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OCR for page 37
Chapter 2
INTRODUCTION
It is well known that ultraviolet radiation (W) can be
harmful to plants and animals including humans. The
effects of UV on living cells and organisms depend on the
wavelength of the radiation. The ultraviolet portion of
the electromagnetic spectrum is conventionally divided
into three parts--UV-A, UV-B, and UV-C--in order of
decreasing wavelength (Figure 2.1). The divisions are
somewhat arbitrary, based largely on our understanding of
how UV affects humans. For the purposes of this report,
UV-A is the wavelength region from 320 nanometers (nary) to
400 nm (near- W); W -B. wavelengths from 290 nm to 320 nm
(mid-UV); and WV-C, wavelengths from 190 nm to 290 nm
(far-UV).
m e known harmful effects per unit dose of the shorter
wavelengths, UV-C and UV-B, are greater than those of the
longer wavelengths, UV-A (Blum 1959; Harm 1980b; NRC
1975, 1976a, 1979a; Parrish et al. 1978). A familiar
effect of UV on humans is sunburn (Figure 2.1). UV also
affects the metabolism of, kills, and mutates cells in
culture, and is carcinogenic for animals, including
humans.
The ozone layer provides protection from W by
absorbing the most harmful wavelengths. The spectrum of
solar radiation reaching the surface of the earth for the
current atmospheric distribution of ozone is shown
schematically in Figure 2.1. Radiation in the UV-C band
is essentially completely absorbed by stratospheric ozone
and does not reach the surface of the earth; even with
large reductions (tens of percents) in the concentration
of stratospheric ozone almost no UV-C would be trans-
mitted to the earth. Most of the solar W -B also does
not reach the surface of the earth. Absorption in the
W -B band is a sensitive function of the amount of ozone,
37
OCR for page 38
38
1.0
in
z
~ 0.6
-
o
J
O 0.4
m
>
'5 0.2
UJ
Or:
\ UV-C I UV-B I UV-A | Visible
~ Human Sunburn (Erythema)
_ ~ 1
\ .1/'
\
ol I I I
260 280 300 320 340
/
Current | /
Solar Radiations
at Earth's / |
it/ 1
1
WAVELENGTH (nary)
1 .0
0.8 `,,
z
UJ
A
0.6 I
J
0.4 u'
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-
0.2 LL
360 400 420
O
FIGURE 2.1 Schematic plots of the W portion of the solar electromagnetic radiation
currently reaching the surface of the earth and the biological sensitivity curve for
human sunburn (erythema) are shown as functions of wavelength.
however, and so if ozone concentrations decrease, either
as a result of natural causes or as a result of human
activity, the amount of W -B reaching the surface of the
earth will increase and the harmful effects of W will
also increase. The amount of UV-A reaching the surface
of the earth is not sensitive to changes in ozone
concentration.
Changes in ozone abundance resulting from the release
of chlorofluorocarbons, and other human activities, would
take place only over a long period of time, probably
decades. It is conceivable that many living creatures
with relatively fast reproductive cycles could adapt
biologically to a slow increase in the average intensity
of W. because they would go through many generations in
the time it takes for the intensity to reach some new
steady state value. Humans, on the other hand, could not
adapt biologically nearly as rapidly. Furthermore, if an
increase in W gives rise to an increased incidence of
skin cancer, the increased cancer incidence is not likely
to be detected for many years after the increase in W.
Thus, the continued release of chlorofluorocarbons may
lead to reductions in stratospheric ozone some time in
the future, and that may lead to increases in the
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39
incidence of cancer in humans even farther in the
future. The effects of human activities on stratospheric
ozone are of concern for the long term, but the effects
of current events on one rucuLe won. ~ '=~, ~=
reversible. The second part of this report addresses the
biological effects to be expected from changes in solar
W . The uncertainties in understanding are large in
spite of substantial advances in basic knowledge. These
advances have not answered all of the important questions.
A long-term commitment to research designed to answer the
remaining critical questions is needed to facilitate
predictions about the effects of enhanced W on biological
systems.
With new knowledge comes the possibility of reduced or
increased concerns about ozone reduction, either from
changes in understanding of the effects currently
recognized or from previously unknown effects. Continuous
surveillance of the problem by knowledgeable photo-
biologists is highly desirable, not only directly but
also indirectly via basic research. For example, a
number of years ago the fact that visible light can
ameliorate the damaging effects of UV on human cells was
not suspected. Now, as a result of experiments of a
hectic nature on cells in culture (Harm 1980a, Sutherland
et al. 1974), this amelioration is recognized as an
important factor (D'Ambrosio et al. 1981b, Sutherland et
al. 1980b).
_ .
~ ~ ~ _ _ A ~ ~
-
THE PROBLEM
At the surface of the earth the intensity of sunlight is
a strong function of wavelength, decreasing rapidly for
wavelengths below 320 nm (Figure 2.1). Intensities at
wavelengths below 320 nm are affected most by changes in
stratospheric ozone. Figure 2.2 shows the effect of
large reductions in ozone on the spectrum of light
reaching the earth. The net effect is a shift in the
entire spectrum of UV at the surface of the earth toward
shorter wavelengths; that is, the intensity of the
short-wavelength W increases. While the reductions of
ozone illustrated in Figure 2.2 are much larger than is
generally anticipated, the figure illustrates the point.
For example, an approximate 50 percent decrease in
stratospheric ozone gives rise to a change in intensity
that increases from a factor of about 2 at 305 nm to a
factor of about 50 at 295 nm. In general, for any change
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40
1 .0
0.1
z
`~, 0.01
o
o
oh 0.001
>
-
LD
?~
It
0.0001
1.0
>G'` Sunlight
/\.? Through Ozone
If.
\
\
\ \(b)
(a)\ \
\ \
\
\ \
\ \
\
\
-
UV-B ~ ~UV-A
290 300 310 320 330
WAVELENGTH (nary)
0.1
en
in
UJ
0.01 Z
J
z
en
0.001 >
J
LL
0.0001
FIGURE 2.2 The relative intensity of sunlight (solar elevation of 60°) reaching the
surface of the earth for different amounts of stratospheric ozone (the normal amount
is close to 3.4 atmosphere mm). The shapes of two biological sensitivity curves are
also shown: (a) damage to DNA multiplied by the transmission of human epidermis,
and (b) human erythema or sunburn. Curve (c) is the response of the Robertson-Berger
meter (discussed in Chapter 5~. (Source: The three curves of sunlight intensity are from
U.S. Congress, Senate (1975~; the two biological sensitivity curves are from Setlow
(1974) and Scott and Straf (1977~; the Robertson-Berger meter curve is from Berger
et al. (l975~.)
OCR for page 41
41
in ozone concentration, one can compute with reasonable
confidence the change in the W spectrum striking the
surface of the earth. Hence a Predicted decrease in
stratospheric ozone will give rise to predicted increases
in intensity as a function of wavelength of solar UV
(Johnson et al. 1976).
The extent of the known deleterious effects of UV also
depends strongly on wavelength and, as a rule, increases
rapidly for wavelengths below 320 nm. Figure 2.2 shows
two curves of biological sensitivity (Scott and Straf
1977, Setlow 1974). The figure illustrates the findings
that UV-A wavelengths are much less biologically
effective for damaging DNA or causing sunburn than W-B,
and that in the W-B region the biological sensitivity
per unit dose is an extremely sensitive function of
wavelength. Thus, even if the increase in the absolute
amount of UV penetrating the ozone layer is small, the
changes will occur in a region of the spectrum that is
very effective biologically.
Plots of biological sensitivity as a function of
wavelength--so-called action spectra--are obtained
experimentally. These experiments are difficult to do on
simple biological systems and even more difficult to do
on animals, plants, and ecosystems. Thus there are
uncertainties in our understanding of the dependence of
effects on wavelength. Furthermore, ethical considera-
tions prevent the controlled investigation of some action
spectra, specifically those for various types of skin
cancer in humans.
Most of the available data do not derive from direct
experiments on the biological systems of interest. For
example, the basic data on human skin cancer are epidemio-
logical: incidence, prevalence, and mortality at a
relatively small number of locations in the United
States. The locations differ in many ways, for example,
in the average UV-B exposure during the year, the maximum
UV-B exposure at any time during the year, the amount of
visible light, and the ethnic and occupational backgrounds
and life styles of the populations. Without data from
many more locations that differ widely in the variables
that might affect skin cancer incidence, it is not
possible to use epidemiological data alone to determine
the important variables or the action spectrum responsible
for skin cancer. Thus we must draw inferences about the
action spectrum for human skin cancer from animal experi-
ments and molecular theories. Without knowing the action
spectrum for a particular effect, that is, without knowing
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42
the biological sensitivity curve, it is not possible
make even rough predictions. For example, if curve (a)
in Figure 2.2 were not the proper one to use because the
major effect arose from wavelengths in the UV-A region,
there would be no real consequences of ozone depletion on
the biological system of interest. But if the sensitivity
curve were as given in curve (a) of Figure 2.2, the
depletion of ozone would have a large effect.
There are two general approaches to measuring and
predicting the effects of increased UV on biological
systems.
to
1. A straightforward approach is to irradiate a
system with solar simulators, which mimic the spectrum of
the sun, as a function of time and for various concentra
tions of ozone. _: ~~~~
This approach is useru' ror scuay~ng
ertects on crop plants and small animals, but, even if
there were large numbers of such simulators available, it
is impractical for studying effects on ecosystems because
they are too large. In addition, the experimental
irradiation of people is not ethical, even though large
segments of the U.S. population willingly participate in
a natural experiment of a sort through their propensity
for sunbathing.
2. A second approach is to expand arid apply photobio-
logical theories of effects on molecules, microorganisms,
cells in culture, plants, and animals in order to improve
the data base and our understanding. Predictions about
the effects of ozone depletion on complex biological
systems, such as humans and ecosystems, can then be made
from fundamental principles.
THE UNDERLYING BIOLOGICAL QUESTIONS
Part II of this report builds on the large amount of
photobiological data accumulated in the U.S. Department
of Energy's Climatic Impact Assessment Program and in two
extensive National Research Council reports: Environ-
mental Impact of Stratospheric Flight (NRC 1975) and
Protection Against Depletion of Stratospheric Ozone by
Chlorofluorocarbons (NRC 1979a).
Since those reports
were written, there have been important additions to the
basic knowledge of photobiological processes and some
modest increases in basic epidemiological data. The
changes and refinements in knowledge are summarized in
the chapters that follow.
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43
Because straightforward solar-simulation experiments
cannot be used to estimate most of the~biological effects
likely to result from a change in stratospheric ozone,
the problem must be approached by determining directly,
or indirectly by extrapolation from simpler biological
systems, the answers to four key questions. With the
answers to these questions, models and theories can be
constructed from which reasonable predictions of photo-
biologi al responses can be made.
1. What is the shape of the dose-response curve? An
.
increasing dose of UV produces an increasing biological
effect, but the effect is usually not linearly propor-
tional to the dose. The quantitative relationship between
dose and response may be described by a dose-response
curve. Figure 2.3 shows the general shapes of three
possible dose-response curves. If the dose-response
curve were a straight line, a 10 percent change in dose
would give a 10 percent increase in effect. If, on the
other hand, it were curved sharply upward, as in curve
_
(c) in Figure 2.3, a 10 percent increase in dose would
give rise to different increases in the biological
effect, depending on the initial dose. For an initial
dose of 2.5 arbitrary units per year, a 10 percent
increase in dose would give a 70 percent increase in the
biological effect. If the dose-response curve were curved
downward, as in curve (a) in Figure 2.3, a 10 percent
increase in the same initial dose would give rise to only
about a 4 percent increase in the biological effect.
Hence it is necessary to know the actual form of this
relationship for the shortest wavelengths of W that
penetrate the ozone layer, say 290 nm, to the longest
that have an important biological effect on the system
being investigated. For the induction of cancer in mice,
for example, this longest wavelength is near 320 nm. If
the dose-response curves have similar shapes for all
wavelengths investigated, one can have confidence that
the fundamental photobiological processes are the same at
all wavelengths. On the other hand, if the curves do not
have the same shape at all wavelengths, different types
of photochemical or photobiological mechanisms must
operate at different wavelengths.
2. IS there a reciprocal relationship between
intensity and duration of exposure in responses? In a
number of biological systems, low intensities delivered
for a long time give the same result as high intensities
delivered for a short time, as long as the same total
OCR for page 44
4
UJ
11
UJ
° 2
o
cr.
LO
-
~:
J
U]
CC
o
S
~'
/
44
4 Percent
/ 10
/ 70 Percent
l(c)
I
10 Percent
Dose Increase
1 1 , 1
0 1 2
RELATIVE DOSE PER YEAR
3 4
FIGURE 2.3 Hypothetical dose-response curves (a), (b), and (c), illustrating the
effect on the changes In anticipated biological effects resulting from a 10 percent dose
increase. (In the W-B spectral region, a 5 percent change In ozone concentration will
probably produce an approximate 10 percent change in dose.)
dose is given--the response is simply the product of
intensity and time, or the total, time-integrated dose.
Exposure time and dose rate are then said to be related
reciprocally, and the reciprocity law holds. If the
reciprocity law does not hold, one must know not only the
dose-response relationship, but also the dependence of
the response on the exposure time and dose rate. For
example, in simple cellular systems, a given effect
usually requires a higher dose at low intensities than at
high intensities, presumably because during low-intensity
irradiation repair processes take place and little damage
accumulates (Harm 1980b). In rats a single dose is more
tumorigenic than an equal dose fractionated over 12 weeks
(Strickland et al. 1979). On the other hand, to produce
tumors in 50 percent of mice by W irradiation, a higher
dose is required at high intensities than at low inten-
sities (see Chapter 5). It is not known how intermittent
exposures, as might actually be experienced by humans at
it.
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45
work or during recreation, affect the dose-response
relationship.
A difficulty in extrapolating the effects of
laboratory-type experiments to the outside world is the
fact that most laboratory experiments involve acute
exposures usually taking only a fraction of a cell cycle
time. In sunlight, however, many biological systems are
exposed to low intensities for long times (chronic
exposure). Since exposures to sunlight, and in particular
to W -B. may often be weak, early or late in the day, or
during the winter, many chronic exposures may be at dose
rates well below those used in the laboratory to
determine whether reciprocity holds.
3. How does biological sensitivity depend on
wavelength? It is clear from descriptions given above
that the specific biological effects resulting from a
change in amount of ozone depends critically on the action
spectra. If these curves are not known from direct
experiment or cogent theory, there is no theoretical
basis for making a prediction of the effects of ozone
depletion. The answers to questions (1) and (2) above
must be known before the shape and the wavelength
dependence of each action spectrum, that is, the relative
effectiveness of different monochromatic wavelengths in
producing the observed effect, can be determined. The
product, wavelength by wavelength, of the action spectrum
and the spectrum of sunlight at the surface of the earth
gives the relative effectiveness of sunlight in producing
the specific biological effect (Caldwell 1971, NRC 1979a,
Setlow 1974), provided that interactive effects (see
question (4) below) are small. Any ozone depletion will
change the spectrum of sunlight at the surface of the
earth. This change, when multiplied by the action
spectrum, will give the radiation amplification factor,
i.e., the percentage increase in biologically damaging W
per percentage decrease in ozone. The radiation
amplification factor depends on the action spectrum.
~ Ar" there effects at different wavelengths that
interact? The studies that have been conauccea In one
three areas discussed above have used single wavelengths
of W . An extrapolation to the effects of sunlight on
crops, ecosystems, and humans from experiments in which
the effects of single wavelengths are studied can be made
only if the effects of the isolated wavelengths are
purely additive, and not synergistic or antagonistic.
Hence it is crucial to determine whether biological
systems irradiated with a range or band of wavelengths
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46
act as one would predict from the sum of the effects at
discrete wavelengths. In addition, there may be other
synergistic or antagonistic agents in the environment to
consider, such as visible light, temperature, and
chemicals. As will be discussed in Chapter 3, there are
large synergistic effects between W -B and longer
wavelengths in many simple photobiological systems.
Despite the present uncertainties in understanding,
there has been impressive progress in the extent of
knowledge and in the delineation of the types of questions
that can be answered easily. Certain questions may take
several years and much data accumulation to answer, and
some appear at present to be unanswerable but perhaps
could be answered in the future with the help of a strong
program of basic research.
Representative terms from entire chapter:
biological sensitivity
