# Potential Possibilities of Using Peat, Humic Substances, and Sulfurous Waters in Cosmetology
## 4 Medicinal Waters
### Sulphurus Water--Mechanism of Action on the Skin  
Sulfurous medicinal waters are widely used in balneology. The therapeutic properties of these waters depend on the presence of divalent sulfur, mainly, H\({}_{2}\)S (hydrogen sulfide), HS\({}^{-}\) (bisulphides), S\({}^{2-}\) (sulfides), Sn\({}^{2-}\) (polysulphides) as well as other sulfur species [103]. [PERSON] and [PERSON] [104] showed that the biological activity of divalent sulfur in sulfurous waters depends on many environmental factors such as pH, temperature, and redox potential. In natural waters, the oxidation processes take place, leading to the formation of polysulfides and thiosulfates. The main active molecule in sulfurous water is hydrogen sulfide (H\({}_{2}\)S). H\({}_{2}\)S is a colorless gas with weak gas properties and is highly soluble in water. It has a characteristic smell of rotten eggs. H\({}_{2}\)S is heavier than air, flammable, and corrosive. The concentration hazardous to health is considered to be 6 mg/m\({}^{3}\). At a concentration of 300 mg/m\({}^{3}\) it becomes undetectable due to the immediate paralysis of the olfactory nerve. H\({}_{2}\)S burns in air to form sulfur dioxide (SO\({}_{2}\)) or free sulfur (at low temperature and insufficient oxygen supply). Its stability depends on pH, temperature, and oxygen concentration in the environment. Sulfur species change at two critical pH values, pKa = 7.04 and pKa = 11.96. At physiological pH, the ratio of hydrogen sulfide to bisulphide (HS-) is 1:3. Identical H\({}_{2}\)S solutions occurring at different ambient temperatures and in an oxygen environment show differences in concentration, and oxygen promotes the reduction in H\({}_{2}\)S to hydrogen sulfate. At acidic pH, H\({}_{2}\)S is the only form of sulfur [104]. The synthesis of H\({}_{2}\)S in the body is based on the transformation of L-cysteine with the participation of the two most important enzymes: cystathionine \(\beta\)-syntase (CBS) and cystathionine \(\gamma\)-lyase (CSE). CBS converts L-cysteine and homocysteine to H\({}_{2}\)S, and CSE catalyzes cystathionine cleavage [105]. Both mRNA and protein expressions of the enzymes CSE and CBS were observed in normal human keratinocytes. CSE and CBS also promote the resolution of inflammation in skin exposed to subcytotoxic concentrations of formaldehyde. The increase in the activity of both enzymes causes an increase in the concentration of H\({}_{2}\)S, which may in turn inhibit the upregulation of pro-inflammatory activity mediators (such as MMP-1, PGE2, and IL-8) [106]. Endogenous H\({}_{2}\)S regulates many important processes in the skin, such as inflammation [106], cell proliferation [107], apoptosis [108], and vasodilation [109]. The effect of H\({}_{2}\)S on keratinocytes depends on the concentration of this compound. At low concentrations, it stimulates the proliferation of keratinocytes, while at high concentrations, it intensifies the apoptosis process. NO has a similar effect, which allows us to assume that the effects of H\({}_{2}\)S in the skin (e.g., vasodilator) may be similar [108]. Studies have shown that hydrogen sulfide (H\({}_{2}\)S) can contribute to the stimulation of angiogenesis and thus contribute to the activation of the wound-healing processes [110]. It was proven that, in mice with type 2 diabetes, the administration of sodium hydosulfide (NaHS) for 18 days resulted in the activation of Ang-1 (angiopoietin-1) and restoring of EPC (endothelial progenitor cell), which has a pro-angiogenic effect [111]. H\({}_{2}\)S possesses also the ability to quickly move through cell membranes without the help of transmitters, which cause a variety of biological reactions [110]. The skin is a tissue that stores sulfur. It is estimated that it accumulates approximately 60 mg per 100 g of tissue. A part of sulfur penetrates the human body and forms chondro-sulfuric acid as a result of combining with hydrocarbons, which is a component of cartilage and synovial fluid. Moreover, as a result of these combinations, mucoitin-sulfuric acid is formed--a component of mucus.