A simple sulfur spring of the hydrogen-sulfide type rising at Kojigoku in Unzen. At the source the water is milky and slightly turbid, with a faintly acidic taste and the characteristic rotten-egg note of H₂S. It is a weakly acidic, hot spring carrying 80.5 mg/kg of metasilicic acid, which leaves the skin smooth. The posted indications cover neuralgia, muscle and joint pain, sensitivity to cold and recovery from fatigue. Because the source temperature is high, fresh water may be added to cool it.
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
- acute illness (especially when fever is present)
- active tuberculosis
- malignant tumors
- severe heart disease
- respiratory failure
- renal failure
- hemorrhagic disease
- severe anemia
- any other illness in a progressive stage
- pregnancy (especially early and late stages)
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⁺7.1mgP21 national26%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺6.8mgP1 national22%
- AmmoniumNH₄⁺4.2mgP90 national17%
- Magnesiummild; a laxative if you drink itMg²⁺2.3mgP36 national14%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.10mgP54 national7%
- Aluminumshows up in acidic volcanic waterAl³⁺0.80mgP77 national6%
- Potassiumminor; matters for drinking limitsK⁺2.5mgP25 national5%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺1.0mgP62 national3%
- StrontiumSr²⁺0.0000mg0.0%
- Manganesetrace; companion to iron springsMn²⁺1.0mgP76 national
- CopperCu²⁺NDmg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻72.1mgP48 national90%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4.4mgP6 national7%
- Thiosulfateoxidized sulfurS₂O₃²⁻2.0mgP84 national2%
- FluorideF⁻0.0000mg0.0%
- BicarbonateHCO₃⁻0.0000mg0.0%
- Hydrogen sulfideHS⁻0.0000mg0.0%
- NitrateNO₃⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃80.5mgP47 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂239mgP90 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S3.5mgP83 national
- CadmiumCdNDmg
- LeadPbNDmg
Measured at the source
On the certificate
- chronic digestive disease
- sensitivity to cold
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