Thermal water is naturally heated groundwater that rises to the surface carrying a unique blend of dissolved minerals. It’s defined by temperature: to qualify as “thermal,” the water must be noticeably warmer than the average annual air temperature of the surrounding area. Unlike regular tap water, thermal water picks up minerals like calcium, magnesium, silica, selenium, and sulfur compounds as it filters through layers of rock deep underground, sometimes over thousands of years. This mineral profile is what gives thermal water its reputation in both traditional medicine and modern skincare.
How Thermal Water Forms
Rainwater seeps deep into the earth, sometimes reaching depths where geothermal heat warms it significantly. As it travels back toward the surface through various rock formations, it dissolves minerals along the way. The specific minerals it absorbs depend entirely on the geology of the region, which is why thermal springs in different parts of the world have distinct mineral compositions. A spring in the French countryside will have a different chemical fingerprint than one in Hungary or Japan.
Common dissolved minerals include calcium, bicarbonate, silicates, iron compounds, sodium and magnesium salts, and sulfur compounds. Trace elements like selenium, copper, and zinc also appear in varying concentrations. The pH, mineral concentration, and purity all vary by source, which is why different thermal water brands marketed for skincare aren’t interchangeable. Some are heavily mineralized, others only lightly so. One well-known French thermal spring water, for instance, is so low in minerals and high in silicates that locals historically called it “velvet water.”
Why Minerals in Thermal Water Matter for Skin
The therapeutic interest in thermal water comes down to its trace elements and what they do at the cellular level. Selenium, one of the most studied components, acts as a potent free radical scavenger. In small amounts, selenium is essential for cell function. It forms the active center of enzymes that neutralize oxidative damage and reduce inflammation. Copper and zinc contribute to the skin’s own antioxidant defenses, supporting enzymes that protect cells from oxidative stress.
These aren’t dramatic drug-like effects. The mineral content of thermal water contributes to what researchers describe as improved skin comfort, meaning the skin feels softer, more supple, and less reactive. The antioxidant properties of thermal water also appear to help neutralize free radicals caused by UV exposure, potentially reducing some sunburn-related damage.
Effects on Inflammation and Immune Response
Thermal water’s anti-inflammatory properties go beyond simple soothing. In animal studies modeling atopic dermatitis (a severe form of eczema), hot spring water treatment significantly reduced levels of IL-4, a key cytokine that drives allergic inflammation and weakens the skin barrier. The treatment brought IL-4 levels down to ranges comparable to healthy skin. It also reduced the number of certain immune cells involved in excessive inflammatory responses. In practical terms, this suggests thermal water helps dial down the overactive immune signaling that keeps conditions like eczema flaring.
Clinical Results for Eczema and Psoriasis
The strongest clinical evidence for thermal water comes from balneotherapy, the practice of bathing in thermal springs for therapeutic purposes. This is distinct from hydrotherapy, which uses plain tap water. In a large observational study at a French hydrotherapy center, nearly 6,000 patients with atopic dermatitis showed a 41.6% improvement in their symptom severity score over just 18 days of treatment. Among roughly 4,900 psoriasis patients in the same study, disease severity dropped by 54.4% in the same timeframe. About 64% of psoriasis patients achieved at least a 50% reduction in their scores.
These results appear to last well beyond the treatment period. In a separate study of psoriasis patients with chronic itching, three weeks of thermal water bathing reduced itch intensity by about 50%. That relief held steady at both three and six months of follow-up. Disease severity scores dropped by 40% with hydrotherapy and remained lower over the full six-month observation period. For a condition that typically requires ongoing medication to manage, sustained improvement from bathing is notable.
How Thermal Water Changes Skin Bacteria
One of the more interesting findings involves the skin microbiome. In a controlled pilot study, 30 healthy women were split into two groups: one bathed in thermal mineral water for 30 minutes across 10 sessions, while the other followed the same routine in plain tap water. The thermal water group showed measurable shifts in their skin bacteria at both the genus and species level. Certain beneficial bacteria increased, while several species associated with inflammation and infection decreased. The tap water group showed no significant microbial changes at all.
This distinction matters because an imbalanced skin microbiome is increasingly linked to conditions like eczema, psoriasis, and acne. The fact that thermal water selectively reduces inflammatory microbes while encouraging beneficial ones suggests a mechanism beyond simple hydration. The minerals themselves appear to actively reshape the microbial environment on the skin’s surface.
Thermal Water Sprays and Skincare Products
You’ll most commonly encounter thermal water as a facial mist sold in pressurized cans. Brands like Avène, La Roche-Posay, and Vichy each source from different thermal springs, so their mineral profiles differ. These sprays are typically used after cleansing, after sun exposure, over makeup to set it, or anytime your skin feels irritated or dry. They’re popular for calming redness and post-procedure sensitivity.
The effects of a spray are more modest than soaking in a thermal bath for weeks. You’re getting a fine mist of mineral-rich water rather than prolonged immersion. Still, regular use can contribute to improved skin comfort and reduced reactivity, particularly for sensitive or inflammation-prone skin types. Some people use thermal water spray before applying a serum or moisturizer to help with absorption, though the primary benefit remains the mineral delivery itself.
One practical note: thermal water sprays don’t replace moisturizer. Water on its own can actually increase dryness as it evaporates if you don’t seal it in with a cream or oil afterward. Think of the spray as a mineral treatment step, not a hydration step.
Who Benefits Most
Thermal water is broadly well tolerated, but the people who notice the biggest difference tend to be those with reactive or chronically inflamed skin. If you have eczema, psoriasis, rosacea, or skin that flares easily from environmental triggers, the anti-inflammatory and microbiome-balancing properties are most relevant to you. People with generally healthy skin may find it pleasant but won’t see dramatic changes.
For anyone considering balneotherapy at a thermal spa, the clinical data supports multi-week treatment courses for chronic skin conditions. The typical protocol in European studies involves daily bathing for about three weeks, with benefits that can persist for months. Many European countries, particularly France, Germany, and Hungary, have medical systems that recognize balneotherapy as a legitimate treatment and sometimes cover it through health insurance.