MEDICAL AND ENVIRONMENTAL EFFECTS OF UV RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 44

dimers in DNA in alfalfa seedlings [6] and in human skin, respectively [7], exposed to
narrow band UV radiation. These data are compared with those for T7 DNA in
solution (diamonds) [29]. These spectra clearly show that in the shorter UV
wavelength region, much lower damage levels are formed in both alfalfa and in human
skin than in isolated DNA. Again, the high sensitivity of the number average length
analysis method allows the quantitation of dimer induction at long wavelengths. At
longer wavelengths, the spectra are rather similar to that of isolated DNA, indicating a
measurable but very low level of damage induced by UVA radiation.
The new action spectra for UV- induced damage to plants obtained by Caldwell
and his associates (this volume) resemble the alfalfa action spectrum of Quaite et al.
[6] (it should be noted that Caldwell's previous 'generalized plant action spectrum' [30-
32] differed strikingly from the alfalfa action spectrum, principally because no
biological damage data from wavelengths longer than 313 nm was considered in
construction of the `generalized plant action spectrum.').
6. Biological Effects of Long Wavelength Environmental UVA
The action spectra for DNA damage induction discussed in Section 5 clearly show that
UVA radiation can induce photoproducts in DNA. Most studies evaluating the effect of
ozone depletion have centered on UVB, since conditions of ozone depletion increase the
number of photons in this wavelength region reaching the surface of the earth. The
predominant source of UV in solar radiation is UVA, and the levels of UVA will not be
altered significantly by ozone depletion. However, evaluating the biological effect of
increased UV requires that we know the damage induced by UVA under normal (in
absence of ozone depletion) solar conditions. That is, if UVA under normal (in absence
of ozone depletion) solar conditions. That is, if UVA induced no damage, and all DNA
damage resulted from the low levels of UVB reaching the biosphere, then a
biologically-weighted increase (e.g., 10%) in UVB would be expected to increase DNA
damage by 10%. However, if the high level of UVA in normal solar conditions induces
80% of the total DNA damage, and UVB induces only 20%, an additional 10%
UVB damage would result in only ~2% increase in total damage.
Current UVA sources limit the accuracy and resolution of action spectra in this
critical wavelength range. First, there are few Hg emission lines in this wavelength
range: 334, 365, 391 and 398 nm. To obtain radiation from a specific line, a
monochromator or narrow band filters are used. It should be noted that Hg-Xe lamps
and even Hg lamps have some emission between these lines, and thus it is essential to
determine the wavelength distribution of the light emerging from the monochromator
and to use appropriate cut-off filters as necessary. It is not valid to assume that the
wavelength specified on a dial of a light source is the only wavelength (or wavelength
range) emitted by the source, that the wavelength calibration is accurate or that there are
no contaminating wavelengths in the beam.
Some UVA wavelengths are available from lasers, but one must be careful that
the high peak power does not produce two-photon effects that can result in different
photochemical reactions from the one-photon mediated effects from lower intensity
sources. Further, some lasers have high brightness (intensity per unit wavelength per
unit timer per unit source area per unit solid angle), but do not provide sufficient total
photon flux to irradiate a sufficient area for most biological samples. The paucity of