Free CO₂ Onsen Analysis | Carbonated Springs & Circulation at 38°C

Free carbon dioxide: Main Effects & Top 5

Table of Contents

What is Free carbon dioxide?
Main Effects
Free carbon dioxide Top 5 Hot Springs

What is Free carbon dioxide?

Hot springs with ≥ 1,000 mg/kg of free CO₂ are classified as "CO₂ springs." CO₂ bubbles cling to the skin, are absorbed transdermally, and dilate peripheral blood vessels.

Main Effects

Promotes circulation and peripheral vasodilationWhen absorbed through the skin, the carbon dioxide in the water may help to directly dilate peripheral capillaries. This action improves blood flow throughout the body, a signature effect of these so-called "bubble baths."
Indicated for poor circulationThe chemical vasodilation caused by carbon dioxide helps to warm the body from its core. This effect is achieved without relying on high water temperatures, making it particularly suitable for those who tend to feel cold.
Warms the body even at lukewarm temperaturesCarbon dioxide gas is volatile and dissipates in hot water, so these springs are typically kept at a lukewarm 35–38°C. This lower temperature allows for longer soaks that warm the body thoroughly without putting excess strain on the heart.

Free carbon dioxide Top 5 Hot Springs

Top 5 hot springs registered on this site with the highest content.

RANKING BY THIS MINERAL CONTENT

References

SPRING TYPEMay 5, 2026· 1min read

Carbon Dioxide Springs: "The Bubble Waters" for Gentle Circulation

Carbon dioxide springs contain CO₂ at 1,000 mg/kg or more — bubbles adhere to skin on bathing, earning the nickname "the bubble waters." CO₂ absorbed through skin dilates capillaries, warming the body even in lukewarm water (35–38°C) with minimal cardiac strain.

SYMPTOMSMay 6, 2026· 2min read

Hot Spring Therapy for Mild Hypertension | All 10 Types & the Special Role of CO₂ Springs

Mild hypertension is a general indication shared by all 10 spring types in Japan's Ministry of the Environment guidelines. Carbon dioxide springs are especially noteworthy for transiently lowering blood pressure through peripheral vasodilation. Severe hypertension is a contraindication — always consult your doctor.

MINERALMay 5, 2026· 2min read

Metasilicic Acid: The "Beauty Spring" Standard Mineral & Top 5 Onsen

Metasilicic acid (H₂SiO₃) is a natural moisturizer also found in skincare lotions. Hot springs containing more than 50mg/kg are called "beauty springs" — leaving skin hydrated long after bathing. This is one of the therapeutic spring criteria in Japan's Mineral Spring Analysis Guideline (Table 1-1).