## 3 Results
### Inorganic Filters (Minerals)  
These filters are the safest and most recommended, since they are not absorbed through the skin. It has been found that these filters have lower penetration into Langerhans cells, keratinocytes, and melanocytes (living epidermis), and thus present a lower risk of inducing allergic contact reactions [69]. U.S. Food and Drug Administration (FDA) has approved two inorganic (mineral) filters: titanium dioxide (TiO\({}_{2}\)) and zinc oxide (ZnO) [41; 60; 70]. These particles act by absorbing, reflecting, and refracting UV photons but its primary function in photoprotection is absorbing UV radiation [23]. Sunscreens utilize ZnO and TiO\({}_{2}\) since they reflect and absorb UV photons. These particles have the ability to protect against UV exposure, directly related to particle size.  
The size of ZnO and TiO\({}_{2}\) particles ranges from 200 to 400 nm and 150 to 300 nm, respectively [71]. White and chalky texture on the skin's surface is due to larger particle size (EWG's Sunscreen Guide) [62]. Patient dissatisfaction and cosmesis have encouraged new formulations with nanoparticles with decreased particle size. These nanoparticles of ZnO and TiO\({}_{2}\) have helped to create of a non-greasy formulation, transparent, cheaper and which is not degraded with UV radiation exposure [71]. In 2018, the EWG reported that a large increase in these inorganic filters with \(\sim\)41% of sunscreens in the United States designated as mineral only (EWG's Sunscreen Guide) [62].  
ZnO and TiO\({}_{2}\) are able to penetrate the stratum corneum, enter the dermis and ultimately the systemic circulation, risking oxidative stress and cellular toxicity. However,  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Inorganic Filters** & **Organic Filters** & **Biological Filters** \\ \hline Titanium dioxide & PABA and derivates & Lignin \\ \hline Zinc Oxide & Cinnamates & Silimarine \\ \hline  & Salicylates & Marine antioxidants \\ \hline  & Octocylene & Plant-derived extracts \\ \hline  & Ensulizole & \\ \hline  & 4-Methylbenzylidene Camphor & \\ \hline  & Benzophenones & \\ \hline  & Anthranilates & \\ \hline  & Avobenzene & \\ \hline  & Mexoryl SX & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Classification of different compounds used as filters.  
in vivo and in vitro studies have found that these minerals do not permeate the skin to any degree [70,72, 73, 74, 75, 76].  
The lungs are unable to clear nanoparticles. Inhalation of these substances is considered to be a potential risk, so the EWG has recommended not use powdered products or spray sunscreens containing ZnO and TiO\({}_{2}\)[77, 62].  
One possible UV filter that has been suggested is cerium oxide (CeO\({}_{2}\)). Some photo-protective formulations commercialize this substance with silica coating because of its high photocatalytic activity, responsible for oxidation and degradation of other formulations' components [78]. [PERSON] and [PERSON] have studied cerium phosphate (CePO\({}_{4}\)) similarly to CeO\({}_{2}\)[79]. CePO\({}_{4}\) possesses high photocatalytic activity, low amount of white residue when applied on the skin, and increased stability being a potential future novel, stable, and efficient inorganic UV filter [79].