Evaluation of a sunscreen during a typical beach period.

PURPOSE: Amongst the radiations reaching the Earth's surface, the ultraviolet rays are the ones receiving most attention from the scientists, given their damaging potential for humans exposed to them. To minimize the harm caused by such exposure, human beings are strongly recommended to use sunscreens, which are pharmaceutical preparations containing filters that confer protection against radiation. As this protection is strongly dependent on the properties of these filters, it is very important to ensure their stability even when under aggressive conditions, such as the typical high temperatures of summer in South Europe. In this study, a commercial sunscreen emulsion was tested in vitro for a period of time intended to simulate a beach period of 15 days, with regard to the maintenance of its sun protection factor (SPF). Moreover, the organoleptic characteristics were also monitored by macroscopic analysis.
MATERIALS AND METHODS: To perform this study, temperature conditions similar to those observed from June to August in Faro (Portugal) were simulated in vitro. The SPF was determined by spectrophotometry, with subsequent application of the Mansur equation. RESULTS AND
CONCLUSION: No significant alterations were observed during the considered period under the specific conditions of this study.

The need of using a sunscreen to protect humans from the damaging solar radiation and, particularly, the ultraviolet (UV) rays, is now an irrefutable reality, which has resulted from the gradual depletion of the protective ozone layer.[1] Among the two UV categories reaching the Earth (UVA: 320–400 nm; UVB: 290–320 nm), UVB is generally considered the most deleterious, being responsible for skin burn though it penetrates only the outer skin layers, because it has higher energy.[23] It is important to mention that UVB exposure is associated with benefits such as in vitamin D metabolism, though the harmful effects stand out for their severity causing erythema (sunburn), cutaneous inflammation and skin cancer.[14] To confer protection from radiation, the skin has natural mechanisms such as sweat secretion, melanin production and the proper layer of stratus corneum.[1] However, when there is an excessive sun exposure, the body may not be able to completely neutralize the generated free radicals.[5] In fact, chronic photodamage may lead to photoaging and photocarcinogenesis[6] and the application of sunscreens is, nowadays, mandatory to minimize these harmful effects.[57]

Sunscreens are formulations for topical application, classified as cosmetics, which contain one or more of the 27 UV filters allowed in Europe.[89] These filters absorb or reflect radiation and, as they usually are selective for a determined band region, the best approach for providing adequate protection relies on the combination of filters,[2] thus being usual that the same formulation has various UV filters, either organic or inorganic. Their application is strongly advised by World Health Organization (WHO), recognizing the advantage of protecting skin from damaging radiation, avoiding skin irritation.[10] The efficacy of sunscreens is measured in terms of their capacity to prevent erythema, which is expressed by the sun protection factor (SPF), considered as a universal indicator.[711] The SPF was created in 1956 by Schulze and it reflects the ratio between the lower amount of UV energy required to produce a minimal erythema on sunscreen protected skin and the amount of energy required to produce the same erythema on unprotected skin.[111] Officially, the SPF has to be determined in vivo in human volunteers, but the in vitro determination by spectrophotometry is widely accepted for preformulation studies, as it is simple and fast, further reducing the risks associated with in vivo tests. Nevertheless, in vitro tests do not replace in vivo determination, because they do not consider any interaction with the skin, but a good correlation between results obtained from both the methods has been demonstrated.[12] In vitro determination of the SPF usually involves the application of the equation developed by Mansur et al.[12] which is often reported as accurate, showing good correlation with the in vivo results.[113] It has been reported that inorganic filters, such as zinc oxide and titanium dioxide, hamper the application of spectrophotometry, because they are not absorbents, thus limiting SPF determination by this technique. Moreover, the use of unsuitable solvents, as well as the measurement of absorbance on sunscreens of high SPF (>15) is not appropriate and might lead to wrong results.[1414]

If one considers that sunscreens are usually directly exposed to the sun on their own package, for instance when the users are in the beach, it becomes logical to wonder about their stability under these conditions. In fact, the stability of these cosmetics and, consequently their efficacy, might be affected by a prolonged sun exposure because temperature is known to affect their properties and effectiveness, especially if the formulation is an emulsion.[15]

In this study, we aim to evaluate if the SPF of a commercial sunscreen remains unaltered in a simulated beach period of 2 weeks. For this purpose, the sunscreen was submitted in vitro and in its proper package to different temperatures typically occurring in Faro (Portugal) during the summer, to simulate sun exposure, and its SPF was monitored. Furthermore, some organoleptic characteristics were also evaluated.

Materials and Methods

Materials

The sunscreen used here was an emulsion with an SPF of 8, formulated with a combination of organic filters (UVB: ethylhexyl methoxycinnamate, 4-methylbenzylidene camphor and phenylbenzimidazole sulfonic acid; UVA: butyl methoxy dibenzoylmethane). The selection of this formulation was based on the fact that emulsions are the most used formulations and due to the need to have a formulation with an SPF that allowed its monitoring by spectrophotometry (SPF< 15). Distilled water was used throughout.

Definition of the experimental conditions

The experimental temperatures used to simulate the beach period corresponded to room temperature in summer (25°C), maximum average temperature (29°C) and an extreme temperature (40°C). The maximum average temperature was calculated from data provided by the Portuguese National Institute of Meteorology and corresponded with the maximum average temperature registered in Faro between June and August of 2003–2007.[16] The extreme temperature was used to simulate accelerated studies.

Three experimental groups were defined, corresponding to each of the different temperatures, and samples were exposed in vitro to the defined temperatures, in their proper package, using an incubator (P-Selecta®, Barcelona, Spain). Groups of maximum average and extreme temperatures (29 and 40°C, respectively) were exposed to these conditions for a period of 5 h for predetermined days (1, 2, 5, 6, 7, 8, 9, 12, 13, 14 and 15), staying at 25°C in the remaining time. The rational of selecting these days was based on the consideration that, frequently, people on beach holidays do not go to the beach everyday and instead perform other activities on some days of the period. Experimental group corresponding to room temperature (25°C) remained under this temperature for the whole study (15 days).

Evaluation of sunscreen organoleptic characteristics and other relevant parameters

The evaluation of organoleptic characteristics was performed on all samples of each experimental group, on days 1, 2, 5, 8, 12 and 15, on the basis of macroscopic observation, which was used to indicate formulation stability. Observed parameters included coloration, smell and texture.[1] Furthermore, consistency and homogeneity were also evaluated.

Evaluation of the sun protection factor

SPF was determined in the samples in which the previous assay of organoleptic characteristics’ evaluation was performed, as well on days 1, 2, 5, 8, 12 and 15. To do so, samples were prepared according to the method proposed by Dutra et al. which was adopted for our study.[14] In this manner, 0.5 g of each sample was mixed with an appropriate amount of distilled water to obtain a final concentration of 0.2 × 10−4 g/ml. Briefly, samples were dispersed in 100 ml of distilled water and were homogenized by ultrasonication (Ultrasons-H®, P selecta, Spain), during 5 min. The obtained dispersion was filtered with filter paper and the first 10 ml rejected. Then 2 ml of filtered solution was made to 50 ml using distilled water. The absorbance of each sample was determined by spectrophotometry (UV–visible spectrophotometer, Pharmaspec UV-1700®, Shimadzu, Japan) in the range of 290–320 nm (UVB), with 5 nm intervals, using distilled water as blank. A fresh sunscreen sample (not submitted to temperature effect) was used as control, in order to establish initial SPF. Three replicates of each group were performed.

The SPF of each sample was determined with the data obtained by spectrophotometric analysis, using the Mansur equation:

Where CF is a correction factor (equals 10); EE(λ) is the eritematogenic effect of radiation at wavelength λ; I(λ) is the intensity of sun light at wavelength λ; and Abs(λ) is the absorbance measured at wavelength λ. Values of EE × I are constants which were previously determined and reported.[12]

Statistical analysis

Mann–Whitney test was performed to compare the results obtained. All analyses were run using the SPSS statistical program (SPSS 16.0 for Windows, Microsoft Corporation, USA) and differences were considered to be significant at a level of P< 0.05.

Results and Discussion

This study proposes the evaluation of SPF alterations of a commercial sunscreen when used in a typical beach period of 2 weeks, which was simulated in vitro. Further analysis relied on observation of organoleptic characteristics of the sunscreen as well as other relevant parameters.

Evaluation of organoleptic characteristics and other relevant parameters

Observation of organoleptic characteristics revealed that upon 15 days of exposure to the different assayed temperatures (room T: 25°C; maximum average T: 29°C and extreme T: 40°C), no alterations were registered (data not shown). In fact, all the samples maintained a homogeneous aspect, being white colored and evidencing the characteristic fragrance of the formulation, as perceived for the control. Texture and consistency also remained unaltered. The literature describes few studies addressing evaluation of organoleptic characteristics, in simulated temperature conditions. Nevertheless, these results are consistent with those observed by Deccache, who described an absence of organoleptic alterations in a sunscreen gel formulation which was exposed to temperatures of 25 and 40°C for 15 days.[15] Therefore, results indicate that in a beach period of 2 weeks with temperatures similar to those simulated in this study, no alterations would be perceived by the user, since organoleptic characteristics remain unaltered.

The SPF of each sample was calculated in predetermined days by applying Mansur equation.[12] Figure 1 represents the variation of SPF of the sunscreen emulsion determined upon exposure to different temperatures (25, 29 and 40°C) during the course of study (15 days), simulating in vitro the conditions of a beach period. An initial SPF determination was performed in a fresh sample of sunscreen (prior to any temperature exposure), which was considered to correspond to 100% SPF.

Figure 1

Sun protection factor (SPF) variation during sunscreen incubation under different conditions: (O) room temperature, 25°C; (O) maximum average temperature, 29°C and (Δ) extreme temperature, 40°C. Dotted line represents 100%. The sunscreen is tested inside the package

As can be seen, generally, SPF values remained stable throughout the whole period of study. However, when the sunscreen was exposed to the temperature of 25°C, upon 24 h, a slight decrease of approximately 5% in SPF was identified (P< 0.05), as compared to the initial SPF value. A similar SPF reduction (4.2%) was perceived in the group of 29°C when comparing initial SPF with that measured on day 15 (P< 0.05). Nevertheless, in spite of the statistical significance of the values, these determinations do not compromise the general trend of results, which indicate the maintenance of SPF.

The SPF variation for each experimental group (25, 29 and 40°C), upon 15 days of exposition is depicted in Figure 2. Displayed values were obtained by comparison with the fresh sample not subjected to temperature effect, assumed as 100%. It is thus demonstrated that, generally, final SPF does not display accentuated alterations either when comparing the result of the experimental groups with the initial SPF or when comparing experimental groups themselves. An exception occurs for the maximum average temperature as compared to the initial SPF value, as previously referred, which is significant (P< 0.05) but not compromising.

Figure 2

Sun protection factor (SPF) variation of sunscreen upon fifteen days exposition to different experimental conditions, as compared to control (fresh sample): 25°C - room temperature; 29°C - maximum average temperature; and 40°C - extreme temperature. Dotted line represents 100%. The sunscreen is tested inside the package

Although there are many studies concerning the determination of SPF in sunscreen of various semisolid dosage forms (milk, lotion, cream), most of them do not address the issue of their behavior when packages are exposed to the effect of high temperatures. Results obtained in this work are in agreement with those reported by Deccache, who described that a sunscreen in the form of gel did not exhibit significant SPF variations during a similar period (15 days) either at 25°C or at 40°C.[15] Nevertheless, attention should be paid to the fact that the formulation is not the same; our work studies an emulsion, whereas this author used a gel.

The overall analysis of these results indicates that the studied emulsion sunscreen maintains its SPF and, thus, its protecting effect against UV radiation, upon exposure to usual temperatures observed in Faro (25 and 29°C) during summer. Protection would also be granted under high temperatures (40°C), as demonstrated in the accelerated studies, although this is not representative of a typical reality. In fact, generally, no significant alterations are observed in either organoleptic characteristics or SPF for a 15-day period. The narrow size of the sample used could be a limitation of this study and, in addition, it is important to highlight that other properties of the formulation, such as pH and viscosity, are essential to evaluate the sunscreen stability and to ensure the effective protection of individuals. Thus, further assays should be conducted to fully characterize the formulation behavior. Moreover, although according to the literature there is a good correlation between spectrophotometric and in vivo analyses,[13] the performance of in vivo studies should be considered, since the interaction of the formulation with the human skin is not addressed in in vitro tests.