The UV spectrum of sunlight is conventionally divided into short wavelength UVC (λ < 280 nm), mid-wavelength UVB (λ 290–320 nm), and long wavelength UVA (λ > 320–400 nm).46 The UVC fraction of sunlight is entirely absorbed by stratospheric oxygen (O2), which subsequently undergoes decomposition and recombination reactions, giving rise to ozone (O3). The resulting O3 molecules can function as a filter and absorb the majority of sunlight UVB.47,48 Thus, the solar UV wavebands that reach the surface of the earth, and as such are of relevance for photocarcinogenesis, are UVA and UVB, which comprise 95% and 5%, respectively, of the terrestrial sunlight UV48,49 (Fig. 1).
