Legislation
In the EC regulation n.1223 / 2009 of November 30, 2009 on cosmetic products, UV filters are defined as "substances intended exclusively or mainly to protect the skin from certain UV radiation through absorption, reflection or diffusion of UV radiation" (Article 2).
In the USA, according to the list by the FDA (Food and Drug Administration), however, only 16 UV filters are allowed, as they are considered not as cosmetics but as OTC drugs (Cosmetic News, 2001).
The solar filters are divided into two main categories: physical filters and chemical filters .
Physical Filters
Physical filters are opaque pigments to light radiation and reflect and / or diffuse ultraviolet light and visible radiation.
The most common are: titanium dioxide (TiO 2 ), zinc oxide (ZnO), silicon dioxide (SiO 2 ), kaolin, iron oxide or magnesium. Of these, only TiO 2 is present in Annex VI (relating to authorized UV filters) of the New Regulation on cosmetic products; the others, in particular zinc oxide, are widely used in solar products but cannot be declared responsible for the filtering action.
The physical filters are photostable, do not react with organic filters and are often used in association with these, even at high concentrations, resulting in a synergistic effect that allows to reach very high SPF values.
In the past, the physical filters, having a considerable solid consistency, were totally reflective and presented the problem of creating a white effect when the solar product was applied to the skin; currently there are micronized titanium dioxide and zinc oxide forms on the market which, by reducing the particle size to the order of magnitude of the nanometers, allow shielding of low-wavelength radiation such as UV, but not visible light, thus avoiding any white effect. However, some studies have shown that micronisation can increase the penetration of the physical filter into the innermost layers of the epidermis, where it can trigger reactions of oxidative stress with consequent depletion of collagen, photo-aging and photocarcinogenesis (Jianhong Wu, Wei Liu, Chenbing Xue, Shunchang Zhou, Fengli Lan, Lei Bi, Huibi Wu, Xiangliang Yang, Fan-Dian Zeng "Toxicity and penetration of TiO2 nanoparticles in airless mice and porcine skin after subchronic dermal exposure" Toxicology letters 191 (2009) 1-8).
To prevent the agglomeration of microparticles as a result of electrostatic attraction, titanium dioxide is coated (allimina, stearates, simethicone, dimethicone) and optionally pre-dispersed and stabilized in water or in a lipophilic vehicle (caprylic / capric triglyceride, C12- 15 alkyl benzoate). The pre-dispersions, which are easier to manipulate and incorporate into the formula, generally offer greater protective performance. In fact it has been shown that the size of the particles and the absence of macroscopic aggregates (the surface of interaction with the incident light) influence the SPF value. Also zinc oxide, able to reflect both UVA and UVB radiations, is available on the market both in powder form and in pre-dispersed form.
Chemical Filters
To date, approved chemical filters can be classified as derivatives of the following compounds: PABA and derivatives, cinnamates, anthranilates, benzophenones, salicylates, dibenzoylmethane, anthranilates, camphor derivatives and phenyl-benzimidazolsulfonates.
They are synthetic substances with a chemical structure that generally consists of an aromatic ring and two functional groups capable of acting as donors or electron acceptors. They selectively absorb short wavelength UV rays and convert them into longer wavelength and less energy radiations. The energy absorbed by the filter corresponds to the energy required to cause its photochemical excitation to a higher energy state than the one in which it is located; returning to the initial energy state, it emits radiation of a greater wavelength, which is not harmful to the skin. Energy can be emitted as fluorescence if it falls in the visible region, as heat if it is in the IR, or it can damage the chemical structure of the filter itself with consequent loss of filtering activity and production of potentially harmful degradation products ( Maier T. & Korting HC, "Sunscreens - Which and what for?", Skin pharmacology and physiology, 2005; 18: 253-262).
Features of a Solar Filter
The general requirements that a good solar filter must possess are:
- broad absorption spectrum (280-380 nm). If it is not possible to cover the entire spectrum with a single filter, use a mixture;
- have good chemical stability;
- have good photostability;
- have a good toxicological profile (very low acute, long-term toxicity, absence of phototoxicity, non-sensitizing, non-photosensitive, absence of percutaneous absorption);
- be as odorless as possible;
- have a good tolerability on the part of skin and mucous membranes;
- don't be irritating;
- have good solubility, compatibility and stability in the finished product (including packaging );
- have a surface action;
- have a high extinction coefficient
- have maximum wavelength and extinction coefficient not influenced by solvent or pH;
- it must not cause discoloration of the skin and tissues.
