Suncare Science - Part 3: Chemical Absorbers
Chemical absorbers are usually soluble in oils or water. They filter UVB and/or UVA radiation to varying efficiency. No chemical absorber completely blocks 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 (310nm – 312nm). 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.
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 (300nm –315nm) 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.
An old and safe but overall weak absorber, menthyl anthranilate absorbs moderately in the UVB range from about 300nm and somewhat more strongly into the UVA range (up to about 340nm). It can somewhat enhance the UVB and lower (320nm to 340nm) 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 (320nm – 360nm). They are quite stable and can enhance effectiveness of stronger UVB absorbers.
Avobenzone (Parsol â1789)
This solid (powder) absorber exhibits marginal UVB and lower (320nm – 330nm) UVA absorption. It gives good UVA absorption from about 330nm to 340nm 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 can convert to its inactive form in the presence of sunlight and readily loses more than one-third of its active form 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.
An emollient, water resistant UVB/UVA absorber; while octocrylene is a relatively weak sunscreen, it gives some protection in the UVB and lower (320 – 350 nm) UVA range. Most important, octocrylene is a very stable absorber and both protects and augments other UV absorbers, while improving their uniform skin coating.