# Unveiling the Role of Minerals and Trace Elements of Thermal Waters in Skin Health
## 1 Introduction  
Thermal spring waters (TSWs), also called \"mineral-medicinal waters\", \"natural mineral waters\", or \"spa waters\" in some countries, are characterized by their mineral content and healing properties. According to their mineral composition, TSWs can be classified as chloride, bicarbonate, sulfate, sulfide or sulfutrous, carbon-dioxide-rich, and weakly mineralized waters. They can also contain soluble minerals such as Mg\({}^{+2}\), Ca\({}^{+2}\), Na\({}^{+}\), silicates, SiO\({}_{2}\), etc., and trace minerals such as manganese, selenium, zinc, boron, etc. Taking into account the temperature, several classifications can be used (from a geological or medical point of view); the most widespread is based on the mean annual soil temperature (Ts) or the mean annual air temperature (Ta) as follows: hypothermal (temperature \(<\) Ts mean annual + 4 \({}^{\circ}\)C), mesothermal (temperature \(=\) Ts mean annual + 4 \({}^{\circ}\)C), and hyperthermal (temperature \(>\) Ts mean annual average + 4 \({}^{\circ}\)C). But for therapeutical purposes, the most useful TSW is that related to body temperature: hypothermal (\(<\)37 \({}^{\circ}\)C), mesothermal (=37 \({}^{\circ}\)C), and hyperthermal (\(>\)37 \({}^{\circ}\)C). Ballecology is defined by the ESPA (European Spas Association) as \"the use of local natural resources such as natural healing water, peloids, gases, and climatic conditions for treatment, which is only available at medical spa facilities that are recognized and regulated by state authorities. The treatments offered at these facilities have been thoroughly researched and approved by scientists and medical doctors\" [1]. Among the uses of thermal waters, rheumatological, respiratory, and dermatological diseases stand out, but TSWs are also used for well-being and relaxing purposes in so-called wellness centers. Additionally, TSWs are included in dermocosmetics as an excipient and active ingredient. The role of TSW in the treatment of dermatological diseases has been described by several authors. [PERSON] et al. [2] described recent evidence of major dermatologicdiseases that are frequently treated by balneotherapy with a remarkable rate of success. The authors considered that balneotherapy offers several advantages, including the fact that no chemicals or potentially harmful drugs are needed; there are almost no side effects during and after treatment; and there is a low risk to the patient's general health and well-being. The effects of TSW were linked to its mineral and trace element contents [3]. The protocols for the treatment of dermatologic diseases vary depending on the thermal spa center, but spa techniques include bathing in thermal water (temperatures ranging from 30 to 35 \({}^{\circ}\)C) [4] and other techniques such as showers, pulverizations, and catalgrams [5]. Thermal mineral baths are sometimes combined with natural or artificial ultraviolet (UV) light to provide a synergistic effect of both treatments, a type of therapy known as balneo-phototherapy [4]. It has been suggested that the observed clinical efficacy of TSW may be related to the penetration of some water-soluble minerals into human skin. Minerals such as selenium, magnesium, sulfur, calcium, and zinc are directly related to the skin structure [6; 7]. The mechanisms involved in thermal healing are complex and include chemical, thermal, mechanical, and immunological effects on different dermatologic diseases [4; 8], such as atopic dermatitis, contact dermatitis, seborrhea, seborrheic dermatitis, psoriasis, and ichthyoses, as well as skin conditions such as sensitive skin, rosacea, and acne. All these mechanisms seem to be related to each other and act synergistically. Although the hormetic mechanism linked to temperature cannot be ignored, the chemical composition of thermal water is crucial in its dermatological effects.