Ultraviolet radiation produces free radicals in Skh-1 mouse skin, contributing to photoaging and carcinogenesis. If a mouse model is a general indicator of free radical processes in human skin photobiology, then radical production observed in mouse and human skin should be directly comparative. In this work we show that UV radiation (lambda > 300 nm, 14 microW/cm2 UVB; 3.5 mW/cm2 UVA) increases the ascorbate free radical (Asc.-) electron paramagnetic resonance (EPR) signal in both Skh-1 mouse skin (45%) and human facial skin biopsies (340%). Visible light (lambda > 400 nm; 0.23 mW/cm2 UVA) also increased the Asc.- signal in human skin samples (45%) but did not increase baseline mouse Asc.-, indicating that human skin is more susceptible to free radical formation and that a chromophore for visible light may be present. Using EPR spin-trapping techniques, UV radiation produced spin adducts consistent with trapping lipid alkyl radicals in mouse skin (alpha-[4-pyridyl 1-oxide]-N-tert-butyl nitrone/alkyl radical adduct; aN = 15.56 G and aH = 2.70 G) and lipid alkoxyl radicals in human skin (5,5-dimethylpyrroline-1-oxide/alkoxyl radical adduct; aN = 14.54 G and aH = 16.0 G). Topical application of the iron chelator Desferal to human skin significantly decreases these radicals (approximately 50%), indicating a role for iron in lipid peroxidation; Desferal has previously been shown to decrease radical production in mouse skin. This work supports the use of the Skh-1 mouse as a predictive tool for free radical formation in human skin. These results provide the first direct evidence for UV radiation-induced free radical formation at near physiological temperatures in human skin and suggest that iron chelators may be useful as photoprotective agents.