Bernd Herzog

Photostability of UV absorber systems in sunscreens.

Sunscreens are used to protect the human skin against harmful UV radiation. Today there is a trend toward higher sun protection factors (SPF) and better UVA protection. Methods for the assessment of SPF and UVA protection involve irradiation of the product, and the photostability properties of the sunscreen have an influence on its performance. Sunscreens often contain more than one UV filter. Thus it is important to understand the photostability properties of the complete system. The filter combinations used may exhibit destabilizing, stabilizing or inert interactions. For that reason, besides assessment of the properties of the single filters, photostabilities of binary filter combinations are investigated. Destabilization occurs when two UV absorbers undergo a chemical reaction after absorption of UV radiation. Stabilization may be achieved when the optical density of the system is very high, giving rise to a self‐protection effect of the sunscreen film. Photounstable UV absorbers may be additionally stabilized by employing triplet quenchers. Being aware of these mechanisms and applying them for specific UV filter combinations can help in designing efficient sunscreens.

Global state of sunscreens

The use of sunscreen is embedded in a hierarchy of sun protection strategies consisting primarily of sun avoidance by seeking shade and covering up with clothing. Sunscreens are, however, important means of protection; thus, understanding how they work and knowing their limitations are crucial. This review explains the role of ultraviolet (UV) filters, emollients, emulsifier systems and other components in a sunscreen, as well as trends in formulations in Europe, North America, Latin America, and Asia Pacific. Furthermore, it explains how sunscreen performance in terms of sun protection factor, UVA protection, and other metrics can be simulated. The role of sensory characteristics in assessing and improving compliance is also discussed. In the final chapter, Facts and Fiction, five of the most common myths about sun exposure and sun protection by sunscreen are debunked.

A Sunscreen's Labeled Sun Protection Factor May Overestimate Protection at Temperate Latitudes: A Human In Vivo Study

We lack comparative data on sunscreens with comparable sun protection factors (SPFs), but with different levels of UVA protection, especially against cumulative erythema from repeated suberythemal exposure. Thus, we compared the protection from cumulative sunburn with two sunscreens labeled SPF 6, but with different UVR-absorbing properties, one that absorbs the UVB waveband and the other that absorbs UVB+UVA wavebands. We simulated sunlight typical of temperate latitudes to expose skin daily to suberythemal doses for 13 consecutive days. The study population consisted of eight fair-skinned sun-sensitive healthy young adults. Erythema was assessed by eye and objectively, and the SPF of each sunscreen was modeled with changes in solar UVR with time of day and latitude. The broad-spectrum sunscreen gave much better protection against cumulative erythema than the UVB sunscreen. The explanation for this is that UVA makes a greater contribution toward sunburn at temperate latitudes than under the laboratory conditions in which SPF is tested and assigned. The data support the current trend toward broad-spectrum sunscreen protection. They also show that labeled SPF is much more reliable with broad-spectrum sunscreens because SPF with primarily UVB sunscreens is dependent on time of day and latitude.

On the assessment of photostability of sunscreens exposed to UVA irradiation: from glass plates to pig/human skin, which is best?

Photostability of suncare products is a great area of interest since several sunscreens on the market are photounstable, and this is primarily a problem concerning the UVA region (320-400 nm). Here we report a comparative study on the photostability assessment of two commercial sunscreens with same SPF, spread onto glass plates or onto full thickness pig ear skin or human/pig SCE membranes, and exposed to 183 kJ/m(2) UVA. Absorbance spectra and lipid peroxidation (measured by TBARS production) were determined. The results indicate: (a) sunscreen performance consequent to UVA exposure is independent of whether it is spread onto a non-biological and chemically inert substrate such as glass, or on biological substrates such as skin/SCE membranes; (b) despite the same SPF, sunscreen performance and photostability can be very different; (c) the data on human SCE membranes are similar to those on pig SCE membranes, indicating the suitability of the latter as a model for human skin. However, since the results obtained using skin membranes, akin to the more realistic conditions of use in vivo, do not substantially differ from those obtained on glass plates, the method proposed here using the latter may be applied for rapid, inexpensive, efficacy screening of photostability of sunscreens. Photostability testing should be a mandatory requirement for safer sunscreen protection products, since the results clearly show that some are still far from perfect.

Assessment of the photo-degradation of UV-filters and radical-induced peroxidation in cosmetic sunscreen formulations

Abstract Photo-instability of common UV-filters is a well documented phenomenon. This study develops a method for concomitant measurement of photostability and photo-induced ROS generation in cosmetic formulations. Oil-in-water formulations containing three common UV filters (OMC, BMDBM, EHT), individually or combined, were further supplemented with phosphatidylcholine and exposed to UVA. All filters show spectral decrease after UVA exposure. OMC and EHT do not induce significant lipid-peroxidation (as measured by TBARS production) while BMDBM does. In the latter case, this is reduced when BMDBM is combined with OMC but not with EHT. Neither OMC nor EHT stabilize BMDBM with respect to loss of absorbance. ROS-generation assessed via TBARS formation was supported by EPR experiments. The UV-induced changes in UV-filter performance, as monitored in the model formulations and in commercial sunscreens, demonstrate that this is a simple and effective method for stability assessment of sunscreen filters under conditions of use.

Simulation of sunscreen performance

Abstract Sunscreens are used to protect the human skin against harmful effects of solar UV radiation. The most important quantity characterizing sunscreen performance is the sun protection factor (SPF). At the stage of development of new sun protection formulations quick and inexpensive methods for estimation of the UV screening performance are highly desirable. The most convenient approach towards this goal is given by computational simulations. Models for the calculation of the SPF employ the same algorithm as used with in vitro SPF measurements, but replace the transmittance measurement by the calculation of the overall absorbance of the UV filters in an irregular sunscreen film. The simulations require a database with quantitative UV extinction spectra of the relevant UV filters as well as a mathematical description of the film irregularity. The simulation algorithm implies also the consideration of photodegradation properties of the UV filters in the sunscreen composition. Besides using such simulations for designing new sunscreen formulations, the calculations can also support the understanding of sunscreen performance in general.

Film thickness frequency distribution of different vehicles determines sunscreen efficacy.

Abstract. Sun protection factor (SPF) frequently differs between sunscreens containing the same composition of ultraviolet (UV) filters that primarily define sunscreen efficacy. We tested the hypothesis that the thickness frequency distribution of the sunscreen film is also responsible for and can explain the divergence in the measured SPF. For this, we developed a method to measure film thickness from the difference of topography before and after application of 2  mg/cm2 of sunscreen on pig ear epidermal membrane. The influence of five vehicle formulations and of application pressure and spreading time on mean thickness (Smean), Smean to median ratio, and SPF in vitro was investigated. The vehicle had a significant impact, low vehicle viscosity resulting in a smaller Smean, larger Smean to median ratio, and lower SPF in vitro than high viscosity; continuous oil phase produced the largest Smean and SPF values. A long spreading time reduced Smean and SPF and increased application pressure reduced SPF. There was a positive correlation between Smean and SPF in vitro, underlining the relevance of film thickness for interpreting UV protection differences of formulations with the same filter composition. This work demonstrated a strong influence of vehicle and application conditions on sunscreen efficacy arising from differences in film thickness distribution.

Comparison of ultraviolet A light protection standards in the United States and European Union through in vitro measurements of commercially available sunscreens

Background The importance of adequate ultraviolet A light (UVA) protection has become apparent in recent years. The United States and Europe have different standards for assessing UVA protection in sunscreen products. Objective We sought to measure the in vitro critical wavelength (CW) and UVA protection factor (PF) of commercially available US sunscreen products and see if they meet standards set by the United States and the European Union. Methods Twenty sunscreen products with sun protection factors ranging from 15 to 100+ were analyzed. Two in vitro UVA protection tests were conducted in accordance with the 2011 US Food and Drug Administration final rule and the 2012 International Organization for Standardization method for sunscreen effectiveness testing. Results The CW of the tested sunscreens ranged from 367 to 382 nm, and the UVA PF of the products ranged from 6.1 to 32. Nineteen of 20 sunscreens (95%) met the US requirement of CW >370 nm. Eleven of 20 sunscreens (55%) met the EU desired ratio of UVA PF/SPF > 1:3. Limitations The study only evaluated a small number of sunscreen products. Conclusion The majority of tested sunscreens offered adequate UVA protection according to US Food and Drug Administration guidelines for broad‐spectrum status, but almost half of the sunscreens tested did not pass standards set in the European Union.

Calculation of the sun protection factor of sunscreens with different vehicles using measured film thickness distribution - Comparison with the SPF in vitro.

The sun protection factor (SPF) depends on UV filter composition, and amount and type of vehicle of the applied sunscreen. In an earlier work, we showed that the vehicle affected the average thickness of sunscreen film that is formed upon application to a skin substrate and that film thickness correlated significantly with SPF in vitro. In the present study, we quantitatively assess the role for sunscreen efficacy of the complete film thickness frequency distribution of sunscreen measured with an oil-in-water cream, an oil-in-water spray, a gel, a water-in-oil, and an alcoholic spray formulation. A computational method is employed to determine SPF in silico from calculated UV transmittance based on experimental film thickness and thickness distribution, and concentration and spectral properties of the UV filters. The investigated formulations exhibited different SPFs in vitro and different film thickness distributions especially in the small thickness range. We found a very good agreement between SPF in silico and SPF in vitro for all sunscreens. This result establishes the relationship between sun protection and the film thickness distribution actually formed by the applied sunscreen and demonstrates that variation in SPF between formulations is primarily due to their film forming properties. It also opens the possibility to integrate the influence of vehicle into tools for in silico prediction of the performance of sunscreen formulations. For this, the use of the Gamma distribution was found to be appropriate for describing film thickness distribution.

Über den Lichtschutzfaktor hinaus - neue effiziente und photostabile UVA-Filter

ZusammenfassungVon Sonnenschutzpräparaten wird heute nicht mehr nur Schutz vor Sonnenbrand, sondern auch Schutz vor Langzeitschäden erwartet. Damit kommt den UVA-Filtern eine wachsende Bedeutung zu. Bis vor kurzem war die Auswahl dieser Filter sehr begrenzt. Im Jahre 2000 verbesserte sich diese Situation zusehends. Mit MBBT (Methylene Bis-Benzotriazolyl Tetramethylbutylphenol), BEMT (Bis-Ethylhexyloxyphenol Methoxyphenyl triazin) und DPBT (Disodium Phenyl Dibenzimidazole Tetrasulfonate) wurden gleich drei neue, photostabile UVA und Breitbandfilter in die Positivliste der kosmetischen UV-Filter der EU aufgenommen. Zur Einschätzung des UVA-Schutzes sind in Europa zahlreiche In-vivo- und In-vitro-Methoden in der Diskussion. Die Anwendung der gängigsten relativen In-vitro-Methoden zeigt, dass Höchstwerte bei der “Kritischen Wellenlänge” wesentlich einfacher zu erreichen sind als beim “UVA/UVB-Verhältnis”. Anhand der in Japan bereits zur Norm erhobenen In-vivo-Methode, PPD (Persistent Pigment Darkening), kann die unterschiedliche Photostabilität des konventionellen, photolabilen UVA-Filters BMDBM (Butyl Methoxydibenzoyl methane) im Vergleich zu der des modernen photostabilen UVA-Filters MBBT eindrücklich demonstriert werden. Die neuen photostabilen UVA-/Breitbandfilter werden diskutiert, wobei auch erste kommerzielle Anwendungsbeispiele Berücksichtigung finden.AbstractToday sunscreens are expected to protect not only against sun burn, but also against long term damaging effects. This means that UV-A filters become more and more important. Until recently the offering of such filters was very limited. With the year 2000 the situation started improving steadily. MBBT (Methylene Bis-Benzotriazolyl Tetramethylbutylphenol), BEMT (Bis-Ethylhexyloxyphenol Methoxyphenyltriazine) und DPBT (Disodium Phenyl Dibenzimidazole Tetrasulfonate) three new, photostable UVA- and Broadband filters have been added to the positive list of the European cosmetic UV filters. Since the methods to assess UV-A protection are not harmonized in Europe, the Australian standard AS/NZS 2604 is frequently claimed in Germany. The application of the two relative in vitro methods “critical wavelength” and “UVA/UVB ratio” shows that the highest protection class is significantly easier to achieve with former one. The PPD method (Persistent Pigment Darkening) has become the in vivo standard in Japan. Its advantage is the absolute value and the fact that photostability is taken into account implicitly. This allows a comparison of the conventional, unstable UVA Filter BMDBM (Butyl Methoxydibenzoylmethane) with the modern photostable UVA-Filter MBBT. The new photostable UVA-/Broadband filters are discussed and the first commercial applications of their use are also taken into consideration.