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Hoshi Sei Onsen Yama Megumi No Yu

Water profile

Hoshi Sei Onsen Yama Megumi No Yu

Spring chemistry and what it does, with the research behind it.

星生温泉(蒸気泉)Hoshisei Onsen (Steam Spring)

湯SimpleSodium-Calcium Sulfate-Bicarbonate spring

A remarkably hot steam-style source at 94.8 °C, classified as a simple thermal spring with neutral pH and a light mineral load. It sits among Japan's hottest natural sources — well above the 65 °C "top decile" line — and must be cooled substantially before bathing.

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

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.

Cationsshare of charge (mval%)
  • Sodium19.6mgP4 national
  • Calcium11.0mgP27 national
  • Magnesium3.7mgP42 national
Anionsshare of charge (mval%)
  • Sulfate48.6mgP39 national
  • Bicarbonate47.1mgP26 national
  • Chloride6.9mgP10 national
  • Fluoride0.20mgP17 national

Measured at the source

Source temperature
94.8°C
Hot

Evidence

  1. 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.
  2. Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
  3. Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
  4. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.

星生温泉(冷鉱泉源)Hoshisei Onsen (Cold Mineral Source)

黄SulfurCalcium-Magnesium Sulfate-Chloride spring

A cold mineral spring at 17.0 °C, classified as a simple sulfur source with a weakly acidic pH of 4.6. Because it sits below the 25 °C threshold, it is heated for bathing; the sulfur component carries the characteristic faint rotten-egg note.

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

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.

Cationsshare of charge (mval%)
  • Calcium39.4mgP52 national
  • Magnesium14.6mgP67 national
  • Sodium18.9mgP4 national
Anionsshare of charge (mval%)
  • Sulfate147mgP61 national
  • Chloride70.4mgP32 national
  • Fluoride0.10mgP9 national

Measured at the source

Source temperature
17.0°C
Cold mineral

Evidence

  1. 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.
  2. Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
  3. Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
  4. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  5. Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
  6. Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
  7. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.
  8. 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

星生温泉(楊風呂)Hoshisei Onsen (Yō-buro Bath Blend)

酸AcidicCalcium Sulfate spring · +含鉄(II,III)-単純泉

A blend of source ① (94.8 °C steam spring) and source ③, brought down to a more bathable 48.1 °C for the Yō-buro bath. The water is strongly acidic at pH 2.8 — sharp enough to prickle slightly on the skin — and iron-rich (Fe(II,III) at 14.2 mg/kg), the kind that oxidizes reddish-brown on contact with air. Sampled from the mixing tank rather than a wellhead.

Good for

Skin conditions

Sulfur and acidic waters are antibacterial and keratolytic, studied for atopic dermatitis and psoriasis.

atopic skin, eczema

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.

Cationsshare of charge (mval%)
  • Calcium52.5mgP58 national
  • Hydrogen1.6mgP70 national
  • Sodium23.4mgP5 national
  • FeTotal14.2mgP92 national
  • Manganese0.60mgP68 national
  • Strontium0.30mgP55 national
Anionsshare of charge (mval%)
  • Sulfate522mgP88 national

Measured at the source

Source temperature
48.1°C
Warm

Evidence

  1. 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.
  2. Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
  3. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  4. Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
  5. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.
  6. 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

星生温泉(岩風呂)Hoshisei Onsen (Iwa-buro Bath Blend)

黄SulfurSodium-Calcium-Magnesium Sulfate spring

A blend of source ③ and source ④ (the cold-mineral well) feeding the Iwa-buro bath, classified as a simple sulfur spring. 47.1 °C — a comfortable bathing temperature — with a slightly acidic pH of 5.8 and the characteristic faint rotten-egg note from the sulfur component. Sampled from the mixing tank.

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

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.

Cationsshare of charge (mval%)
  • Sodium27.1mgP6 national
  • Calcium22.0mgP40 national
  • Magnesium11.7mgP63 national
Anionsshare of charge (mval%)
  • Sulfate140mgP60 national
  • Chloride19.2mgP17 national
  • Bicarbonate22.0mgP13 national
  • Fluoride0.30mgP24 national

Measured at the source

Source temperature
47.1°C
Warm

Evidence

  1. 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.
  2. Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
  3. Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
  4. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  5. Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
  6. Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
  7. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.
  8. 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

Educational, not medical advice. If you have a health condition, consult a physician before onsen therapy.