title: Cumulative effects from repeated exposures to suberythemal doses of UVB and UVA in human skin.
abstract: BACKGROUND
  The skin is repeatedly exposed to solar UV radiation. Long-term photodamage is a consequence of cumulative UV radiation injury. Hence an examination of the repetitive effects of UV exposure is more likely to yield clues to the early alterations that lead to photoaged skin than a single exposure.
  
  
  OBJECTIVE
  We examined the effects of repetitive low-dose UV irradiation on human skin with the aim of identifying UVA-induced effects that may have a different wavelength dependence than acute erythema.
  
  
  METHODS
  Areas on the lower part of the back were each exposed to a suberythemal dose (0.5 minimal erythema dose [MED]) of solar simulated radiation (290 to 400 nm) and of UVA (320 to 400 nm) once daily, 5 days a week, for 28 doses. One site was also treated daily with a sunscreen having a sun protection factor of 22 and then exposed to 11 MEDs of solar simulated radiation for the same duration. Epidermal and dermal changes were analyzed and quantified by histochemical stains in combination with computer-assisted image analysis of tissue sections.
  
  
  RESULTS
  At equal 0.5 MED doses, UVA induced greater cumulative changes than solar simulated radiation, as assessed by development of a greater cumulative erythema response in the first week of treatment, the presence of epidermal hyperplasia and stratum corneum thickening, depletion of Langerhans cells, dermal inflammatory infiltrates, and deposition of lysozyme on elastin fibers. These changes were not prevented by the sunscreen. A single short-term dose of UVA did not elicit these changes.
  
  
  CONCLUSIONS
  These findings suggest that UVA may contribute significantly to long-term actinic damage and that the spectral dependence for cumulative damage does not parallel the action spectrum for acute injury (erythema) in human beings.
authors: Department of Dermatology, University of Pennsylvania School of Medicine, Philadelphia 19104.
authors: R M
authors: RM
authors: Lavker
authors: G F
authors: GF
authors: Gerberick
authors: D
authors: D
authors: Veres
authors: C J
authors: CJ
authors: Irwin
authors: K H
authors: KH
authors: Kaidbey
mesh_terms: D000328:Adult; D001706:Biopsy; D004817:Epidermis / Q000187:drug effects / Q000378:metabolism / Q000473:pathology / Q000528:radiation effects; D004890:Erythema / Q000209:etiology / Q000473:pathology / Q000517:prevention & control; D005260:Female; D006644:Histiocytes / Q000378:metabolism / Q000528:radiation effects; D006801:Humans; D006965:Hyperplasia; D008214:Lymphocytes / Q000378:metabolism / Q000528:radiation effects; D008297:Male; D008407:Mast Cells / Q000378:metabolism / Q000528:radiation effects; D008543:Melanins / Q000378:metabolism / Q000528:radiation effects; D009113:Muramidase / Q000378:metabolism / Q000528:radiation effects; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000209:etiology* / Q000378:metabolism / Q000473:pathology / Q000517:prevention & control; D012867:Skin / Q000187:drug effects / Q000378:metabolism / Q000473:pathology / Q000528:radiation effects*; D013473:Sunscreening Agents / Q000494:pharmacology / Q000627:therapeutic use; D013997:Time Factors; D014466:Ultraviolet Rays / Q000009:adverse effects*
chemical_list: D008543:Melanins; D013473:Sunscreening Agents; D009113:Muramidase