## 3 Sunscreens and Filters against UV Radiation and Its Detrimental Effects on the Skin  
In recent years, the incidence of skin damage due to solar radiation has increased. Exposure to UV radiation plays a major role in acute and chronic skin damage, as well as in many skin conditions, such as sunburn, pigmentation, wrinkles, dermatitis, urtic, aging, immune system suppression, and skin cancers (Figure 1). Covering the skin with the use of sunglasses, a hat, or other external clothing are no longer sufficient solutions for sun protection. Therefore, the use of sunscreen is essential. Sunscreen agents protect against the sun, by absorbing ultraviolet and visible sun rays. Solar Ultraviolet Radiation (UVR) is divided into three categories: UVA (320-400 nm), UVB (380-320 nm), and UVC (200-280 nm) [27; 28; 29]. Of the three forms of UVB, UVC is the form with the highest energy. It would therefore be evident that it is the one that causes the most biological damage. However, this is not the case, since it is effectively filtered by the ozone layer [1]. The amount of UVB and UVA radiation reaching the earth is affected by latitude, altitude, season, time of day (10 am-4 pm is strongest), cloud cover, and the ozone layer. UVB is mainly associated with erythema and sunburn, and it cannot be excluded from being able to cause immunosuppression and photocarcinogenesis. During a summer day, the spectrum of UV radiation reaching the earth's surface consists of 3.5% UVB and 96.5% UVA (ratio: 1:20) [1]. UVA radiation is less affected by the conditions mentioned above (latitude, altitude, etc.) because of its longer wavelength, compared to UVB. UVA has been shown to penetrate deeper into the  
Figure 1: Illustrating the negative effects associated with solar radiation. skin, passing through glass and 50% of its exposure is from shaded areas. It is also known to have multiple adverse effects on the skin including immunosuppression, photoaging, eye damage, and skin cancer. But in addition to the damage it causes, it stimulates the production of vitamin D3 through the irradiation of 7-dihydrocholesterol and enhances the dark color of preformed melanin that promotes tanning. UVR can cause several disorders (Table 1), by inducing a plethora of molecular and cellular manifestations, including the increase in reactive oxygen species (ROS), expression of genes, and production of proteins that induce aging, inflammation, and oxidative stress and associated manifestations, DNA photo-destruction, melanogenesis, and several other pathological manifestations. Thus, erythema, tanning, modulation of immunity, and reduction in blood pressure are some of the observed clinical manifestations, as well as some more severe ones, including photoaging and several types of skin cancer. On the other hand, UVR induces the production of beneficial molecules like vitamin D, and thus insufficient exposure to UVR may cause vitamin D deficiencies, as observed in several places all over the world during winter time. In these cases, and since vitamin D can be obtained by ingestion, the dietary supplementation of vitamin D is suggested, either through incorporating diet foods with a rich content in vitamin D, or through the supplementation of vitamin D with food supplements, nutraceuticalals, and nutticrosmetics, or even cosmeceuticals and cosmetics that can increase the levels of vitamin D in the body and especially in the skin. In all these cases, exposure to UV radiation without proper protection is the main factor leading to damage to both cellular components, such as proteins, and human DNA. This damage can occur through two mechanisms: either by the direct absorption of UV light by cellular components, leading to damaging chemical reactions, or by photosensization. In one mode of photosensization (type 1), light activates entropic factors that affect cells through electron transfer and hydrogen removal, whereas in the second mode (type 2), cells are affected through oxygen energy transfer, generating a reactive oxygen singlet.