Illustrative example · not a congress contribution
Penetration into the skin
Shorter UV wavelengths interact more strongly with surface layers and penetrate less deeply; longer wavelengths reach deeper structures of the dermis.
UVC, when present, is essentially absorbed within the stratum corneum and superficial epidermis. UVB acts predominantly within the epidermis, where it produces direct DNA photoproducts in keratinocytes. UVA, by contrast, penetrates to the dermis, where it exerts effects primarily through oxidative pathways.
Figure 2. Relative penetration depths of UVC, UVB and UVA.
Cellular and molecular effects
UVB induces the formation of cyclobutane pyrimidine dimers and 6-4 photoproducts in cellular DNA; these lesions, if unrepaired, contribute to mutagenesis and cutaneous carcinogenesis.
UVA acts largely indirectly, generating reactive oxygen species that damage lipids, proteins and DNA bases, and contributes substantially to photoageing and to immunomodulation in the skin.
Clinical implications
An understanding of wavelength-specific behaviour underpins much of clinical photodermatology: the choice of phototesting protocols, the design of phototherapeutic regimens, the formulation of sunscreens with appropriate UVA–UVB coverage, and the public-health messaging around photoprotection.
Conclusion
The cutaneous effects of ultraviolet radiation are not uniform but tightly coupled to wavelength. Continued investigation of these wavelength-specific mechanisms remains central to the work of photodermatology in clinic, laboratory and public health alike.