title: Diurnal and seasonal variations of the UV cut-off wavelength and most erythemally effective wavelength of solar spectra.
abstract: BACKGROUND
  Biologically effective solar ultraviolet radiation is defined as the product of the intensity of the solar spectrum and the erythema action spectrum at each wavelength. In this way we may arrive at the weighted effectiveness of each wavelength of solar radiation to produce a sunburn reaction. There have been many measurements of the variation of the solar spectrum with the time of the day and the time of the year, but questions remain as to the variation of the quality of the spectrum and the contribution of the shortest wavelengths of solar terrestrial radiation. The purpose of the present study was to determine the variation of the biologically effective solar spectrum with the time of the day and the time of the year and to determine the variation of the shortest wavelength that contributes to the sunburn reaction with the time of the day and the time of the year.
  
  
  METHODS
  Spectroradiometric measurements were made at ground level over the period of one year (1988-1989) and at different times of the day at latitude 29.5 degrees north. The measured spectral irradiance was multiplied wavelength by wavelength by the erythema action spectra.
  
  
  RESULTS
  We determined that the biologically effective solar spectrum remains essentially the same over the times of the day that sunburn may be experienced. The maximally effective wavelength of biologically effective solar radiation was determined to be 308 nm. The cut-off wavelength for biologically effective solar radiation (defined as the wavelength at which the biologically effective solar radiation is at 1% of its maximum) varied from 291 to 295 nm over the time of the year and from 292 to 296 nm over the day.
  
  
  CONCLUSIONS
  For all practical purposes the biologically effective spectrum of solar ultraviolet radiation may be considered to remain constant over the period when sunburn may occur and the minimal wavelength of sunlight that contributes to sunburn is in the range of 291-296 nm.
authors.affiliation: Johnson and Johnson Consumer Product Worldwide, 199 Grandview Road, Skillman, NJ 08558, USA.
authors.forename: N
authors.initials: N
authors.lastname: Kollias
authors.forename: A H
authors.initials: AH
authors.lastname: Baqer
authors.forename: H
authors.initials: H
authors.lastname: Ou-Yang
mesh_terms: D006801:Humans; D011874:Radiometry; D012621:Seasons; D012993:Solar Energy; D013057:Spectrum Analysis; D013471:Sunburn / Q000209:etiology* / Q000473:pathology; D014466:Ultraviolet Rays / Q000009:adverse effects* / Q000145:classification*