# Incorporation of UV Filters into Oil-in-Water Emulsions--Release and Permeability Characteristics
## 1 Introduction  
UV filters are a group of compounds, which protect the skin from ultraviolet light that can cause redness, burn, photoaging, and cancer [1]. They can be divided into inorganic (physical) and organic (chemical) agents [2]. UV blockers are sunscreen formulations' components and protect against harmful radiation by reflecting, scattering, or absorbing UV rays [3, 4, 5]. Depending on the type of photoblocker, the mode of their action is different. Inorganic filters usually reflect or scatter ultraviolet radiation via an optical mechanism [6, 7]. In turn, organic UV blockers absorb the light that implicates chemical changes in the molecules, such as isomerization. These ultraviolet filters can also release the incident energy as heat and emit radiation at a higher wavelength [6]. In the USA, sunscreen agents are treated as over-the-counter drugs and are subjected to the regulation of the Food and Drug Administration (FDA) [8]. The FDA ensures that UV filters meet safety, quality, and efficacy standards. The availability of novel sunscreen agents in the USA is limited mainly due to a more complex and long-term approval process than in the European Union [9]. Lately, sunscreen agents have gained concerns due to reports in which it was proved thatsome UV filters were photounstable, undergo photodegradation upon irradiation, and can cause allergies and toxic reactions, as well as penetrating the stratum corneum [10]. They were detected in plasma, urine [11], and even in human milk samples [12]. It was also reported that some UV filters can act as endocrine disruptors [13]. Therefore, the ideal sunscreen formulation should ensure skin accumulation of a UV filter, creating a stable film on its surface with negligible permeation to the systematic circulation [14]. The effectiveness of sunscreen agents depends on their ability to remain on the skin surface after the application of the formulation. UV filters should not permeate into the deeper layers of the skin. The formulation type may influence the safety and efficacy of sunscreen agents. In this study, two commonly applied sunscreen agents were selected: avobenzene (butyl methoxydibenzoylmethane-Avo), which provides protection against UVA and octyl methoxycinnamate (ethylhexyl methoxycinnamate-OMC), used as a UVB absorber [15]. These UV filters are components of sunscreen formulations, such as lolions, sprays, and other topical products, which protect against UV light. They are approved by regulatory authorities in the USA, Europe, Japan, and Australia [16]. However, it has been established that avobenzene is not photostable [17]; therefore, its photodegradation products may cause photoulergies [18]. To overcome this problem octocrylene is usually added to sunscreen formulations [19]. In turn, OMC is the most widely applied lipophilic UVB filter in cosmetic formulations because it exhibits the ability to impair water resistance in the final sunscreen product [20]. When octyl methoxycinnamate is exposed to sunlight, it is subjected to photoisomerization between the _cis_-(Z) and _trans_-(E) isomers [20]. The chemical structure and geometric properties of both UV filters are presented in Table 1. [PERSON] et al. created a theoretical approach that enables the selection of molecules with a high skin permeability [21]. The following criteria were considered: the molecular weight should be lower than 180 Da; the melting point below 100 \({}^{\circ}\)C; log K\({}_{\text{ow}}\) higher than 0.3; and topological polar surface lower than 40 A\({}^{2}\). It was suggested that if the molecule follows at least two of these criteria, it will exhibit a high skin permeation. Based on these assumptions and considering the data presented in Table 1, both Avo and OMC should be able to permeate through human skin. However, [PERSON] et al. proved that avobenzene exhibited a low skin permeability both in single and repeated applications [22]. The rate-limiting step of Avo diffusion was the release from the vehicle. In other studies, [PERSON] et al.