All it takes is one trip to the sunscreen aisle of the local drug store to realize With so many labels making similar claims, how does one judge? The best way is to know a little something about the chemistry behind this class of products.
Eighteen 2/0 active ingredients are currently FDA-approved for sunscreen use in the United States. They fall into two categories: physical blockers (a.k.a. inorganic physical ingredients) and chemical absorbers (a.k.a. organic chemical ingredients). Physical blockers work by scattering and reflecting UV radiation; chemical absorbers from the organic chemistry lab work by absorbing UV radiation before it can penetrate the skin.
1. Titanium Dioxide
This white pigment powder is widely used in cosmetics. The purpose of large particle titanium is to give opacity and lighten (or whiten) the color of the products containing it. Opaque titanium dioxide highly reflects and strongly scatters all UV and visible rays. It also reflects much of the skin damaging infrared waves, which keeps the skin cooler, reducing “heat” damage and its subsequent photoaging.
To photo-stabilize titanium dioxide, it must be micro-coated with its own protectant such as silicone or aluminum oxide. Alternatively, breakdown is inhibited by incorporating other appropriate blockers together with titanium dioxide since titanium dioxide spreads poorly on the skin. Micro-coating the titanium dioxide is also a common way to achieve cosmetic elegance and usefulness; designing a vehicle to assure good, even application to the skin is essential. Large particle titanium dioxide products produce a very white, opaque appearance when applied on the skin, but submicronizing titanium dioxide powder creates small particles that absorb visible light and are invisible on the skin while enabling products to protect the skin from most UVB and some UVA.
Transparent (sub-micronized) titanium dioxide works by absorbing, reflecting and scattering UVB and some UVA rays. However, protection against UV, visible and infrared is significantly limited when sub-micronized titanium dioxide is the primary protectant.
2. Zinc Oxide
Zinc Oxide has been known and used topically for centuries as a skin protectant and wound healing adjuvant and is a recognized mild antimicrobial agent. More than 50 years ago, zinc oxide was indicated as a block for ultraviolet light (UVB/UVA). Like titanium dioxide, it also reflects infrared from the skin. Unlike titanium dioxide, it has a much higher ability to protect in the long UVA range (300 – 400 nm). Thus, used in combination with titanium dioxide, ultrafine zinc oxide “closes the window” in the UVA range. Zinc oxide both complements titanium dioxide’s protective abilities and extends photoprotection to the skin where titanium dioxide is insufficient. The optimal particle size range for ultraviolet-blocking zinc oxide (without blocking visible wavelengths) is approximately 80 to 150 nanometers.
3. Iron Oxides
We most commonly encounter iron oxide either as rust on exposed iron or in cosmetics, where it is used to give the cover-up color desired. While not approved by the FDA as an active ingredient in sunscreens, many companies use iron oxides in sunscreen products. Cosmetic iron oxides are man-made to obtain very high purity.
Iron oxide pigments for cosmetic use are micronized powders. By controlling the purity, particle size, temperature and rate of drying during manufacture, they become available in a number of shades and tones of red, yellow, black and brown (and blends of these basic colors). If incorporated at adequate concentration and when properly dispersed in well-designed vehicles, these cosmetic pigments not only add color to the lotion (or cream, powder, etc.), but also significantly contribute to protecting the skin from multiple wavelengths of light.
Ultra-submicronized iron oxides offer the advantage of protecting against visible light waves while adding little color to the finished product. Thus, higher levels of infrared protecting iron oxide can be used without compromising the cosmetic elegance and shade of the final preparation. Submicronized iron oxides are reported to considerably block ultraviolet rays, further complementing a product’s primary UV blocking agents.
*TIZO SUNSCREENS CONTAIN NO CHEMICAL ABSORBERS*
Chemical absorbers are usually soluble in oils or water. They filter UVB and/or UVA radiation to varying efficiency. No chemical sunscreen filter completely absorbs the UVB and/or UVA rays from the skin. Further, the actual protection offered by any and all sun-protective products relates directly to their level of concentration, how long they remain stable when exposed to sun, the film thickness applied to the skin, as well as the careful, total coverage of the exposed skin sites.
The most common chemical absorbers used in sunscreens include:
Salicylates are the oldest class of sunscreens, with octyl salicylate being the most widely used. While it is strictly a UVB absorber, and a weak one at that, it offers several positive qualities, including that it is virtually nonirritating and nonsensitizing to skin, and cosmetically, it is an easy to handle emollient “oil” that acts as a good solvent (solubilizer) for other, solid organic sunscreens, such as the benzophenones.
Octyl Dimethyl PABA (Padimate O)
This oil-like UVB absorber is the most efficient for this ultraviolet range, absorbing best at the maximum sunburn frequencies (310 nm – 312 nm). It was the most popular UVB sunscreen in the United States, but adverse (perhaps unfounded) reports have reduced its use. Padimate-O is a PABA derivative, but it is quite distinct. Today’s purified material is essentially free of PABA.
Octinoxate (Octyl Methoxycinnamate)
Currently, this oily liquid is the most widely utilized organic UVB absorber in the world. It is second in efficiency to Padimate-O, but offers broader protection (300 nm – 315 nm) in the sunburn region of UVB. It has a very good safety record and is relatively easy to use in formulations. Additionally, it is moisturizing and water insoluble, adhering tenaciously to the skin.
Meradimate (Menthyl Anthranilate)
An old and safe but overall weak absorber, menthyl anthranilate absorbs moderately in the UVB range from about 300 nm and somewhat more strongly into the UVA range (up to about 340 nm). It can somewhat enhance the UVB and lower (320 nm – 340 nm) UVA absorption of more active absorbers.
Oxybenzone (Benzophenone-3) and Sulisobenzone (Benzophenone-4)
These are closely related solid (powder) absorbers. Oxybenzone is water–insoluble, while the acid form, sulisobenzone, can be made soluble in water when it is neutralized. While these compounds are classified as UVA absorbers they are also UVB absorbers. Overall, they offer only moderate protection through both the UVB range and part of the UVA range (320 nm – 360 nm). They are quite stable and can enhance effectiveness of stronger UVB absorbers.
This solid (powder) absorber exhibits marginal UVB and lower (320 nm – 330 nm) UVA absorption. It gives good UVA absorption from about 330 nm to 340 nm and very good absorption in the UVA range up to about 370nm, where it rapidly loses effectiveness. Because of its irritation potential, the allowed concentration level is low, limiting the actual level of protection obtained. In addition, avobenzone is reported to be photo-unstable. It can convert to its inactive form in the presence of UV radiation and readily loses more than one-third of its active form (negating its photoprotective ability) rather quickly. Therefore, avobenzone (Parsol 1789) is a useful, but limited, UVA protector. Its usefulness can be enhanced when combined with UVB absorbers and physical protectors, such as zinc oxide, but without careful formulation, it may decrease the protective ability of UVB sunscreens. The combination of avobenzone with zinc oxide is presently not allowed by the tentative FDA monongraph; evaluation by the FDA to allow this combination is underway. [FDA-Federal Register OTC Sunscreen Labeling Update; p10]
An emollient, water resistant UVB/UVA absorber. While octocrylene is a relatively weak sunscreen, it gives some protection in the UVB and lower UVA range (320 – 350 nm). Most important, octocrylene is a very stable absorber and both protects and augments other UV absorbers while improving their uniform skin coating.