What are sunscreens and what is the difference between physical and chemical sunscreens? Also list down additional ingredients of sunscreen with product forms of sunscreens

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    Sunscreens represent a practical approach to photoprotection. Sunscreen ingredients are usually referred to as UV filters and are identified on product labels as active ingredients.

    Additional ingredients in sunscreen products are listed as inactive ingredients in the Drug Facts section on the information panel.

    UV Filters
    UV Filters are classified into two groups based on their mechanism of action;

    1. Physical Sunscreens
    2. Chemical Sunscreens

    A single product usually contains more than one active ingredient, both physical and chemical filters, to achieve the desired SPF value and broad-spectrum protection. It should be noted, that the OTC monograph includes restrictions for the combination of certain ingredients.

    Physical Sunscreens
    Otherwise known as inorganic UV filters, reflect and scatter UV radiation. There are two approved physical filters in the US, namely Titanium dioxide (TiO2) and Zinc oxide (ZnO). Both of these ingredients are white powers that are insoluble in the sunscreen product base. Therefore, they are suspended from such products.

    Inorganic sunscreens can only penetrate the outer layer of the skin. Therefore, they have an excellent safety profile. Additionally, inorganic filters are photostable, independent of the sunscreen base and other ingredients. Inorganic filters provide broad-spectrum protection since they reflect and scatter both UVA and UVB radiations. TiO2 offers UVB and UVA II protection, while ZnO protects against UVB, UVA II, and UVA I radiations.

    The main disadvantage of inorganic filters with regular particle size is that they reflect and scatter UV radiations into the visible spectrum (>400nm), which provides a white appearance on the skin after application. This can make sunscreens cosmetically less appealing and can lead to the reduced application by consumers. 

    The reflection spectrum of TiO2 and ZnO, and therefore, the white appearance can be, however, modified by decreasing their particle size. The smaller the particle size, the less reflection occurs in the visible spectrum. Therefore, today, TiO2, and ZnO are commonly used in micronized (i.e., with a particle size of 1-100nm) forms for an aesthetically more appealing cosmetic look. 

    Another advantage of smaller particles is that they feel lighter on the skin. Products that contain nanoparticles no longer reflect visible light and therefore do not appear white but transparent on the skin. However, there is an important fact that should be emphasized; particle size reduction leads to a shift in the UV radiation protection profile of inorganic sunscreens towards the UVB and UVA II range, and the UV protection becomes imbalanced. 

    Using nanosized and microsized particles in combination may improve with organic filters to provide broad-spectrum protection while maintaining good aesthetics.

    Chemical Sunscreens
    Otherwise known as organic UV filters, are generally aromatic compounds. Their molecular structure is responsible for absorbing UV rays. Organic filters absorb UV rays, which produce excitation of the sunscreen chemical to a higher energy state.

    Then, they return to the ground state and convert the absorbed energy into longer, lower energy wavelengths (heat). Currently, there are 15 organic sunscreen ingredients approved in the US. For Example;

    1. Octinoxate
    2. Octisalate
    3. Meradimate, etc

    Organic filters are often combined to achieve the desired broad-spectrum protection. According to the OTC monograph for sunscreens, a sunscreen product must have a minimum SPF of not less than the number of active sunscreen ingredients used in combination multiplied by 2.

    Organic sunscreens can penetrate the skin due to their lipophilic nature, which may cause safety concerns. An issue with many organic sunscreens is their photostability. Upon exposure to UV radiation, the structure of UV filters may be negatively affected and/or destructed. As a result, instead of returning to the ground state, they lose their absorption capacity. Therefore, most formulations contain photo stabilizers.

    As discussed earlier, inorganic and organic UV filters are often combined to provide optimal photoprotection. Inorganic UV filters are stable upon exposure to sunlight. However, these metal oxides may produce free radicals after UV exposure, which can degrade organic filters. Therefore, when organic and inorganic filters are combined, surface-treated inorganic filters should be used, for example, silica-coated aluminum-coated, or silicon-coated TiO2 or ZnO.

    Additional Ingredients of Sunscreens
    Inactive ingredients generally found in sunscreen products depend on the dosage form. The vehicle determines which UV filters can be used based on their polarity and solubility characteristics. The most commonly used ingredient types are summarized here;

    • Waterproofing agents are included in formulations to increase their water-resistance properties. As O/W emulsions are one of the most popular formulations, the incorporation of waterproofing agents is important in their case to help the product withstand water in any form, such as sweat, sea, or a swimming pool. The choice of sunscreen can also influence water resistance.
      Examples include silicone oils, such as dimethicone 350, Cyclomethicone, and dimethicone/trimethyl siloxy silicate. They are very resistant to water penetration and are easy to spread and form a continuous thin water-repellent film on the skin surface. Additionally, polymeric film-formers are gaining more interest as waterproofing agents. Alkylated polyvinylpyrrolidone (PVPs), which are closely related to the film-formers found in hair sprays, are also excellent for imparting water resistance o a formulation.
       
    • Photostabilizers are ingredients that can prevent the degradation of organic UV filters, otherwise initiated by UV exposure. As discussed earlier, certain chemical filters are very sensitive to UV radiation and may suffer significant changes in their structure. Photostabilizers can help stabilize UV filters through chemical bonds or help them dispose of the UV energy more quickly, thus reducing or even eliminating the possibility of a chemical reaction.
      Examples include UV filters, such as octocrylene (for avobenzone specifically) and TiO2, as well as other ingredients, such as polyester-8).
       
    • Emollients are lipophilic ingredients that help increase water resistance since they are not washed off easily. They can also serve as solvents for lipophilic organic sunscreens. The maximum absorbance wavelength of sunscreen agents can be shifted to either longer or shorter wavelengths depending on the polarity of the sunscreen and that of the solvent. This may be either beneficial or detrimental for a formulation affecting the protection it can provide.
      Examples include mineral oil, shea butter, castor oil, cocoa butter, isopropyl myristate, isohexadecane, paraffin, and silicones.
       
    • Water is an essential ingredient in O/W and W/O emulsions as well as in gels. It also serves as a solvent for water-soluble ingredients. For aerosol products usually, alcohol is used as a solvent.
       
    • Usually, a combination of emulsifiers is used to provide the appropriate stability for the products. Examples include glyceryl stearate, PEG-100 stearate, cetyl alcohol, polyglycerol;1-3-methyl glucose dissertate, and cetyl dimethicone copolyol.
       
    • Thickeners act as rheology modifiers in the formulations; they affect the application, spreadability, and efficacy of sunscreens.
      Examples include carbomers; cellulose derivatives, such as hydroxy propyl cellulose; gums, such as can than gum; and other polymers, such as acrylates/C10-30 alkyl acrylate crosspolymer. Lipophilic thickeners, such as waxes, are also often used in sunscreens.
       
    • Film-forming ingredients help form an even and uniform film on the skin after application and drying. This way they allow for a higher SPF factor. They are sometimes referred to as "SPF boosters" for this reason.
      Examples include hydrolyzed wheat protein/PVP crosspolymer, methylcellulose, polyester-7, and acrylates/octyl acrylamide copolymer.
       
    • Antioxidants: The damage attributed to UVA radiation, for example, aging, is attributed to the formation of free radicals in the deeper skin layers. Although the body has natural antioxidant defenses against the reactive oxygen species, this endogenous system is quickly overwhelmed when faced with excessive oxidative stress. Antioxidants can help prevent oxidative reactions and therefore can be added to sunscreens, although their impact is not universally accepted.
      Examples include Vitamin E and C.
       
    • Preservatives are necessary when water is present in the formulations.
      Examples include parabens, benzyl alcohol, methylchloroisothiazolinone, methylisothiazolinone, and phenoxyethanol.
       
    • Humectants provide moisturization.
      Examples include sorbitol, glycerine, and propylene glycol.
       
    • Chelating agents contribute to the stability of the system by forming complexes with metal ions that may cause premature deterioration of the products. 
      Examples include EDTA and its derivatives.
       
    • Propellants are essential ingredients in aerosol sunscreens. They help expel the content of the aerosol can.
      Examples include isobutane and dimethyl ether.
       
    • Additional ingredients may include pH modifiers (neutralizers), such as citric acid and triethanolamine; fragrances; and colorants, TiO2 could act as a pacifier in certain cases; however, this effect is reduced with size reduction. Natural ingredients, such as allantoin, aloe vera, panthenol, and vitamins can also be added to formulations.

    Product Forms of Sunscreens
    Sunscreens are available in a variety of dosage forms, including O/W or W/O emulsions; anhydrous systems, such as ointments, sticks, oils, and silicone-based aerosols; wipes; and gels.

    Emulsions are the most popular form since they offer a variety of textures (such as sprayable lotions, thicker lotions, and creams). Additionally, another advantage of emulsions is that both oil and water-soluble ingredients can be incorporated into them. O/W emulsions are generally more popular as the skin feel they offer is cosmetically more appealing, and are easier to manufacture at a relatively low cost. products with good spreadability are generally applied more uniformly to the skin, which results in a smooth surface with low variation in SPF and a greater application thickness. W/O emulsions are greasier and stickier however, they have been shown to have higher efficacy. Additionally, due to the oily outer phase, they provide a higher level of water resistance. Lower viscosity emulsions may be packaged into nonaerosol spray containers. It can make application easier and more comfortable.

    Sticks are available as lipsticks and lip balms as well as sticks or babies. They are suitable for smaller surface areas only, such as the face and lips, and for babies. However, applying them to a larger surface area, such as the legs or back of adults, may be time-consuming; therefore, sticks are usually not offered for such purposes. Due to their anhydrous nature, they provide water resistance and higher efficacy but are often expensive to prepare for this reason. The choice of waxes determines the melting point of the stick and its performance on the skin. Melting point can be reduced by the level and type of emollients used, although firmness must be maintained in hot weather conditions.

    Aerosol sprays are one of the most popular formulations. The multi-position spray nozzles allow for quick and easy application by covering a relatively larger area on the body, so even hard-to-reach areas can be covered more easily. A common problem however is that they are not applied appropriately and the amount of product applied is far less than what is recommended by the FDA.

    Aerosol spray sunscreens should be evenly spread on the skin and rubbed onto the skin after application (not just sprayed on). Concerns arose with regard to the sale see of sunscreens after severe burning cases were reported from consumers using aerosol products near a flame source.

    Aerosols are considered a safe dosage form; however, users should read the labels before using a product and take precautions when applying such products. Aerosols sprays are generally alcohol-based products containing alcohol-soluble organic UV filters As physical sunscreens are not alcohol soluble, they are usually not formulated into aerosol sprays or would otherwise clog the valve. They usually contain film-forming agents to ensure the formation of an even film on the skin.

    Ointments and oils were quite popular years ago when most people used low-SPF products and wanted a deep-colored tan. Today, they are not as popular as other forms. Ointments and oils are greasier than W/O emulsions, which are not desired by consumers. Additionally, they are based on waxes, fats, and oils and are therefore more expensive than other forms.

    Gels provide a nice skin feel and absorb quickly; however, providing water resistance properties for such formulation is quite difficult. Additionally, active ingredients that can be used for this dosage form are very limited. Another disadvantage may be an irritation if alcohol is used in the formulation.

    Wipes are the latest sunscreen formulations that are similar to wet facial wipes. These are pre-moistened by the manufacturer. The formulators can place emollients and other moisturizers onto the cloth wipes, which may, therefore, provide additional benefits.

    Cosmetic products containing sunscreens (which are considered drugs for this reason), such as facial liquid, foundation, face powder, BB creams, and CC creams, are also available today. These products are a convenient way for women to provide photoprotection on the facial region every day.

    The FDA currently considers sunscreens in the form of oils, creams lotions, gels, butter, pastes, ointments, sticks, and sprays to be eligible for potential inclusion in the OTC sunscreen monograph i.e., they can be marked without individual product approvals. However, wipes, towelettes, powders, body washes, and shampoos are not eligible for the monograph. Therefore, they cannot be marketed without an approved application.

    Products formulated specifically for babies are worth mentioning at this point. Baby skin is much more sensitive than adult skin and penetration of ingredients into baby skin can be much higher and faster. Therefore, ideally, babies under 6 months should avoid sun exposure and no sunscreens should be used on their skin. The best sun protection for them is to keep them in shade as much as possible in addition to wearing long sleeves, pants, a wide-brimmed hat, and sunglasses.

    For babies older than 6 months, sunscreens can be used to protect their skin from the sun. These are normally similar to adult formulations but are usually perfume-free to reduce the chance of irritation and sensitization.

    Additionally, irritation and sensitization potential can also be minimized using physical sunscreens, which do not penetrate the viable skin layers. Products designed for babies should have high SPF and water resistance properties. In addition, baby skin is more susceptible to moisture loss; therefore, formulations should have additional moisturizing benefits as well to promote the protective skin barrier. Sunscreen products for babies are available in a variety of dosage forms, including aerosols, creams and lotions, pump sprays and sticks.

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