QUANTIFICATION OF BIOLOGICAL EFFECTIVENESS OF UV RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 54

radiation. This requires UV radiation sources of high intensity and also long exposure
times. During these extended periods of irradiation with UV-A bands repair and
restoration processes may occur that lead to a dose rate dependence. Furthermore,
intermediate molecules. so-called photosensitizers, might be involved in genetic effects
that absorb the incident UV-A photons and transfer the effect to the DNA 22 .
In most cases, relative action spectra are used for weighted spectrometry, that are
scaled to one at 300 nm (Figure 4). Depending on the biological endpoint under
consideration, the action spectra may vary by orders of magnitude. Therefore, different
biological effective irradiances and BEDs are obtained for different endpoints. These
cannot be compared with each other, if relative action spectra have been used. However,
they can be used for trend estimations as e.g., the sensitivity of DNA damaging
irradiance to UV 30 .

10 1

plants

10 0 erythema (CIE)

melanoma (fish)

10 -1

10 -2

10 -3

erythema (laser)

10 -4

10 -5

skin cancer

DNA damage

10 -6

250 275 300 325 350 375 400 425

Wavelength nm

Figure 4. Action spectra for biological responses to UV utilized as biological weighting functions in
weighted spectroradiometry: DNA damage (data from ref. 18,24), skin cancer (data from ref. 24,25),
erythema (data from refs. 26, 27), malignant melanoma (data from ref. 28), and plant damage (data
from ref. 29). Reference wavelength is O = 300 nm.

Figure 5 shows, how relative action spectra can lead to erroneous assumptions. It
shows absolute action spectra for normal fibroblasts and those of the DNA repair
deficient strain Xeroderma pigmentosum (XP), obtained from the inactivation rate
constant for different wavelengths as shown in Figure 3 23 . They clearly show that XP
cells are more sensitive to UV-C and UV-B than normal cells - which is caused by the
excision repair defect associated with XP cells -, however both cells have nearly
identical sensitivities to UV-A. Probably other repair pathways take over in responses to
UV-A which are equally efficient in both cells. In contrast, if both action spectra are
expressed in relative units, e.g., scaled to one at the reference wavelength of 254 nm or
300 nm, both cell types, the normal as well as the XP one, appear to have the same