# Chaves Thermal Spring Water Impact on Skin Health: Potential Cosmetic Application
## 1 Introduction  
Thermal water is naturally formed under specific geological conditions, and it is characterized as being rich in minerals and bacteriologically pure [1]. Depending on their geological, geochemical, and geothermic conditions, different springs can have very diverse chemical and physical water properties [2]. Since ancient times, thermal water has been associated with therapeutic effects; however, it is only more recently that some of these effects have been scientifically verified [3]. Bathing in thermal water has been proven to be beneficial to the skin and to improving dermatologic disorders such as atopic dermatitis [4; 5] and psoriasis [6; 7]. Such properties prompted the World Health Organization to officially recognize hydrotherapy and balneotherapy's efficiency within its Pain Management Protocol [8; 9]. Given the growing emphasis on sustainable and natural remedies in modern society [10], there has been an observed and noticeable increase in the interest in and demand for the use of these types of therapies.  
When undergoing balneotherapy, the first contact point between the human body with the water is the skin. As the largest human organ, the skin's primary functionis to protect the body against the external environment, therefore making it the organ most impacted by the properties of thermal waters. The skin consists of three layers: the epidermis (the most exterior layer), the dermis (the middle layer), and finally, the hypodermis (subcutaneous tissue) [11]. The epidermis is a stratified epithelium composed of four layers of keratinocytes in progressive stages of differentiation [12]. The dermis is predominantly composed of fibroblast cells, but it also contains structures such as blood vessels, lymphatics, nerves, sweat glands, and pilosebaceous units. Lastly, the hypodermis is formed by subcutaneous fat and connective tissue [13]. Additionally, the skin supports a community composed of diverse bacteria, fungi, and viruses, which is known as the skin microbiota. In healthy individuals, the skin microbiota is in equilibrium with the host and is essential for skin health [14].  
Exposure to pollution can cause oxidative stress, premature aging, inflammation, and diseases, not only in the respiratory and cardiovascular systems but also on the human skin [15]. Since most people's skin is exposed to pollution, protecting it is important [16]. Thermal water is rich in minerals that are assimilated by the skin and, depending on the mineral water composition and concentration, improve its physiological functions [8]. Sulfur-rich thermal waters have been proven to have antioxidant and anti-inflammatory effects [17; 18; 19] and potential antiaging properties for skin [20]. Thermal water also protects and repairs the skin barrier [21; 22], being able to soothe sensitive skin [23]. Regarding the skin microbiota, there is evidence that thermal water increases its diversity and decreases inflammatory infectious agents [24; 25].  
The beneficial effects of thermal spring water on the skin and the skin microbiota have led to its incorporation into cosmetic formulations as an active ingredient [26]. In the present work, the potential of using Chaves thermal water as a cosmetic ingredient is investigated. Chaves thermal spring water is in the north of Portugal, and it has been used as a local spa since Roman times. Its waters are hot waters (ranging from 55 to 76 \({}^{\circ}\)C), with total diluted solids of 1600-1800 mg/L and a neutral pH, and they are bicarbonate-/sodium-/CO\({}_{2}\)-rich [27]. Even though Chaves thermal spa has been functioning for several decades and its customers are satisfied with the medicinal quality of the thermal spring water [28], there is no published evidence, as far as we are aware, on the specific effects that this thermal spring water has on the skin. Therefore, the goal of this work was to assess the Chaves thermal spring water antioxidant, antiaging, and antipollution potential, and its impact on the skin biometric parameters and microbiota.