ひらゆの森の湯Hirayu no Mori no Yu Spring
The certificate classes this water explicitly as a hydrogen-sulfide-type sulfur spring, laid over a bicarbonate base with a chloride edge. The pH is neutral and the water is soft to the touch, yet it reaches the wellhead visibly gassy — fine bubbles, a clear rotten-egg note of hydrogen sulfide, and a weakly carbonated tang. It carries noticeably more free hydrogen sulfide than Suiseki no Yu, the other source certified at this site, so the sulfur character is the stronger of the two. Metasilicic acid is generous enough to leave the thin silica film that makes skin feel smoother afterwards, and the certificate states plainly that water is added to the bath because the source runs too hot.
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺163mgP43 national38%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺135mgP79 national36%
- Magnesiummild; a laxative if you drink itMg²⁺49.8mgP85 national22%
- Potassiumminor; matters for drinking limitsK⁺15.9mgP55 national2%
- LithiumLi⁺0.70mgP63 national0.6%
- ManganeseMn²⁺1.7mgP88 national0.3%
- StrontiumSr²⁺1.2mgP81 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻717mgP85 national62%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻151mgP50 national22%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻144mgP60 national16%
- Hydrogen sulfideHS⁻1.6mgP75 national0.3%
- FluorideF⁻0.60mgP35 national0.2%
- NitrateNO₃⁻0.60mgP69 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃62.2mgP38 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂18.2mgP73 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂358mgP93 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S7.4mgP90 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —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.
- 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