To conclude the Sunscreen & Skin Cancer blog series, I am going to tie up some loose strings by discussing the role of the avobenzone sunscreen active in commercial formulations and methods used in order to optimize its ability to attenuate UVR.
Evaluating the photostability of Avobenzone
Using the leftover avobenzone from the original experiment [see Sunscreen & Skin Cancer: Analysis of Single Ingredients], the authors conducted further tests to determine efficacy of different ingredients on maintaining avobenzone’s structure during additional radiation. 14-18 mg of sunscreen formulas containing avobenzone were spread onto glass microscope slides with a density of 2 mg/cm2 and left to air dry in the dark. Glass slides that were not irradiated glass slides were used as the controls for this experiment. Each experimental slide was irradiated 120 cm from the source of UV light coming from a “1000 watt xenon solar stimulator filtered with a WG320 filter”. According to the article, this configuration is extremely specific due to the fact that its specificity prevents heating issues and ensures that all slides are maintained at room temperature. The authors tested every product in triplicate and irradiated with varying doses: 20, 40, 60, 80 joules/cm2. Loss of avobenzone was recorded by observing changes in absorbance at 360 nm. The amount of radiation exposed to the slides was comparable to the strength and duration of an hour of midday June sun in Memphis, TN. All three determinations of the standard deviation yielded less than 2%.
A second method measured the photostability of commercial sunscreen containing avobenzone over the full UV spectrum. Using Vitro Skin, absorbance after UVR exposure was taken in 12 non-overlapping sections of the synthetic skin, numbers which were then used to calculate the areas under UVB and UVA regions. Calculating change in area under UVB and UVA spectral regions over UVR dose then generated photostability numbers.
I believe both methods to be convincing and trustworthy. This conclusion is based on three aspects of its procedure: the specificity of the instruments used, the preciseness of the amounts of radiation distributed and results collected, and all three standard deviation calculations equaling less than 2%. The article is very descriptive about the experimental tools used and leaves no doubt that meticulous care was taken when setting up the equipment, which is a necessary precaution when performing this experiment.
Formulation Strategies: Maintaining Avobenzone’s Structure
1) Avoid using photochemically incompatible ingredients
2) Using the stabilizing abilities of sunscreen ingredients in an “ebb and flow” manner
3) Adding additional photostabilizers when individual sunscreen ingredients are not enough for adequate protection
The first strategy is exemplified by the author’s usage of octinoxate.
As seen through the results, octinoxate reacted irreversibly, destroying absorption abilities. The authors speculate that this may be due to octinoxate’s exocyclic double bond that reacts with avobenzone and consequentially collapses to form a bunch of degradation products.
The second strategy is presented in commonly used sunscreen products in the US, in which additional sunscreen actives are added to increase the photostability of avobenzone. 10% octocrylene caused avobenzone to remain intact after irradiation, while 6% oxybenzone allowed for a 30% degradation of avobenzone after 4 hours of UV exposure. The results below show how avobenzone is capable of photostabilization, but the addition of other individual sunscreen active ingredients can assist avobenzone in reaching complete stabilization during UVR exposure.
The third strategy comes into play when individual sunscreen actives are insufficient and require the addition of special photostabilizers, namely diethylhexyl-2,6-naphthalate (DEHN) 5% or diethylhexylsyringylidene malonate (DESM) 2%.
Overall I believe this article to be reliable and trustworthy. The authors thoroughly explain the methods used with utmost detail and provide plenty of figures to draw conclusions from. While the sample sizes are on the smaller end of the scale, the standard deviations do not exceed 2%, which indicates consistency and repeatability.
According to the article, the study conducted is the first of its kind. Never before has there been an experiment that compared UVA protection for avobenzone, ZnO, and TiO2 using real-world applications and comparisons. This interests me because it indicates that this research contains breakthrough, up and coming findings that have the potential to lead to other groundbreaking discoveries about sunscreen and skin cancer.
Beasley, DG, and TA Meyer. "Characterization of the UVA protection provided by
avobenzone, zinc oxide, and titanium dioxide in broad-spectrum sunscreen
products." Am J Clin Dermatol, December 1, 2010.
Fat, Michelle. "Sunscreen and Skin Cancer: Analysis of Single Ingredients."
Cancer Biology (blog). Entry posted May 8, 2014. Accessed May 24, 2014.