Biological action spectra

Ultraviolet (UV) radiation induces a wide variety of biological responses: ranging in humans from well-known shortterm effects like sunburn to long-term effects like skin cancer. The wavelength dependences ('action spectra') of the responses can differ significantly, depending on the W-targeted molecules (their absorption spectra), their localisation (transmission to the target depth) and the photochemical reactions involved (e.g. quantum yields, competing reactions). An action spectrum (e.g. of sunburn) is usually determined in a wavelength by wavelength analysis of the response. This is not always possible (e.g. in case of skin cancer), and an action spectrum may then be extracted mathematically from differences in responses to broadband UV sources of various spectral compositions (yielding 'biological spectral weights'). However, relative spectral weights may shift with exposure levels and contributions from different wavelengths may not always add up. Under these circumstances conventional analyses will yield different action spectra for different experimental conditions.