Hokokukan draws Nigayu — the certificate's name for it means "bitter water" — a naturally free-flowing source at Manza, classified as an acidic, sulfur-bearing sodium sulfate-chloride spring of the hydrogen sulfide type. The acidity is sharp enough to prickle on the skin, and the unmistakable rotten-egg note of H₂S rises straight off the water; it looks colorless and clear, yet carries a heavy load of free hydrogen sulfide. The source ranks among Japan's hottest natural springs, and a generous 158.7 mg/kg of metasilicic acid leaves the kind of thin silica 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
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, netto 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.
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺6.3mgP79 national38%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺105mgP30 national27%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺39.0mgP52 national12%
- Magnesiummild; a laxative if you drink itMg²⁺22.6mgP72 national11%
- Aluminumshows up in acidic volcanic waterAl³⁺10.5mgP90 national7%
- Potassiumminor; matters for drinking limitsK⁺25.4mgP71 national4%
- Iron (II)Fe²⁺4.3mgP81 national0.9%
- ManganeseMn²⁺2.0mgP90 national0.4%
- AmmoniumNH₄⁺0.30mgP39 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻563mgP89 national70%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻132mgP47 national22%
- HSO4HSO4120mgP78 national7%
- FluorideF⁻1.0mgP46 national0.3%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃159mgP72 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.4mgP46 national
- H2SO4FreeH2SO4Free1.9mgP75 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S49.7mgP98 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂31.2mgP61 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.
- 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