A blended source at Umegashima Onsen classified as a simple sulfur spring. At pH 9.76 it is strongly alkaline, giving the water a pronounced slippery feel, and at the source it carries a faint greenish-yellow tint with the unmistakable faint rotten-egg note of H₂S. With 65.4 mg/kg of metasilicic acid, it leaves skin feeling smooth.
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺69.8mgP21 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.4mgP9 national4%
- PotassiumK⁺0.80mgP11 national0.9%
- Magnesiummild; a laxative if you drink itMg²⁺0.09mgP7 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻135mgP47 national67%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻9.6mgP50 national10%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻14.4mgP27 national9%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1.4mgP2 national1%
- HydroxideOH⁻1.4mgP89 national1%
- BO2BO₂⁻—mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻—mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃65.4mgP41 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
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