avoiding the energy transfer to molecular oxygen and the consequent formation of singlet oxygen (3,4). What makes the evaluation of a sunscreen’s photostability important is its special application type and the chemical structure of the UV fi lters used (5,6). According to the European Cosmetics Regulation No. 1223/2009 (7), sunscreen products are classifi ed as cosmetic products, whose defi nition is “any preparation, such as creams, oils, gels, and sprays, intended to be in contact with the human skin, to protect it from UV ra- diation by absorbing, scattering, or refl ecting radiation.” Table summarizes the UV fi lter substances allowed by European Union (EU) (Annex VI, last update November 24, 2020). In the United States, sunscreens are classifi ed as over-the-counter (OTC) drugs. This means that they must comply with all the requirements listed in the Food and Drug Administration’s (FDA) OTC sunscreen monograph. Individual sunscreen active ingredi- ents are reviewed by the FDA, and only those that are on the FDA’s monograph approved list may be used in sunscreen products marketed in the United States (8). On February 21, 2019, the FDA issued a proposed rule describing the conditions under which the FDA proposes that OTC sunscreen monograph products are generally recognized as safe and effective (GRASE) and not misbranded. The FDA has proposed the following catego- ries for the 16 sunscreen monograph ingredients (Table II). It is worth mentioning that formulations of sunscreens are applied to a wide area on the skin when used at the beach ( 1.5 m2), where they remain for a long time (generally, after 2 h they have to be reapplied because they wash off because of the sweat, bathing, rubbing, etc.). The sunscreen-containing formulations are the source of a continuous and high input of the chemical substance it contains, permeating the viable skin strata and possibly the systemic circulation. Consequently, a development of formulas based on non-penetrating photo protectors is considered to be more than relevant. A non-harmful sunscreen application, which will offer the greatest possible protection from UV rays, is important. Protection against UV radiation should be combined with minimal skin penetration. This explains why the biggest challenge cosmetologists face is the development of appropriate products that could hinder skin penetration. Mi croencapsulation of sunscreens is considered to have an advantage because it is safer and more effective to use. Microencapsulation exhibits no percutaneous absorption and also reduces photo degradation, while at the same time, it has a lasting effect and makes the sun- screen stable. This process leads to the creation of capsules with a diameter between one to few micrometers (9). The benefi t of this technique lies on the fact that encapsulated UV fi lters do not come in direct contact with the skin, preventing any possible toxicological risks. In the ongoing endeavor of overcoming problems caused by sunscreens, nanotechnology plays an important role because nanosystems are often used as vehicles to sunscreens. Cyclodextrins and nanoemulsions, liposomes, and nanoparticles (lipid, polymeric, and inorganic) are the most thoroughly examined nanosystems in photo protection (10). Mi crospheres (11), micro- and nanocapsules (NCs) (12,13), lipid particles (14,15), hydrotalcite-like anionic clays (16), and inclusion complexes (17–20) have caught the attention recently as they are considered suitable vehicles for sunscreens. UV chemical blockers were integrated into microparticulate carriers, using hydrophilic (chitosan and gelatine) and hydrophobic (polymethylmetacrylate) polymers (21). Lipid carriers minimize skin penetration and retain their satisfactory photo-protective proper- ties. Being under investigation, as drug carrier systems for insuffi ciently, water-soluble DISTRIBUTION OF UV FILTERS ON THE SKIN 299

Table I UV Filter Substances Agreed by the EU Chemical name/INN/XAN Name of common ingredients glossary Maximum concentration in ready-for-use preparation (%) N,N,N-Trimethyl-4-(2-oxoborn-3-ylidenemethyl) anilinium methyl sulfate Camphor benzalkonium methosulfate 6 Benzoic acid, 2-hydroxy-, 3,3,5-trimethylcyclohexyl ester/Homosalate Homosalate 10 2-Hydroxy-4-methoxybenzophenone/Oxybenzone Benzophenone-3 (BP-3) 6 2-Phenylbenzimidazole-5-sulphonic acid and its potassium, sodium and triethanolamine salts/Ensulizole Phenylbenzimidazole sulfonic acid 8 (as acid) 3,3’-(1,4-Phenylenedimethylene) bis(7,7-dimethyl-2-oxobicyclo[2.2.1] hept-1-ylmethanesulfonic acid) and its salts/Ecamsule Terephthalylidene dicamphor sulfonic acid 10 (as acid) 1-(4-tert-Butylphenyl)-3-(4-methoxyphenyl) propane-1,3-dione/Avobenzone BMDBM 5 Alpha-(2-Oxoborn-3-ylidene)toluene-4-sulphonic acid and its salts Benzylidene camphor sulfonic acid 6 (as acid) 2-Cyano-3,3-diphenyl acrylic acid 2-ethylhexyl ester/Octocrilene OCR 10 (as acid) Polymer of N-{(2 and 4)-[(2-oxoborn-3-ylidene)methyl]benzyl}acrylamide Polyacrylamidomethyl benzylidene camphor 6 2-Ethylhexyl 4-methoxycinnamate/Octinoxate Ethylhexyl methoxycinnamate (EHMC/OMC) 10 Ethoxylated Ethyl-4-Aminobenzoate PEG-25 PABA 10 Isopentyl-4-methoxycinnamate/Amiloxate Isoamyl P-methoxycinnamate 10 2,4,6-Trianilino-(p-carbo-2′ ′ EHT 5 Phenol, 2-(2H-Benzotriazol-2-yl)-4-methyl-6-(2-methyl-3-(1,3,3,3- tetramethyl-1-(trimethylsilyl)oxy)disiloxanyl)Propyl Drometrizole trisiloxane 15 Benzoic acid, 4,4-{[6-[[[(1,1-dimethylethyl)amino]carbonyl]phenyl]amino]- 1,3-5-triazine-2,4-diyl]diimino}bis-, bis (2-ethylhexyl)ester/Iscotrizinol DEBT 10 3-(4′ 4-Methylbenzylidene camphor 4 2-EHS/octisalate) EHS 5 2-Ethylhexyl 4-(dimethylamino)benzoate/padimate O (USAN:BAN) Ethylhexyl dimethyl PABA 8 2-Hydroxy-4-methoxybenzophenone-5-sulfonic acid (benzophenone-5) and its sodium salt/sulisobenzone Benzophenone-4 (BP-4) benzophenone-5 (BP-5) 5 (as acid) 2,2′ bis(6-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethylbutyl) phenol)/bisoctrizole Methylene bis-benzotriazolyl tetramethylbutylphenol (MBBT) 10 Methylene bis-benzotriazolyl tetramethylbutylphenol (nano) Methylene bis-benzotriazolyl tetramethylbutylphenol (NANO) 10 JOURNAL OF COSMETIC SCIENCE 300