薬師泉源(さくら湯飲用)Yakushi Spring (Sakura-yu Drinking)
The Yakushi spring at Sakura-yu is classified as a weakly radioactive simple thermal water — light on total dissolved solids (270 mg/kg) but carrying a substantial 141 Bq/kg of radon, which places it in Japan's hormesis-tradition radon-spring lineage. Alkaline at pH 8.86, the water is soft on the skin, and a generous 71 mg/kg of metasilicic acid leaves the faint moisturising film that makes the touch noticeably smooth. Sampled from a drinking fountain, this is the source served for insen (drinking spring) (drinking) at the bath, with a long-standing reputation for chronic GI complaints.
Good for
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
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
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⁺43.7mgP11 national76%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺9.0mgP25 national18%
- Magnesiummild; a laxative if you drink itMg²⁺1.3mgP28 national4%
- Potassiumminor; matters for drinking limitsK⁺2.4mgP24 national2%
- Manganesetrace; companion to iron springsMn²⁺0.03mgP19 national
- StrontiumSr²⁺0.03mgP13 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.01mgP12 national
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- BariumBa²⁺NDmg
- CopperCu²⁺NDmg
- LithiumLi⁺NDmg
- ZincZn²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻114mgP45 national70%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻12.0mgP56 national15%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻11.2mgP23 national9%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3.6mgP5 national4%
- Fluoridetrace; drinking limits applyF⁻0.70mgP40 national1%
- Hydrogen sulfideHS⁻0.80mgP66 national0.8%
- HydroxideOH⁻0.10mgP50 national0.4%
- Bromidetrace, sea-originBr⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃71.4mgP44 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.40mgP8 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂NDmg
- ArsenicAsNDmg
- CadmiumCdNDmg
- 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.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- 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.