*2.3. Result Discussion and Interpretation Bases and Criteria*

Different regulations could be used for the interpretation of the obtained results. In this paperwork, the discussion and interpretation of the results will be centered on the documents summarized in Table 1.


**Table 1.** Documents used during the interpretation and discussion of the obtained results.

The HC-SC guideline is focused on heavy metals (As, Pb, Cd, Hg, and Sb). Other metal elements such as Se, Ba, and Cr are considered less significant in terms of toxicity; therefore, no impurity limits are found for these elements in this document [1]. The Natural and Non-prescription Health Products Directorate (NNPHD) [39] is a guidance document intended to give support to stakeholders "in assuring that natural health products are produced in a high-quality manner". NNPHD is focused on natural and non-prescription health products, which is the case of PS9, G30, and ALI ingredients and resultant hydrogels. Acceptable limits for As, Cd, Pb, Hg, Cr, and Sb elemental impurities are defined in this guide, including the limits for topical administration (Figure 1).

**Figure 1.** Acceptable limits for heavy metals in topical products [39].

The ICH guideline for Elemental Impurities Q3D(R1) refers to medicinal products and classifies elements into four groups based on their toxicity and likelihood of occurrence in these products (Figure 2). The fourth group called "other elements" includes elements for which Permitted Daily Exposure (PDE) limits have not been established. As a result of this, these elements have not been included in this manuscript. Regarding the most toxic elements, the Q3D(R1) guideline also reports the PDE limits by oral, parenteral, and inhalation administration routes [30]. PDE is the maximum acceptable intake of the elemental impurity per day. Although this guideline is not specific for cosmetics, the fact that it deals with elemental impurities of drugs (which are intended to reach the bloodstream) makes it useful to also ensure the safety of cosmetics.

**Figure 2.** Element classification of the ICH Q3D(R1) guideline. Based on ICH Q3D(R1) [30].

Finally, the European Regulation EC 1223/2009, from all the documents and guidelines included in this study, is the most restricting one in terms of impurities present in cosmetics [3]. Only those elements considered safe and innocuous for the human being are allowed in cosmetics, while the rest of them are banned without any limit or maximum dose established. Furthermore, according to article 17 of this regulation, "the presence of traces of banned substances will only be allowed if they are technically inevitable and do not impair the safety of the cosmetic product". In this regard, in vitro studies such as Franz cells could help to discern and discuss this point since they allow the study of the mobility of the elements present in the ingredients.

#### Dose of Nanoclay/Natural Spring Water Hydrogels

The toxicity of elemental impurities obviously depends on the administration route of the dosage form. Nanoclay hydrogels prepared in this study are intended to be topically applied over either the healthy or wounded skin of patients subjected to balneotherapy treatments. In thermal stations, natural or artificial clay-based/spring water formulations could be applied in different forms. The most common one includes the administration of semisolid systems at 45–50 ◦C on restricted body regions (mainly isolated joints) with 210 cm of thickness or in the form of total/partial baths. Most of these treatments usually last for 15 to 30 min [40–42]. The density of the hydrogels was obtained by the Minimum Square Method applied to experimental volume, and mass hydrogels measurements (R2 were >0.998 in both cases): -ALIPS9 = 1.0606 g/mL; -ALIG30 = 1.0992 g/mL. These data would be used to calculate safe doses of hydrogels in order to not exceed the PDE limits defined in the Guideline for Elemental Impurities [30] for each element. Moreover, despite the bioavailability of topically administered dosage forms hardly reaching 100%, in the discussion, we will systematically consider the maximum potential bioavailability in order to guarantee safe doses.
