Action spectra for the cytotoxic action of electromagnetic radiation in the solar range 280–434 nm have been determined for human fibroblasts and epidermal keratinocytes derived from the same foreskin biopsy. The spectra for the two cell types are close to identical and coincide with our previously published data for a human lymphoblastoid line indicating that the mechanism of inactivation of the three human cell types is similar at any given wavelength. Using published data for ultraviolet transmission of human skin and sample spectral irradiance data, we have estimated the relative biological effectiveness of the middle ultraviolet (UVB) (290–320 nm), near ultraviolet (UVA) (320–380 nm), and violet (380–434 nm) regions of sunlight for cytotoxicity at the basal layer of the epidermis. We conclude that the UVB component in noon summer sunlight (the most UVB rich spectral conditions tested) may contribute only about 40% of the total cytotoxic effectiveness of sunlight at 290–434 nm. At lower zenith angles, UVA can account for up to 80% of the cytotoxic effectiveness of the combined UVA and UVB regions. Finally, a comparison of published action spectra data for human erythema with cytotoxicity data corrected for ultraviolet transmission to different depths of the human epidermis suggests that UVA erythema could be causally related to cytotoxicity occurring at an average depth of 40–50 µm into the human epidermis.

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This investigation was supported by grants from the Swiss National Science Foundation (3.108.85) and Swiss League against Cancer.

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