Volume XIX - Nr.2 - November 2004

Preliminary tests to assess the effectiveness of sunscreens

Is sunbathing, and for that matter the sun itself, beneficial or harmful? The answers given by both experts and non experts seem to vary depending on who you talk to.

Psychological research carried out in recent years shows that a person's (theoretical) behaviour when faced with certain problems and his/her actual behaviour are not always coherent. The relationship between man and sun was analyzed starting from empirical data, by taking into consideration three levels of observation: knowledge, behaviour , and psychology.

The results of surveys that were carried out in Austria, Germany, France, and Australia show that human behaviour with regards to the sun is not controlled by one factor alone, but by several factors. In some climates, such as the summer climate in Perth, the effects of high temperatures, strong sun shine, low relative humidity, and strong winds on people who are exposed to the sun for several hours at a time because of their jobs can be catastrophic.

The skin's pigmentary reaction to UV rays increases and some signs of degradation appear. These include, dehydration of the horny layer, thickening of the dermal tissue, dilatation of the dermal pores, while the elderly also show uneven pigmentation of the hands, face, and neck.

The effects of UV rays are worsened by infra-red radiation, which increases the temperature of the cutis and of perspiration, thus leading to greater transpiration of the skin itself. This is why effective protection requires a combined, integrated, and simultaneous action against both the harmful effects of UV radiation, especially after swimming or while perspiring, and against the dehydrating effects of heat, humidity, and wind.

The sunscreen index, which is one of the few existing cosmetic parameters, helps consumers who are more interested in protecting themselves than in tanning quickly, to choose the most suitable product. The positive attitude of the cosmetic industry has made it possible to define the concept of the protection index and to introduce this concept into the selection criteria.

However, differing interpretations arose concerning this subject, which then led to investigating the sources of error and suggesting new measurement methods. The problems that had to be tackled in the attempt to define the protection index involve, first of all, the choice of the skin sampling area (the size and the sensitivity of the cutis, which varies depending on part of the body); secondly, the sunscreen film thickness, irradiation sources, and evaluation of the results; these experimental conditions can be perfectly controlled. Relative humidity and temperature are two key factors that must be taken into consideration when performing laboratory tests.

With regards to practical applications, how well the product being tested resists to perspiration or water, based on quantitative variations must be determined.

The ideal way to carry out this type of assessment would be to test the products on individuals who would alternate physical activity with swimming in water whose chemical-physical characteristics are known, and then to make macroscopic observations and take photometric readings.

Though not clearly proved, we can assume that the efficacy of tanning products closely depends on how thick the layer of film applied to the horny layer is, since penetration of the sun screen into the skin reduces its protective efficacy.

A method has been developed to determine the quantity of UV screen that is absorbed by the cutis in the application area. The residue that remains on the surface of the horny layer after various application periods was determined by a solvent recovery technique.

The results showed that various sunscreen types (aminobenzoates, cinnamates, salicylates) penetrate the skin and that the depth of penetration may vary depending on where the sunscreen is applied, the application time, and the composition of the “carrier”. In order to better understand the mechanisms the sun deploys on the cutis, an electronic paramagnetic resonance (EPR) study was carried out on the free radicals that are formed by irradiation of the proteinaceous components of the cutis.

After performing a study on models, peptides and polypeptides that was presented at the X IFSCC Congress in London, a further study was conducted on the keratin of the cutis horny layer and on collagen. With regards to keratin, UV rays cause the production of radicals such as “-NH-CH-R”, which react with the glycine content of adjacent proteinaceous chains, thus forming radicals -NH-CH-CO3; while, for collagen, besides -NH-CH-CO3, free radicals such as alanine radicals (-NH-CH-CH3) at -160ºC and (-NH-C(CH3)-CO-) radicals at -20ºC, can be observed.

These results help us to better understand both the mechanisms leading to early ageing of the cutis when exposed to the sun, and the transformation of collagen into reticulate proteins which D.A. Hall called “false elastins”.

The study that was conducted on the free radicals which are formed in the horny layer keratin also allows for an in-depth investigation of the protective role played by the melanin-based pigments of the cutis. As a preliminary measurement of UV screen affinity with the horny layer, it is possible to determine screen absorption on sheep's wool. It is also possible to measure screen affinity on pig skin by removing the cutis some time after having treated and washed it with water, and then measuring the remaining sun screen residue.

According to Ippen, it is also possible to obtain “in vivo” laboratory test models by determining the sunscreen index before and after washing with water. Tangible results can be obtained by tests performed on humans exposed to the sun by means of the Fitzpatrick, Pathak, and Greiter methods.

Generally speaking, the protection index is measured at intervals of several hours with one or two 15-20 minute swims in between. These methods have shown that water/oil emulsions usually withstand washing better than oil/water emulsions do, although the opposite may occur in some cases. It may also be assumed that UV rays and product application times may affect the screen's affinity with the horny layer.

Another method, which is based on high-performance liquid chromatography, has been developed o analyze optical isomer traces of urocanic acid (UCA). The influence that age, gender, skin area the sample is taken from, and the season have on the quantity of UCA and on the optical (cis-trans) isomer ratio found on the cutis surface was studied. The results showed that: the total amount of UCA is not related to age or gender; it decreases in summer, while the cis isomer ratio increases; the cis-isomer ratio is higher on the forearms and cheeks, while trans isomers prevail on the back.

It has been observed that photochemical irradiation causes cis-trans isomerization both ways. These reactions are quick in water and the equilibrium constant varies depending on the intensity of the light. The protection index of a watery solution of sodium urocanate that is photochemically isomerized is almost equal to the protection index of a trans isomer solution.

These results lead us to believe that both isomers act as screens and that the photoprotective effect is not only caused by isomerization energy but also by photoabsorption. This opinion is supported by the fact that 4-5imidazolyl-methylidene monosodium malonate, which is not photochemically isomerized, has a screening effect.

Therefore, we believe that the amount of UCA in the cutis, the thickness of the horny layer, and the melanin content of the epidermis are all important factors for the sunscreen protection effect.



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