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actinic keratosis; basal cell carcinoma; chromophore; cyclobutane pyrimidine dimer; DNA damage; DNA repair; DNA photoproduct; infrared radiation; melanoma; photoaging; photocarcinogenesis; photodermatoses; photoprotection; phototherapy; squamous cell carcinoma; sunburn; tanning; ultraviolet radiation; UV signature mutation; visible light; vitamin D; xeroderma pigmentosum;



  • Radiation can only cause a photobiologic response, if it is first absorbed by a molecule (chromophore) in the skin.

  • Understanding the photophysical, photochemical, and photobiologic effects of radiation exposure is helpful to understand the wavelength-dependent consequences, including burning, tanning, photodermatoses, formation of skin cancer, photoaging, and effects of phototherapy.

  • Photoreactions have specific action spectra that depend on the absorption characteristics of various different intrinsic and extrinsic chromophores.

  • Cutaneous vitamin D production is mediated by wavelengths within the UVB range. Optimal vitamin D blood levels are essential for good bone health and are increasingly associated with a myriad of other potential health benefits.

  • Formation of DNA damage by ultraviolet radiation (UVR) mediates sunburning, tanning, and skin cancer formation.

  • Skin cells are equipped with a number of damage response pathways that limit the negative impact of radiation exposure, including several different DNA repair mechanisms.

  • Ultraviolet radiation–induced DNA damage can result in mutation formation. C to T and CC to TT mutations at dipyrimidine sites are highly characteristic for an induction by ultraviolet radiation and are called UV-signature mutations. Such UV-signature mutations are found in UV-induced skin cancers.

  • Ultraviolet radiation has both pro- and antiinflammatory properties.

  • Photoaging affects all compartments of the skin and is characterized by both clonal proliferative events and loss-of-function events.

  • Photoaging is largely irreversible, possibly due in part to an UV-induced accumulation of undegradable abnormal proteins in the extra- and intracellular space.

  • Exposure to visible light and infrared radiation also has photobiological consequences, including erythema, tanning, and degradation of extracellular matrix proteins.

  • Rational phototherapy and photoprotection are based on these insights.


As the outermost layer of the human body, skin is heavily exposed to damaging environmental agents, including different types of radiation (Table 17-1 and Fig. 17-1). Ionizing electromagnetic radiation, like, for example, x-rays or gamma rays, carries sufficient photon-energy to completely remove an electron from an atom or molecule (= ionization). Other types of ionizing radiation are alpha particles (2 protons and 2 neutrons) and beta particles (electrons). Alpha and beta particles are not part of the electromagnetic spectrum; they are energetic particles, as opposed to pure energy bundles (photons) of the electromagnetic radiation. Nonionizing radiation, which includes ultraviolet radiation (UVR), visible light, and infrared radiation, is able to move an electron to a higher energy state, but, in contrast to ionizing radiation, cannot remove an electron from atoms or molecules. Cutaneous photobiology is the science that studies the interaction of nonionizing radiation with skin. The term light is commonly reserved for wavelengths of the electromagnetic spectrum that are perceivable by the human eye: visible light.

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