Relationship between UVA protection and skin response to UV light: proposal for labelling UVA protection

Definition and validation of a most relevant method to assess ultravoilet A (UVA) protection is a major concern for industry, authorities and consumers. However, due to the lack of knowledge about all the biological phenomena involved, the level of UVA protection needed, the ways to assess and label it, remain controversial. In order to overcome this situation, the paper deals with the outcomes of a mathematical model to calculate the distribution between ultravoilet B (UVB) and UVA components of skin responses to UV light. Mathematical calculations of UVB and UVA erythemal components of skin response to sunlight are developed from the well-known determination procedure to calculate the sunburn protection factor (SPF) of sunscreens. The model establishes the relationship between the UVA component of skin erythemal response to overall UV radiation received from sunlight and the ratio SPF/PFAe (erythemal protection factor) where SPF is the product and PFAe is related to the UVA part of the sunlight. Depending on the efficacy profile of sunscreens, the skin erythemal response may be mainly promoted by UVB rays as it normally occurs in unprotected skin or on contrary by UVA rays. Therefore, the efficacy profile of sunscreens defines the deepness where biological events induced by sunlight take place. This new relationship pinpoints the tremendous importance of the protection afforded by sunscreen products in the UVA range when erythema is taken as biological response. By extrapolation of the model to any other biological skin response it becomes possible to predict how to improve the efficiency of sunscreen products in the future. UVA protection afforded by sunscreens should be improved until reaching the same level as the SPF protection factor so that all UV-induced biological responses could be prevented or lowered at the same extend. To enforce this improvement, a proposal to classify sunscreen products in relation with their UVA protection is made.     

Poly(D,L-lactide) nanoencapsulation to reduce photoinactivation of a sunscreen agent

The use of sunscreens is the 'gold standard' for protecting the skin from ultraviolet light. Octyl methoxycinnamate (OMC) is one of the most widely used UVB filter but it can act as a sensitizer or photoallergen. When exposed to sunlight, OMC can change from the primary trans-form to cis-form and the isomerization, not reversible, conducts to a reduction of the UVB filtering efficiency because the trans-form has a higher extinction coefficient. Photostability is the most important characteristic of effective sunscreens and it can be influenced by formulation ingredients and by applying technological strategies. In this work, photostability experiments, performed on emulsion-gels containing different percentages of OMC free or loaded in poly(D,L-lactide) nanoparticles, were carried out. The presence of a polymeric envelop may act to protect the active ingredient. In this study, the influence of poly(D,L-lactide) matrices on the photochemical stability of the sunscreen agent was investigated. As highlighted in this study, free OMC in different formulations has different photoisomerization degree. Moreover, a dissimilar behaviour was observed by studying different sunscreen concentrations in the same cosmetic formulation. Photostability results show a significant reduction in photoisomerization degree for formulations containing sunscreen loaded in nanoparticles, highlighting that the encapsulation is a suitable strategy to improve OMC photostability. Moreover, sun protection factor (SPF) results show that the UVB filter protective power is also maintained after encapsulation.     

Quartz plates for determining sun protection in vitro and testing photostability of commercial sunscreens

Testing plate made of optical quartz has been developed for the purpose of determination of sun protection factor (SPF) _in vitro by the method of diffusion transmission spectroscopy; the plates were coarsened by sanding and grinding to surface roughness values (Ra) of 18 μm. The plate was coated with a film of sunscreen by an application of 2 mg cm⁻² as that used for determination of SPF _in vivo by the COLIPA method. The transmission values measured were converted into the SPF _in vitro and the protection factor in ultraviolet A light, UVAPF _{in vitro} . The testing plate was tested with commercial sunscreens. The found values of SPF _in vitro fit well with the values determined by means of the COLIPA method in vivo . The plates coated with sunscreen film were irradiated with light simulating the sun radiation. The values of protection factors obtained before and after irradiation were compared, and the differences were used for estimation of photostability of the UV filters included.     

Research on a natural sunscreen from Chinese herbs

To compare the difference in SPF values between a synthetic sunscreen and a sunscreen derived from Chinese herbs, by using optical and biological methods, we observed the effects of these sunscreens by photochemical reaction, sun protection factor (SPF) values, and UV absorption. The results showed that the synthetic sunscreen was more effective in UV absorption than the sunscreen derived from natural Chinese herbs. When they were tested biologically, it was found that the sunscreen derived from Chinese herbs was able to improve skin tonicity, as well as reduce the itching, photoerythema, squamation, burning, and reddening caused by the excessive UV radiation, while the synthetic sunscreen could not improve these conditions of the skin, and even induced some side-effects in the skin with photoerythema. By comparing the in vivo SPF values of the two types of sunscreen, they were found to be mainly similar. The results indicate that the SPF value was related to both a photo-reaction and a biological reaction. The sunscreen derived from natural Chinese herbs, although having low UV absorption, was able to exhibit the same SPF value as the synthetic sunscreen because of its biological effects.     

Preliminary studies towards utilization of various plant extracts as antisolar agents

The aim of this work was to evaluate several plant extracts with regard to ultraviolet absorption spectra in view of a possible application as antisolar agents. Liquid and dry extracts of Hamamelis virginiana, Matricaria recutita, Aesculus hippocastanum, Rhamnus purshiana and Cinnamomum zeylanicum were prepared by repercolation, maceration and microwave oven extraction. UVB absorption spectra (290–320nm) were obtained and the solar protection factors (SPF) of these preparations were determined by a spectrophotometric method. The results showed that after incorporation to a 2% solution of the synthetic sunscreen octylmethoxycinnamate, the extracts showed an intensification in SPF values, suggesting that this can be an interesting method to intensify SPF. In addition, these extracts can contribute their emollient and moistening properties to the product. These are important characteristics for protecting skin against exposure to the sun.     

Adaptation of the protocol for determining in vitro the sun protection factor of anti-solar sticks

Apart from the protection offered by clothing, the application of sunscreen products suited to each type of skin constitutes one way for decreasing the frequency of skin cancers nowadays. After having adapted an in vitro method for determining the efficacy of sunscreens in emulsion form, we wished to transpose this technique by adapting it for the anti-solar sticks for the evaluation of sun protection factor (SPF) using a spectrophotometer equipped with an integrating sphere. To do this, we tested 14 products in the market as well as sticks that we ourselves fabricated in the laboratory. In a base common to all of these sticks, we added organic (13 filters tested) and inorganic (two filters tested, titanium dioxide and zinc oxide) to their maximum permitted concentration in the European Union. In parallel, emulsions containing the same filters at the same percentage of use were studied; to be in keeping with the results on the products packaging on the one hand, and with the results obtained for the emulsion form on the other hand, we were able to determine the optimal mass which needed to be placed on the support used the in vitro test to determine the SPF.     

New data on skin photoprotection

Absorbed ultraviolet (UV) radiation is responsible for photochemical degradation of the skin. Erythema is the most apparent result of the sunburn reaction, but the most serious results are photoaging and photocarcinogenesis, largely mediated by reactive oxygen species (ROS). To prevent these light-induced reactions, effective protection against both UVB and UVA radiations are essential. The aim of this study was to evaluate the protective efficacy against both UVB and UVA rays of three sunscreen preparations (two creams and one lotion), evaluating also their water resistance. For this purpose SPF (Sun Protection Factor) and UVA protection were tested, respectively, on 40 and 20 pre-selected volunteers (male and female, < 22-years-old with skin types I, II and III) by the Colipa test method and the Mark et al. methodology. A wash-off test was also conducted to determine the resistance to water. Results obtained showed that the three formulations studied underline interesting SPF values from about 20-30 with an excellent correlation between visual and colorimetric results obtained (P < 0.05). The controlled decrease in effectiveness of only 20% after 10 min washing proved that the vehicle is also an important factor in determining the protective efficacy. From the obtained data, the studied formulations seem to have an high SPF and UVA protection efficacy together with a good water resistance. According to several other studies, it is possible to confirm therefore that the regular use of these sunscreens may certainly reduce the harmful effect caused by long-term exposure to UVB and UVA radiation.     

The adverse consequences of not using sunscreens

The adverse effects of solar ultraviolet radiation (UVR) on normal skin are well established, especially in those with poorly melanized skin. Clinically, these effects may be classified as acute, such as erythema or chronic such as keratinocyte and melanocyte skin cancers. Apart from skin type genetics, clinical responses to solar UVR are dependent on geophysical (e.g., solar intensity) and behavioural factors. The latter are especially important because they may result in ‘solar overload’ with unwanted clinical consequences and ever greater burdens to healthcare systems. Correctly used, sunscreens can mitigate the acute and chronic effects of solar UVR exposure. Laboratory studies also show that sunscreens can inhibit the initial molecular and cellular events that are responsible for clinical outcomes. Despite public health campaigns, global trends continue to show increasing incidence of all types of skin cancer. Large-scale epidemiological studies have shown the benefits of sunscreen use in preventing skin cancer, though it is likely that sunscreen use has not been optimal in such studies. It is evident that without substantial changes in sun-seeking behaviour, sunscreen use is a very important part of the defence against the acute and chronic effects of solar exposure. Ideally, sunscreens should be able to provide the level of protection that reduces the risk of skin cancer in susceptible skin types to that observed in heavily melanized skin.     

In vitro evaluation of sun protection factors of sunscreen agents using a novel UV spectrophotometric technique

A method for the in vitro determination of low- and high-value sun protection factors (SPF) of sunscreens using artificial substrates and a novel pseudo double beam (PDB) mode of operation of a standard double beam UV spectrophotometer is described. The method allows transmittance to be calculated from detector responses of reference and sample beams measured at different gain levels and facilitates the accurate quantification of low levels of electromagnetic radiation transmitted through highly absorbing samples. The spectrophotometer was modified to hold quartz diffusing plates on which a substrate [Transpore™ adhesive tape or human stratum corneum obtained from a skin surface biopsy (SSB)] and the sunscreens to be tested were applied. The PDB mode of operation increased the effective linear range of the detector response of the spectrophotometer by a factor of approximately 20000-fold, enabling the in vitro SPF determination technique to be applied to both high and low SPF value sunscreens. Eight commercial sunscreens with known SPF values ranging from 4 to 77, previously determined by in vivo methods, were tested in vitro using both test substrates and correlations between the in vivo and in vitro values were determined. SPF values determined using the in vitro method correlated well with the known in vivo results (Transpore™ tape, R ² = 0.611; SSB, R ² = 0.7928). The in vitro SPF obtained for one of the tested products differed substantially from the cited in vivo SPF value. Independent in vitro and in vivo re-evaluation of the SPF of this product matched the value predicted by the present method much more closely than the originally cited in vivo value. All determined SPFs were ordered correctly in comparison to in vivo ranking and the technique appeared to correctly identify a sunscreen that had a labelled SPF value that was significantly higher than its true SPF.     

In vitro assessment of water resistance of sun care products: a reproducible and optimized in vitro test method

The aim of the study was to develop a simple reproducible and reliable in vitro water resistance (WR) method to assess the sun care products. This paper is the result of a scientific collaboration between seven different international industrial laboratories and testing institutes. The same group has already achieved an in vitro protocol for the sun protection factor (SPF) determination [1]. The in vitro WR of sunscreens was tested by applying the same principle as in vivo, which determines the percentage of retention of sunscreen products by assessing the SPF before and after water immersion. Special care was taken to study the parameters influencing the WR and the possibility to follow the kinetics of sunscreen retention during water immersion. The influence of different water qualities has been tested, and osmosed water (1-3 microS cm(-1)) was chosen for the main ring study. Measurement was carried out after 5, 20 and 40 min of immersion. Histograms of selected products demonstrate the percentage of WR at all measuring times and centres, and the regression coefficient to the in vivo determination was shown and statistical calculations clearly demonstrate the reproducibility of the results between the different evaluation centres. The presented method is a practical, convenient and relevant tool for WR screening of sun care and skin care products. It even has the potential to be the starting point for the replacement of the in vivo method in future.     

Beyond UV radiation: A skin under challenge

Since ancient times, human beings have been trying to protect their skin against the adverse effects of the sun. From the first mineral sunscreens used by Egyptians, to the current more sophisticated ultraviolet (UVA/UVB) organic sunscreens, progress has been made in terms of sun protection and deeper knowledge of skin physiology has been acquired in the process. The solar spectrum is composed of radiations of various wavelengths having specific, as well as overlapping effects on skin. UVB is mainly responsible for sunburn and DNA dimer formation that can lead to mutation. UVA generates oxidative reactions affecting DNA, proteins and lipids, and is also immunosuppressive. Recently, visible light and infrared radiation (IR) have been associated with oxidative damage and IR has been additionally linked to adverse heat effects on skin. Numerous other extrinsic factors, related to environment and lifestyle, also affect the appearance of skin, precipitating ageing. New molecular mechanisms linking sun and environmental factors to skin ageing have been identified: IR affects mitochondrial integrity and specific heat receptors also mediate some of its effects, tryptophan is a chromophore for UVB, and the aryl hydrocarbon receptor (AhR) is activated by light and xenobiotics to alter skin physiology. Integrating all these new elements is changing the way we think about skin extrinsic ageing. Is UVA/UVB sunscreen protection still enough for our skin?     

Nanostructured lipid carriers as novel carrier for sunscreen formulations

Incorporation of sunscreens into lipid carriers with an increased sun protection factor (SPF) has not yet been fully accomplished. In the present paper, the effectiveness of a sunscreen mixture, incorporated into the novel topical delivery systems, i.e. solid lipid nanoparticles (SLN) and nanostructured lipid carriers (NLC), used as ultraviolet (UV) protector enhancers with a distinctly higher loading capacity has been developed and evaluated. SLN and NLC were produced by hot high pressure homogenization technique in lab scale production. Size distribution and storage stability of formulations were investigated by laser diffractometry and photon correlation spectroscopy. Nanoparticles were characterized by their melting and recrystallization behaviour recorded by differential scanning calorimetry. Lipid nanoparticles produced with a solid matrix (SLN and NLC) were established as a UV protection system. The loading capacities for molecular sunscreens reported before now were in the range of 10-15%. It was possible to load NLC with up to 70% with molecular sunscreen, which is appropriate to obtain high SPFs with this novel UV protection system. The developed formulations provide a beneficial alternative to conventional sunscreen formulations. The UV protective efficacy of the lipid particles varied with the nature of lipid and UV wavelength.     

Predicting the efficacy of sunscreens in vivo veritas

A novel in vitro technique for measuring the efficacy of sunscreens over the skin surface is described. It is demonstrated that those products that spread easily are associated with both a low variance in delivered SPF and a subjective assessment of a product that is pleasing to use, which in turn results in a higher application thicknesses and greater delivered photoprotection leading to improved health benefits of sunscreen use.     

Determination of sun protection factors. Correlation between in vivo human studies and an in vitro skin cast method

Sun protection factors were determined by both an in vitro method which used resin casts taken from replicas of human skin and by an in vivo SPF method. Thirty-eight product development samples were tested for the level of sun protection using both methods and the results were compared. The values obtained showed a positive relationship which was closely approximated by a log-linear regression of in vivo data on in vitro data (regression coefficient, r² = 0.86).
It is concluded that the cast technique is quick, convenient, inexpensive and in its present form useful for screening sunscreen products prior to in vivo SPF testing.     

A National Web-Based Survey of Sunscreen Products as a Tool for Industry Self-Regulation, Consumer Awareness Campaigns and Marketing

The objective of the work was to test a web-based tool in the collection and presentation of data for our regular sunscreen surveys and to develop, test and conduct a web-based survey of sunscreen products, with instant data reporting. After the Photobiology Laboratory at MEDUNSA was approached by SurveyIT to pilot a new web-based data collection and reporting tool, companies were contacted via e-mail and invited to visit a secure web-based site to complete the provided questionnaire. Data collection and processing were greatly simplified by the new approach. The survey gleaned responses from 18 companies on 31 brands with a total of 110 primary sunscreen products. The data revealed a reduction in the number of companies marketing sunscreens in South Africa compared with the previous year, despite an increase in the volume of sunscreen sales. Ninety-eight percent of the products were claimed to be in vivo SPF tested. Fifty-seven percent of the products had undergone in vitro SPF testing for determination of their UVA protection. Fifty-one percent of the products were claimed to be water-resistant. Octyl methoxycinnamate was the most popular UV filter (88% of products), followed by benzophenone-3 (60%). Butyl methoxydibenzoylmethane was in 52% of the products and surprisingly only 29% of the products contained titanium dioxide. The major conclusions are that the data collection tool was a success and that sunscreens are becoming more widely used and more thoroughly tested in South Africa.     

The COLIPA in vitro UVA method: a standard and reproducible measure of sunscreen UVA protection

There is a continuing need to measure and communicate reliably the UVA protection offered by commercial sunscreens. To that end, the COLIPA (European Cosmetics Trade Association) ' In Vitro Sun Protection Methods' group has developed a new in vitro method for measuring UVA protection in a standardized, reproducible manner. The method is based on in vitro UV substrate spectrophotometry and convolution of resulting absorbance data with the action spectrum for the in vivo Persistent Pigment Darkening (PPD) endpoint to provide an in vitro UVA protection factor (UVAPF) which is correlated with an in vivo measure. This method has been published as a COLIPA guideline, used currently in European geographies for testing and labelling sunscreen products.
This article summarizes two 'ring' studies, involving eight separate testing laboratories, which both defined critical parameters for the method and validated it. In Ring Study 1, eight laboratories tested the in vitro UV transmission of a total of 24 sunscreens and, from the data, a unit dose of UVA ( D ₀ of 1.2 J cm⁻²) was defined to provide a single irradiation step which, by taking into account potential sunscreen photo-instability, gave the closest agreement with in vivo UVAPF values. In Ring Study 2, eight laboratories tested the in vitro UV transmission of a total of 13 sunscreens using this single irradiation step and established a very good correlation ( r ² = 0.83; slope = 0.84, P  < 0.0001) between resulting in vitro UVAPF values and corresponding values derived from the in vivo PPD method. This new method, therefore, can be used to provide a reliable in vitro metric to describe and label UVA efficacy in sunscreen products, in line with the EU Commission recommendation 2006/247/EC.     

Consequences for sun protection factors when solar simulator spectra deviate from the spectrum of the sun

The sun protection factor (SPF) of two products, one with an expected SPF of 4 and another with an expected SPF of 15 were determined, using two solar simulators: Multiport Solar UV Simulator (xenon, Solar Light, Philadelphia, PA, USA), and Supersun 5000 (metal halide, Mutzhas, Munich, Germany). The mean SPFs using the Multiport were: 4.8 for the low SPF product and 19.4 for the high SPF one. The results using the Supersun were lower: 2.6 for the low SPF product and 7.2 for the high SPF one. Relative emission spectra of the two sources were recorded using a fluorescence spectrophotometer in bioluminescence mode. Efficacy spectra were calculated and compared with the corresponding spectrum of natural sunlight. It was evident that the spectral power of the xenon source is too high in the UVB, leading to overestimation of SPFs, whereas the Supersun irradiator emits too much in the UVA, resulting in too low SPFs. Heat effects and photodegradation of UV filters are discussed as further possible reasons for the discrepancies between the experimentally determined SPFs. Our results confirm a recent publication about theoretical SPFs, calculated with emission spectra of a xenon source and spectra of the sun at different elevation angles, where the authors provide evidence that in natural sunlight the contribution of UVA to total UV radiation is twice as high as in a xenon source. This may contribute to an understanding of why sunscreens tested according to the FDA method (xenon sources) often yield higher SPFs than those obtained from European testing procedures.     

Contact angle measurement - a reliable supportive method for screening water-resistance of ultraviolet-protecting products in vivo

Substantivity of sunscreen formulations is affected by the wash-out rate of ultraviolet-absorber and -reflector compounds in water. Water-resistance of sunscreen formulations is currently determined according to a standardized European Cosmetic Toiletry and Perfumery Association (COLIPA) protocol, encompassing the determination of a minimal erythemal dose before and after a defined immersion step in water. It can be supposed that the higher the wettability of a treated skin area, the higher is the wash-out rate of sunscreen compounds. This present report addresses the validity of determining the wettability of treated skin alone as a measure for the water-resistance of sunscreen products. The report addresses the robustness, accuracy and congruence of a recently developed wettability test, based on the measurement of the contact angle (CA) of a sessile water drop on treated skin areas. Contact angle data of 66 sunscreen formulations are compared with the corresponding results of 81 water-resistance tests, using the sun protection factor (SPF)/immersion/SPF method. Sunscreen products tested by the CA method were applied to the skin of the volar forearm of test subjects at a defined dose and drying-time, using a standardized application and recording device. Contact angles between a sessile water drop and skin were recorded by a Charge-Coupled Device (CCD) camera and subjected to automatic contour analysis. Taking the SPF/immersion/SPF method as gold standard, accuracy parameters of the CA method were determined. By using an appropriate cut-off level of CAs, the CA method has a specificity and positive-predictive value of 100%, and turns out to be a reliable screening method to identify water-resistant formulations. Based on our findings, those formulations that give CAs above 30 degrees may be categorized water-proof without further testing by the COLIPA water-resistance method.     

In vitro testing of zinc oxide sunscreens

The UVA performances of two all-mineral zinc oxide sunscreens are measured following Colipa and ISO procedures and compared to a sunscreen containing only organic actives. It is found that the two sunscreen types yield very different in vitro SPF and UVA results. It shown that the results can be rationalized in terms of the differences in the monochromatic extinction spectra of the two types of sunscreens.     

Determination of sun protection factors in the hairless mouse

A laboratory method for determining sun protection factors (SPF) in the hairless mouse has been developed. The ability of the method to predict the SPF in man has been investigated using sunscreen ingredients and finished products with known activities in man. The results indicate that the hairless mouse can be used to assess sunscreen products although the SPF found appeared slightly higher than that determined in man.
Determination des facteurs de protection solaire sur la souris sans poils