Sunscreen and Skin Cancer: Analysis of Single Ingredients

As discussed in my previous blog post, sunscreen is made up of chemicals that many of us do not pay attention to. While some believe ignorance is bliss, Donathan G. Beasley and Thomas A. Meyer think otherwise. This mindset is exemplified in their analysis of the attenuation profiles of individual sunscreen ingredients. (For those who are confused by the word “attenuation”, here is the Merriam Webster definition of “attenuate”: To make something weaker or less in amount, effect, or force)

Methods

     Beasley and Meyer measured the absorbance profiles of TiO₂, Avobenzone, and ZnO using Vitro Skin®. This synthetic product is developed with similar pH, topography, surface tension, and ionic strength as human skin so that it may be used as a testing substrate for factors such as SPF and UVA. UVA protection factors were measured using delayed erythema, or sunburn, as a gauge for efficacy. The FDA recommends using erythema as a biological endpoint when measuring the extent of protection against UVA provided by sunscreen products.

     Water-in-oil mixtures were used as the model formulas for sunscreen in this experiment. All three formulas contained 5% PEG-12 dimethicone cross-polymer as the emulsifier mixed with either 3% Avobenzone, 5% ZnO, or 5% TiO₂ as the single sunscreen ingredient being tested. Additional chemicals were also added to the oil and water components as well.

     The researchers applied the test lotions via syringe onto the Vitro Skin. In the form of even dotting and subsequent spreading, each formula was placed onto the substrate with equal density (2µL/cm²). For the control, untreated Vitro Skin film served as reference. Average absorbance profiles (290-400nm) for each product film recorded 12 absorbance profiles from nonoverlapping sections of each Vitro Skin mount. Beasley and Meyer tested the variability in area of these sections under UVA and UVB spectra regions on each Vitro Skin and found it to be <4%. This low number reveals that the model formulas were spread evenly across the Vitro Skin surface.

     I am satisfied with the methods that Beasley and Meyer used for determining the absorbance profiles of each sunscreen ingredient; however, I am skeptical of the validity of the results of this study due to the sample sizes used in this experiment. The paper noted only 12 absorbance profiles recorded on each Vitro Skin mount. No overall sample size was given, but I inferred that since there were four film mounts tested (control, +Avobenzone, +ZnO, +TiO₂), the sample size would be 4x12=48 absorbance profiles recorded. Both researchers presented their data in a manner that convinces the reader of the fact that Avobenzone, when combined into a sunscreen formula, provides the most protection against UVA radiation and that TiO₂ cannot be a reliable replacement for either Avobenzone or ZnO. The lack of an adequate sample size (n=48) causes me to feel skeptical about the validity of these assumptions, but I believe there is potential for more research to be conducted that will be able to replicate these results.

Results

     Attenuation profiles for TiO₂, Avobenzone, and ZnO as single ingredients in model sunscreen formulas are shown in the figure below:

The attenuation of radiation levels for the sunscreen formula with 3% Avobenzone concentration surpassed those with 5% ZnO concentration across the entire UVB and UVA spectrums. When compared to the formula with 5% TiO₂ concentration, 3% Avobenzone provided greater attenuation across the UVA spectrum only. 5% ZnO offered greater attenuation than 5% TiO₂ at UVA wavelengths >360nm. Because Beasley and Meyer focus solely on comparing and contrasting UVA protection of Avobenzone, ZnO, and TiO₂, they do not acknowledge the results that yielded higher attenuation levels for 5% TiO₂ across the UVB spectrum.

     While it is clear that Avobenzone has the highest attenuation profile across the UVA spectrum, we must now compare ZnO and TiO₂. Both inorganic particles, ZnO and TiO₂ weaken the effects of UV radiation by scattering and absorption. The efficiency of TiO₂ is dependent on the size of the chemical particles themselves, while ZnO efficiency is based on the difference in refractive indexes for both chemicals. The paper claims that ZnO decreases the UVA radiation more effectively than TiO₂, but why is this the case?

Analysis

     Scattering of visible radiation can also be thought of like the reflection of light. According to Beasley and Meyer, the ideal sunscreen containing TiO₂ maximizes the efficiency of UVR attenuation by optimizing the particle size of the TiO₂ chemical. The attenuation profile of ZnO owes to its lower refractive index. When light, or in this case radiation, enters a medium with a higher refractive index, the angle of radiation dispersion will only differ slightly from the angle that it entered at. The higher the refractive index, the closer to the original direction will the radiation travel. With a lower refractive index (2.0 for ZnO vs. 2.6 for TiO₂), ZnO bends the incoming radiation at a higher degree of difference from the initial angle of entrance than does TiO₂. This basically means that ZnO scatters incoming radiation “better” than TiO₂, indicating its higher efficiency for protecting against UVA radiation.

     I trust the results provided by this paper because the data utilized to prove that Avobenzone and ZnO attenuate UVA radiation at higher levels than TiO₂ did not contradict each other. Beasley and Meyer disregard the UVB spectrum and choosing to focus on the attenuation profiles of three individual ingredients across the UVA spectrum. The specificity of this experiment is comforting for me, since I know that the authors did not attempt to evaluate beyond the scope of their focused lens and that all the data provided is relevant. I believe that using Vitro Skin allows for more replicable results because it does not require human participation and eliminates the negative side effects that these tests could impose on living people. The authors of this paper only recorded 48 absorbance profiles

     Avobenzone is currently labeled as the most effective UVA radiation protection factor ingredient when compared to ZnO and TiO₂. Since no sunscreen ingredient has effects that remain static, I am now interested in learning about how Avobenzone losses are minimized to the point where sustained protection is not affected by long exposure periods to UV rays.

Sources:

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. 

Food and Drug Administration. Sunscreen drug products for other-the-counter use: amendment

     to the tentative final monograph, enforcement policy. Fed Regist 1998 Oct 22; 63 (204):

     56587.

Merriam-Webster, Incorporated. "Attenuate." Merriam-Webster. Last modified 2014.

     Accessed May 7, 2014. http://www.merriam-webster.com/dictionary/attenuate.

IMS Inc. "What is VITRO-SKIN®?" IMS Inc.: Empowering Product Development. Last

     modified 1992-2012. Accessed May 7, 2014. http://www.ims-usa.com/

     vitro-skin.