Fig. 2.1 Events in light-induced effects
Chromophores absorb light energy from the various wavelengths with differing efficiencies. This pattern of absorption is called an absorption spectrum and is characteristic of the type of molecule involved. Figure 2.2 shows absorption spectra for five of the chromophores present in skin and eye tissues that are thought to be important to the biologic effects of UV-B in humans and animals. These are DNA, tyrosine and tryptophan (two amino acids that are largely responsible for the UV absorbance of proteins), trans-urocanic acid (a molecule present in large amounts in the outermost layer of skin), and melanin (the principal pigment of the skin). The gray area in Fig. 2.2 marks that part of the UV spectrum, wavelengths under 290 nm, which is not present in terrestrial energy. Thus only those portions of these absorption spectra appearing in the white area (above 290 nm) are likely to be of any relevance to the effects associated with environmental exposures. As Fig. 2.2 indicates, for all of the molecules except melanin, absorption efficiency drops rapidly within the terrestrial UV-B spectral region with little or no absorbance in the UVA spectral region (above 320nm). Thus the increase in UV-B that accompanies ozone depletion will increase the amount of biologically active radiation present in ambient sunlight. As chapter 1 has discussed in more detail, while it is difficult to predict quantitatively exactly how these increases will be distributed globally, such increases have been observed in a variety of sites across the world. Because of the biologic activity of UV-B, such increases are likely to have marked consequences for humans as well as other living creatures. Some of these consequences could be beneficial, e.g., a greater production of vitamin D in the skin of humans, but far more are likely to be detrimental.
Fig. 2.2 UVR absorption spectra of molecules important to UV-induced health effects.
This chapter presents an overview of the consequences
likely to accompany increases in UV-B. It will focus on the possible health
risks and only briefly mention possible beneficial effects when these might
offset adverse effects or when concerns about them might modify adaptive
strategies. The chapterís design is adapted from a four-step risk assessment
approach. It first identifies the hazards. Second, it discusses a variety
of factors that can modify exposure or susceptibility. Third, it presents
quantitative and qualitative estimates of risk with their attendant uncertainties,
and fourth, it ends with a brief discussion of potential risks associated
with several of the strategies being adopted to manage or mitigate risk.