Effects of Solar Radiation on Human Skin

Sunlight is the most visible and most obvious source of comfort in the environment. The sun provides the beneficial effects like warmth and vitamin D synthesis. But, acute and chronic exposure to sun may also have serious pathologic consequences, like exposure to sunlight is the major cause of human skin cancer and it also can exert immunosuppressive effects as well.

The energy of sun reaching the surface of the earth is limited to components of the ultraviolet (UV), the visible light, and the infrared portion of wavelength. At the short end of the UV is at approximately 290 nm (nanometer) as the cutoff (this is primarily due to stratospheric ozone formed by highly energetic ionizing radiation, which prevents penetration of shorter, more energetic, potentially more harmful wavelengths of solar radiation to the surface of the earth). Due to the concern about destruction of the ozone layer mainly by chlorofluorocarbons released into the atmosphere has led to international agreements to reduce production of chlorofluorocarbons and other ozone layer destroying chemicals.

The major components capable of affecting human skin are the UV and visible wavelengths between 290 and 700 nm. The wavelengths beyond 700 nm in the infrared spectrum primarily emit heat, but under certain circumstances may exacerbate the pathologic effects of energy in the UV and visible spectra.

The UV spectrum reaching the earth represents less than 10% of total solar energy and is divided into two major segments (although arbitrarily), UV-A and UV-B, comprising the wavelengths from 290 to 400 nm and this range of wavelength is responsible for harm of health state of skin. This portion (290 to 400 nm) of the photobiologic action spectrum is the most efficient in producing redness or erythema in human skin and hence is sometimes known as the “sunburn spectrum” and affect patients’ health, mainly skin health state. UV-A consists of wavelengths between 320 to 400 nm and is approximately 1000-fold less efficient in producing skin redness than is UV-B, which consists of wavelengths between 290 to 320 nm.

The wavelengths between 400 and 700 nm are visible to the human eye and is not harmful to humans as the photon energy in the visible spectrum is not capable of damaging human skin in the absence of a photosensitizing chemical.

Photosensitivity occurs when a photon-absorbing chemical (without the absorption of energy by a molecule there can be no photosensitivity) present in the skin absorbs solar energy, becomes excited, and transfers the absorbed energy to various structures or to oxygen.

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