Environmental Engineering Reference
In-Depth Information
Action spectra as biological weighting functions
Action spectra play a pivotal role in characterizing biological effectiveness of
environmental UV radiation
18,22
. They are most simply defined as the measurement of a
biological effect as a function of wavelength. If cell killing is taken as biological
endpoint, dose effects curves are obtained for different narrow band wavelength ranges
and the biological sensitivity is derived either from the inactivation rate constant
k
or
from the reciprocal value of the fluence, producing a given effect, e.g.,
1/F
10
for a dose
producing a survival rate of 10
-1
(Figure 3). The latter procedure is mostly used in case
of non-linear dose effect curves (Figure 3).
10
0
F
10
10
-1
10
-2
k = - (ln N/N
0
) / F
10
-3
Inactivation rate constant (k)
or
1 / fluence reducing survival to 10
-1
(1/F
10
)
10
-4
UV
O
Fluence F
O
, J/m
2
Figure 3.
Typical exponential dose effect curve of cell inactivation (A) and experimental survival
curves for normal human fibroblasts (open circles) and Xeroderma pigmentosum cells (closed
circles) after irradiation with UV at 313 nm (B).(B from ref. 23).
Several criteria have to be met for the experimental determination of an action
spectrum
21,22
:
x similar dose effect curves for all wavelengths;
x same quantum yield for all wavelengths;
x same absorption spectrum of the chromophore in vivo and in vitro;
x negligible or constant absorption or scattering in front of the chromophore;
x low absorption of UV by the sample in the wavelength range studied;
x reciprocity of time and dose rate.
Whereas these criteria hold relatively well when using small transparent cells or
biomolecules in solution, extending such studies to tissues, larger organisms, or even
ecosystems will inevitably lead to substantial absorption of UV by the layers
surrounding the chromophore of interest.
Another practical difficulty arises from the need to extend the action
spectroscopy to the UV-A range, which physically dominates in the solar spectrum but
which is biologically less effective by several orders of magnitude than UV-B or UV-C
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