Different wavelengths of solar radiation stimulate a diverse range of biologic effects within human skin.
Biologic responses of skin to solar radiation are variable not only by the dose and wavelength of the radiation itself, but also by the phototype of the skin absorbing the radiation.
For a biologic process in the skin to result from exposure to solar radiation, there must be a chromophore within the exposed skin capable of absorbing the wavelengths of the incident solar radiation.
Acute effects of solar radiation on human skin include: erythema, pigment darkening, epidermal cell proliferation, vitamin D synthesis, and immunomodulation.
The studies of Sir Isaac Newton in the seventeenth century and his discovery of the visible light spectrum are recognized as a turning point in photobiology.1 More than a century later, Sir William Herschel and Johann Ritter furthered the understanding of optical radiation with their discoveries of infrared and ultraviolet (UV) radiation, respectively.2,3 In the intervening centuries, investigations into the properties of solar radiation have focused primarily on the UV spectrum. More recently, however, interest has arisen concerning the impact of visible light and infrared radiation. The acute effects of exposure to solar radiation, which will be reviewed in this chapter, include erythema, pigment darkening, epidermal cell proliferation, vitamin D synthesis, and immunomodulation.
Biologic responses of human skin to optical radiation vary by the dose and wavelength of the radiation as well as by the phototype of the skin absorbing the radiation.4,5 Radiation is the transfer of energy via particles (photons) and waves that propagate through space.6 The energy of each photon has an inverse relationship to its wavelength, which is the best predictor of the biologic impact that radiation will have on the skin.5 There are three possible ways by which electromagnetic radiation can interact with the skin: reflection, scattering, or absorption.6 Reflection occurs when the radiation bounces off of the surface of the skin. When reflected, there is no biological impact, but this phenomenon can be used to gain information about the topography of the skin surface, and it allows the retina to perceive the color of the skin.6 Scatter occurs when the direction of the radiation wave propagation is physically altered by interaction with components of the skin. Shorter wavelength radiation is more likely to scatter, which means that shorter wavelengths are less likely to penetrate to deeper portions of any substrate, including the skin.6 This is the reason why the shortest wavelength UV rays, UVC, do not reach the surface of the earth because they are absorbed in the atmosphere. Radiation that is not reflected or scattered may be absorbed by molecules (known as chromophores) in the skin.6 When radiation is absorbed by molecules within the skin, the energy of the photons is transferred to these molecules.5 This energy may then be processed in one of three ...