# Chaves Thermal Spring Water Impact on Skin Health: Potential Cosmetic Application
## 4 Discussion  
For centuries, thermal spring water has been used as a treatment for several diseases, including dermatologic diseases [3]. Different studies have reported the absence of side effects of thermal spring water, allowing for its use as an adjuvant in the treatment of various skin disorders, demonstrating the importance of thermal spring waters in the  
Figure 4: Skin biometric parameters. The measurement of (**a**) hydration levels, (**b**) transepidermal water loss (TEWL), and (**c**) pH values was performed on days 0 (before the application) and on days 8 and 16 (after the application). The results are represented as bar graphs (average \(\pm\) SD). * and ** stand for \(p\) < 0.05 and \(p\) < 0.005, respectively. ns, not significant (\(p\) > 0.05). Figure 5: The relative abundances of (**a**) _Staphylococcus_ sp. ; (**b**) _Corynebacterium_ sp. ; (**c**) _Propioniibacterium_ sp. ; (**d**) _Malassezia_ sp. ; (**e**) _Staphylococcus epidermidis_; (**f**) _Propioniibacterium_ _canes_, determined by qPCR (ns, not significant). cosmetics industry [26]. The scientific knowledge about the mechanisms of action and clinical benefits of this natural resource has been increasing over the years, including in Portugal [23; 37; 38; 39; 40]. In the present study, we performed a preliminary screening of Chaves thermal spring water, evaluating its biological potential for use as a therapeutic or cosmetic product. We began by determining the physicochemical composition of Chaves thermal spring water, which was consistent with previous reports [27], indicating that Chaves thermal spring water has been stable over the years. Its pH value was 6.8, an almost neutral value. This is an important characteristic for a potential cosmetic active ingredient that will be topically applied, since skin pH is normally acidic, ranging between 4 and 6, which regulates the maintenance of the stratum corneum homeostasis and the skin barrier permeability [41]. Therefore, skincare products should have a pH similar to that of skin so they do not alter it [42]. Additionally, Chaves thermal spring water contained less than 2 g/L of total dissolved solids and is classified as low-mineralized in comparison to other thermal waters, but this does not impair its potential to benefit the skin, since the absorption of minerals through the skin is limited [43]. For example, Avene thermal spring water is also low-mineralized, and it presents anti-inflammatory, antioxidant, and anti-irrittant proprieties [23; 44; 45]. The main minerals presented in Chaves thermal spring water, sodium, potassium, silicon, and calcium, are also present in other thermal spring waters around the globe, but in different proportions [26]. After the determination of the physicochemical composition, a cytotoxicity assay was performed, demonstrating that Chaves thermal spring water did not interfere with the viability of HaCaT and nHDF cells, allowing it to go through further in vitro tests. The non-toxicity of other thermal spring waters has also been reported by other authors [46; 47; 48]. Recently, [PERSON] et al. [26] reviewed the published data on the dermatologic potential of thermal spring waters, based on our evaluation of the antiaging and antioxidant capacity of Chaves thermal spring water. Our results demonstrated that Chaves thermal spring water inhibited the activity of elastase, which is an enzyme that hydrolyses elastin, and its activity is associated with the mechanical properties of connective tissues, including the firmness and elasticity of the skin [49]. This result suggests that Chaves thermal spring water may contribute to the improvement in skin health while avoiding the degradation of the elastase. The antioxidant activity of thermal spring waters is related to their mineral composition. In fact, minerals like calcium, potassium, and magnesium are involved in human cell defense mechanisms against reactive oxidative species (ROS), since they support the activity of several antioxidant enzymes.