Abstract
The requirements on sunscreens have essentially changed, since some years ago it was demonstrated that approximately 50% of free radicals, that are formed in the skin by solar radiation, originate from the visible and infrared regions of the solar spectrum. In addition, a critical radical concentration threshold could be found. If this concentration, the free radical threshold value (FRTV), is exceeded, sunburn, immunosuppression and skin cancer may develop. Application of sunscreens and lotions protects against sunburn in the UV region of the solar spectrum and therefore is frequently used to extend people's stay in the sun. However, this behaviour can enhance the concentration of free radicals formed in the visible and infrared regions of the solar spectrum, so that the critical radical threshold is exceeded and the skin may be damaged.
- Sun radiation
- skin protection
- sunscreens
- free radicals
- free radical threshold value
- antioxidants
- visible and infrared light
- review
The sun is of utmost importance to life on earth. Solar radiation is the basis of vitamin D production in the human body, and the sun light is considered a source of well-being (1). However, if the skin is exposed to excessive doses of solar radiation, immediate negative consequences may follow, e.g., sunburn and immunosuppression (2). In the long run, premature skin aging and skin cancer can develop (3-6). Thus, sunscreens play a significant role as people are changing their leisure patterns, now exposing themselves longer to solar radiation than in the past (7).
Sunscreens are characterized by the sun protection factor (SPF), which, in simple terms, is a measure of how much longer a sunscreen applicant can stay in the sun without sunburning than an unprotected person. However, the problem with sunscreens is that they are frequently applied in insufficient amounts. To determine the SPF of a sunscreen product, 2 mg/cm2 of the respective product must be applied (8). Averaged over all values this means that one tube of sunscreen is sufficient for 5 or 6 days if used by one applicant. In reality, however, a whole family goes on holidays with only one tube of sunscreen, the content of which has often not been fully consumed at the end of the holidays. Experiments carried out by the Charité on real conditions at the beach have shown, that indeed only 20% of the required amount of 2 mg/cm2 of sunscreen is applied (7).
Using sophisticated combinations of filters, in the past decade a main focus of research has been the development of formulations, which create a comfortable sensory feeling, and novel forms of application, e.g. sprays, in order to stimulate the correct application of the products (9).
In light of the rapid advances in knowledge about the nature of the solar radiation's carcinogenetic action in the human skin, the requirements on efficient sunscreens have increased again. The latest findings and their consequences for real progress in the prevention of skin carcinogenesis are presented in this article.
Action Spectrum of the Radical Formation in the Human Skin
In 2009, Zastrow et al. (10) succeeded in determining the radical formation spectrum in the human skin within the region of the solar radiation reaching the surface of the earth. They found that the highest amounts of free radicals are formed in the ultraviolet (UV) spectral region. Surprisingly, their investigations also revealed that approximately 50% of the free radicals are generated in the visible (VIS) and infrared (IR) spectral regions. These findings were recently confirmed by in vivo studies showing a VIS- and IR-induced degradation of cutaneous carotenoid antioxidants (11-13).
Action spectrum of the formation of free radicals (15).
But this means that sunscreen products, which have so far been proven to be highly efficient protectants in the UV spectral region, tempt applicants to expose their skin longer to solar radiation, although these sunscreens provide no protection against the formation of carcinogenetic-free radicals in the visible and infrared spectral regions. The action spectrum of radical formulation is shown in Figure 1.
However, the occurrence of free radicals in biological systems is not detrimental per se, as free radicals are known for assuming signaling functions (14). Zastrow et al. (10) could show that there is a critical radical concentration named free radical threshold value (FRTV). Only if this FRTV is exceeded, the above described adverse events in the skin occur.
Critical Radical Concentration (Free Radical Threshold Value)
The Free Radical Threshold Value was detected by quantitative ESR-x-band spectroscopy as described in detail by Zastrow et al. (15-17). Spintrap PBN and spin label PCA were used to quantify the total amount of free radicals depending on the irradiation dose. Spintraps DMPO and DEPMPO allowed distinguishing free radical oxygen species (ROS) from carbon-centered lipid peroxide radicals (LOS) Figure 2. All measurements were performed on ex vivo human skin biopsies of skin type 2 according to the Fitzpatrick classification (18).
If the human skin is irradiated with steadily increasing UV+VIS doses, free radicals are formed depending on the dose applied. Up to an amount of approximately 3.5×1012 radicals per mg of tissue (10), this increase is almost linear. Subsequently, there is a small surge in the curve, which at even higher UV+VIS doses is the starting point for a reduced increase in the radical concentration. The radical concentration of 3.5×1012 rad/mg is considered to be the critical FRTV.
The same FRTV concentration, will also be obtained by the dose of solar light necessary for the essential vitamin D synthesis, a dose which is considered to be harmless. UV+VIS doses exceeding the value used for FRTV lead to erythema formation, which is characterized by the minimal erythema dose (MED). This process is recognized as being detrimental. By determining the nature of the mixture of free radicals it could be demonstrated that below the critical concentration (FRTV) the mixture is mainly composed of primary free oxygen radicals (ROS) and above the FRTV it is characterized by secondary lipid oxide radicals (LOS). This excess of LOS, which is dominant in the region of already visible clinical damage (erythema), is considered an obvious sign of free radical-induced damage. These findings clearly demonstrate the necessity to reconsider sun protection and develop sunscreen systems, that provide protection also in the visible and infrared regions of the light and thus protection against skin cancer.
Relation between UV+VIS dose and amount of free radicals.
Sun Protection in the Visible and Infrared Spectral Region
Primarily because of their color, filter substances are unsuitable for providing protection against the visible and infrared light. A very promising approach is the stimulation of the natural protective mechanisms of the human skin. These mechanisms are hyperkeratosis, i.e., the thickening of the stratum corneum, which results in a reflection or improved scattering of the incident photons. The melanin production, which leads to tanning of the skin, also intensifies absorption, reflection and scattering. In addition, the natural antioxidative protection system of the skin can be improved, especially with food and the application of creams rich in antioxidants. Thereby, free radicals are directly neutralized even before they start damaging the body.
Protective action of sunscreens against radical formation (17).
Pigments, such as TiO2 and ZnO, which are frequently used as physical filters in sunscreens, also lead to an enhanced reflection and scattering of the sun light. Furthermore, antioxidants, which are added to sunscreens in order to stabilize the UV filter substances, are effective in the visible and infrared spectral regions.
Optical properties of the sunscreens (17).
In order to find out whether or not commercial sunscreens provide protection in the visible and infrared spectral regions, four such products were tested at the Center of Experimental and Applied Cutaneous Physiology of the Charité – Universitätsmedizin Berlin (17). The results are presented in Figure 3.
In these investigations the radical protection factor (RPF) of the four products was determined. This RPF permits conclusions about the efficacy of the antioxidants to be drawn as shown in Table I.
In addition, the optical properties - reflection and scattering – were determined. The results are shown in Figure 4.
While the untreated skin and the Colipa standard No. 3 (19), i.e., a formulation neither containing pigments nor antioxidants, provide no protection, the investigated sunscreens revealed a differentiated protection in the infrared spectral region. In these investigations the skin samples were irradiated with a water-filtered infrared lamp. In the case of product 4, which provides the best protection in the infrared spectral region, the RPF of 40 is a good although not extremely high value. However, this product exhibits very good optical properties. It strongly reflects and scatters the incident solar radiation. Although the RPF of product 3 is clearly higher, its optical properties are noticeably inferior.
Radical protection factors (RPF) mean±SD of the 5 formulations (17).
Consequently, an efficient protection in the infrared spectral region can be provided by varying the pigments and adding them to the sunscreens together with antioxidants. Because of the similar mechanisms of action and the same species of free radicals, that are generated by visible light, these protection mechanisms are also protecting against radical formation in the visible spectral range.
Conclusion
It has been demonstrated that protection against UV radiation alone is not sufficient to prevent deleterious dermal consequences, that may arise if the human skin is exposed to natural solar light too long. This is mainly due to the fact that free radicals are not only formed in the UV spectral region. Over 50 % of these free radicals are generated in the visible and infrared spectral regions. Free radical biochemistry, action spectrum and critical radical concentration (FRTV) are therefore to be considered to a greater extent in the genesis of skin cancer.
All this makes a rapid transition from the currently dominant “UV protection” to a holistic “light protection” indispensable.
Acknowledgements
The Authors would like to thank N. Groth and F. Klein, of the Galenus GmbH, Germany, D. Kockott of UV-Technik, Hanau, Germany, as well as O. Doucet, of Lancaster – Coty International Inc., Monaco, for their technical support and helpful discussions.
- Received January 22, 2016.
- Revision received February 17, 2016.
- Accepted February 22, 2016.
- Copyright© 2016 International Institute of Anticancer Research (Dr. John G. Delinassios), All rights reserved
