Solar ultraviolet, particularly its UVB component, is responsible for more than 1.2 million cutaneous malignancies diagnosed each year in the U.S. alone. UV radiation causes multifaceted damage to the skin and its adjacent tissues, and it is one of the leading causes of premature skin aging, immunosuppression, and carcinogenesis. According to the International Commission on Illumination, UV radiation is divided into three categories: UVA or long wavelength UV, UVB or medium wavelength UV, and UVC or short wavelength UV.
UVA constitutes about 90-95% of solar radiation reaching the earth. UVA, due to its longer wavelength, has high penetrating power, and reaches deep into the epidermis and dermis of the skin. Intense exposure of the skin to UVA burns sensitive skin and damages the underlying structures, causing premature aging. UVA exposure also leads to the generation of singlet oxygen, hydrogen peroxide, and hydroxyl free radicals, causing damage to cellular proteins, lipids, and DNA. UVB, in contrast, constitutes only about 4-5% of UV radiation but is thought to be the most active constituent of solar radiation reaching the earth.
However, even though UVB is more genotoxic and capable of causing much more cell damage than UVA, it has less penetrating power than UVA and acts mainly on the epidermal basal layer of the skin. UVB induces direct adverse biological effects including DNA damage, oxidative stress, free radical production, photoaging, and skin cancer. At the other end of the spectrum, UVC is extremely damaging to the skin, even with a very short exposure. Fortunately, UVC is prevented from reaching the earth, because it is almost completely absorbed by the molecular oxygen and ozone present in the earth's troposphere.
Nevertheless, during the past few decades, depletion of the stratospheric ozone layer has led to an increase in the amount of solar UV radiation reaching the earth. This has led to a significant increase in the amount of UV radiation that people receive. In addition, compounded by changes in lifestyle related to excessive outdoor activities, UV radiation has consequently led to an increase in the incidence of skin-related disorders.
Skin, the largest organ of the body in terms of surface area, serves as a competent epithelial barrier that interfaces the environment. The major role of the skin is to provide a protective covering at this crucial interface through a variety of passive and active features. Exposure of the skin to UV radiation initiates a photo-oxidative reaction, which impairs the antioxidant status and increases the cellular level of reactive oxygen species. This overwhelms the defense capacity of the skin, thereby impairing the ability of skin to protect itself from the damaging effects of UV, resulting in increased oxidative stress.
The induction of oxidative stress and subsequent imbalance of the antioxidant defense system results in damage to the cutaneous tissues and has been associated with the onset of several disease states. Therefore, additional efforts are needed to protect the skin against the deleterious effects of UV radiation. One such approach to prevent the occurrence of skin damage is to enhance endogenous photoprotection through topical or oral administration of botanical antioxidants that possess photoprotective properties.
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