Usenet 1994 January/usenetsourcesnewsgroupsinfomagicjanuary1994.iso/answers/ozone-depletion/uv

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From: [e-mail] (Robert Parson)
Subject: Ozone Depletion FAQ Part IV: UV Radiation and its Effects
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Summary: This is the fourth of four files dealing with stratospheric
         ozone depletion. It describes the properties of solar UV
         radiation and some of its biological effects.
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Keywords: ozone layer depletion UVB UVA skin cancer phytoplankton
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* Copyright 1993 Robert Parson                                        *
*                                                                     *
* This file may be distributed, copied, and archived. All             *
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* "Caveat". Reproduction and distribution for personal profit is      *
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This file deals with the physical properties of ultraviolet
radiation and its biological consequences, emphasizing the
possible effects of stratospheric ozone depletion. It frequently
refers back to Part I, where the basic properties of the ozone
layer are described; the reader should look over that file first.

The overall approach I take is conservative. I concentrate on what
is known and on most probable, rather than worst-case, scenarios.
For example, I have relatively little to say about the
effects of UV radiation on plants - this does not mean that the
effects are small, it means that they are as yet not well
quantified (and moreover, I am not well qualified to interpret the
literature.) Policy decisions must take into account not only the
most probable scenario, but also  a range of less probable ones.
will probably do, but also the worst that he could possibly do.
There have been surprises, mostly unpleasant, in this field in the
past, and there are sure to be more in the future. In general,
_much_ less is known about biological effects of UV-B than about
the physics and chemistry of the ozone layer.


| _Caveat_: I am not a specialist. In fact, I am not an atmospheric
| scientist at all - I am a physical chemist studying gas-phase
| reactions who talks to atmospheric scientists. In this part in
| particular I am well outside the range of my own expertise.
| I have discussed some aspects of this subject with specialists,
| but I am solely responsible for everything written here, including
| any errors. This document should not be cited in publications off
| the net; rather, it should be used as a pointer to the published
| literature.

*** Corrections and comments are welcomed.


- Robert Parson
  Associate Professor
  Department of Chemistry and Biochemistry,
  University of Colorado (for which I do not speak)

  [e-mail]
  [e-mail]


CONTENTS

 1. What is "UV-B"?

 2. How does UV-B vary from place to place?

 3. If the ozone layer is depleted, won't the UV light just
    penetrate deeper into the atmosphere and make more ozone?

 4. *Is* UV-B increasing?

 5. What is the relationship between UV radiation and skin cancer?

 6. Is ozone loss responsible for the melanoma upsurge?

 7. Does UV Radiation cause cataracts?

 8. Are sheep going blind in Chile?

 9. What effects does increased UV have on agriculture?

 10. What effects does increased UV have on marine life?

 References



1. What is "UV-B"?

 "UV-B" refers to UV light having a wavelength between 280 and
320 nm. These wavelengths are on the lower edge of ozone's UV
absorption band, in the so-called "Huggins bands". They are
absorbed by ozone, but less efficiently than shorter wavelengths
("UV-C").  (The absorption cross-section of ozone increases by more
than 2 orders of magnitude between 320 nm and the peak value at
~250 nm.)  Depletion of the ozone layer would first of all result
in increased UV-B. In principle UV-C would also increase, but it is
absorbed so efficiently that a very large depletion would have to
take place in order for significant amounts to reach the earth's
surface. UV-B and UV-C are absorbed by DNA and other biological
macromolecules, inducing photochemical reactions. UV radiation with
a wavelength longer than 320 nm is called "UV-A".  It is not
absorbed by ozone, but it is not believed to be especially
dangerous. (See, however, question #6.)


2. How does UV-B vary from place to place?

 A great deal.  It is strongest at low latitudes and high altitudes.
At higher latitudes, the sun is always low in the sky so that it takes
a longer path through the atmosphere and more of the UV-B is absorbed.
For this reason, ozone depletion is
likely to have a greater impact on _local_ ecosystems, such as the
Antarctic marine phytoplankton, than on humans or livestock. UV
also varies with altitude and local cloud cover. These trends can
be seen in the following list of annually-averaged UV indices for
several US cities  [Roach] (units are arbitrary - I don't know
precisely how this index is defined though I assume it is
proportional to some integral over the UV-b region of the spectrum)

 Minneapolis, Minnesota         570
 Chicago, Illinois              637
 Washington, DC                 683
 San Francisco, California      715
 Los Angeles, California        824
 Denver, Colorado               951
 Miami, Florida                 1028
 Honolulu, Hawaii               1147

 It should be noted that skin cancer rates show a similar trend.

3. If the ozone is lost, won't the UV light just penetrate
   deeper into the atmosphere and make more ozone?

 This does happen to some extent - it's called "self-healing" - and
has the effect of moving ozone from the upper to the lower
stratosphere. It is not a very effective stabilizing mechanism,
however. Recall that ozone is _created_ by UV with wavelengths
less than 240 nm, but functions by _absorbing_ UV with wavelengths
greater than 240 nm. The peak of the ozone absorption band is at
~250 nm, and the cross-section falls off at shorter wavelengths.
The O2 and O3 absorption bands do overlap, though, and UV
radiation between 200 and 240 nm has a good chance of being
absorbed by _either_ O2 or O3. (Below 200 nm the O2 absorption
cross-section increases dramatically, and O3 absorption is
insignificant in comparison.) Since there is some overlap, a
decrease in ozone does lead to a small increase in absorption by
O2. This is a weak feedback, however, and it does not compensate
for the ozone destroyed. Negative feedback need not imply
stability, just as positive feedback need not imply instability.

Numerical calculations of ozone depletion take the "self-healing"
phenomenon into account, by letting the perturbed ozone layer come
into equilibrium with the exciting radiation. Even the simple
one-dimensional models used in the mid 1970's included this
rather obvious effect.


4. Is UV-B at the earth's surface increasing?

Yes, in some places; no, in others.

Very large increases - up to a factor of 2 - have been seen even
in the outer portions of the Antarctic hole. [Frederick and
Alberts]

Small increases, of order 1% per  year, have been measured in the
Swiss Alps. [Blumthaler and Ambach] These _net_ increases are small
compared to natural day-to-day fluctuations, but they are actually
a little larger than would be expected from the amount of ozone
depletion over the same period.

In urban areas of the US, UV-B
levels showed no significant increase (and in most cases actually
decreased a little)  between 1974 and 1985. [Scotto et al.].  This
is probably due to increasing urban pollution, including low-level
ozone and aerosols. [Grant] Tropospheric ozone is actually
somewhat more  effective at absorbing UV than stratospheric ozone,
because UV light is scattered much more in the troposphere, and
hence takes a longer path. [Bruehl and Crutzen]  Increasing
amounts of tropospheric aerosols, from urban and industrial
pollution, may also offset UV-B increases at the ground.  [Liu et
al.] [Madronich 1992, 1993] [Grant] There have been questions about
the suitability of the instruments used by Scotto et al.; they were
not designed for measuring long-term trends, and they put too much
weight on regions of the UV spectrum which are not appreciably
absorbed by ozone in any case. [WMO 1989] Nevertheless it seems
clear that so far ozone depletion over US cities is small enough to
be largely offset by competing factors.  Tropospheric ozone and aerosols
have increased in rural areas of the US and Europe as well, so
these areas may also be screened from the effects of ozone depletion.

A recent study [Kerr and McElroy] has found convincing evidence of
UV-B increases in Toronto, Canada over the period 1989-1993. The UV
intensity at 300 nm increased by 35% per year in winter and 7% per
year in summer. At this wavelength 99% of the total UV is absorbed,
so these represent large increases in a small number, and do not
represent a health hazard; nevertheless these wavelengths play a
disproportionately large role in skin carcinoma and plant damage
since DNA absorbs strongly there. Total UV-B irradiance, weighted
in such a way as to correlate with incidence of sunburn ("erythemally
active radiation"), increased by 5% per year in winter and 2% per year
in summer. The trends are strongly influenced by the unusually large,
and probably transient, ozone losses in these regions in the years
1992-1993 (see part I) and should therefore not be extrapolated into
the future; they do provide strong evidence of a link between ozone
loss at middle latitudes and total UV-B radiation.

 Indirect evidence for increases has been obtained in the Southern
Hemisphere, where stratospheric ozone depletion is larger and
tropospheric ozone (and aerosol pollution) is lower. Biologically
weighted UV-B irradiances at a station in New Zealand were 1.4-1.8
times higher than irradiances at a comparable latitude and season in
Germany, of which a factor of 1.3-1.6 can be attributed to differences
in the ozone column over the two locations [Seckmeyer and McKenzie].
In the southern hemisphere summer, the noontime UV-B irradiance
at Ushaia in Tierra del Fuego is 45% above what would be predicted
were there no ozone depletion. [Frederick et al. 1993]

 In comparing UV-B estimates, one must pay careful attention to
exactly what is being reported. One wants to know not just whether
there is an increase, but how much increase there is at any given
wavelength, since the shorter wavelengths are more dangerous.
Different measuring instruments have different spectral responses,
and are more or less sensitive to various spectral regions. [Wayne,
Rowland 1991].  Wavelength-resolving instruments, such as the
spectroradiometers being used in Antarctica, Argentina, and Toronto,
are the most informative, as they allow one to distinguish the effects
of ozone trends from those due to clouds and aerosols.
[Madronich 1993] [Kerr and McElroy].


5. What is the relationship between UV radiation and skin cancer?

 There are three kinds of skin cancer, basal cell carcinomas,
squamous cell carcinomas, and melanomas. In the US there were
500,000 cases of the first, 100,000 of the second, and 27,600 of
the third in 1990. [Wayne] More than 90% of the skin carcinomas in
the US are attributed to UV-b exposure: their frequency varies
sharply with latitude, just as UV does. The mechanism by which UV-B
induces carcinomas has been identified - the pyrimidine bases
in the DNA molecule form dimers when stimulated by UV-B radiation.
[Tevini]. Fortunately, these cancers are relatively easy to treat
if detected in time, and are rarely fatal. Skin carcinoma rates vary
sharply with latitude, just as UV-B does. Fair-skinned people of
North European ancestry are particularly susceptible. The highest
rates in the world are found in Queensland, a northerly province of
Australia.

[Madronich and deGruiji] have estimated the expected increases in
skin carcinoma rates due to ozone depletion over the period 1979-1992:

 Lat.    % ozone loss    % increase in rate,    % increase in rate,
    1979-1992    basal cell carcinoma    squamous cell carcinoma

 55N    7.4 +-1.3    13.5 +-5.3        25.4 +-10.3
 35N    4.8 +-1.4     8.6 +-4.0        16.0 +-7.6
 15N    1.5 +-1.1     2.7 +-2.4         4.8 +-4.4

 15S    1.9 +-1.3     3.6 +-2.6         6.5 +-4.8
 35S    4.0 +-1.6     8.1 +-3.6        14.9 +-6.8
 55S    9.0 +-1.5    20.4 +-7.4        39.3 +-15.1

Of course, the rates themselves are much smaller at high latitudes,
where the relative increases in rates are large. These estimates do
not take changes in lifestyle into consideration.

 Malignant melanoma is much more dangerous, but its connection
with UV exposure is not well understood. There seems to a correlation
between melanomas and brief, intense exposures to UV (long before
the cancer appears.) Melanoma incidence is definitely correlated with
latitude, with twice as many deaths (relative to state population)
in Florida or Texas as in Wisconsin or Montana, but this correlation
need not imply a causal relationship. Some claim that UV-A, which is
not absorbed by ozone, is involved. [Skolnick] [Setlow et al.]


6. Is ozone loss to blame for the melanoma upsurge?

 A few physicians have said so, but most others think not.
[Skolnick]

First of all, UV-B has not, so far, increased very much, at least
in the US and Europe.

Second, melanoma takes 10-20 years to develop. There hasn't been
enough time for ozone depletion to play a significant role.

Third, the melanoma epidemic has been going on since the 1940's.
Recent increases in rates may just reflect better reporting, or
the popularity of suntans in the '60's and '70's. (This becomes
more likely if UV-A is in fact involved.)


7. Does UV-B cause cataracts?

While the evidence for this is indirect, it is very plausible.

The lens of the eye is a good UV-filter, protecting the delicate
structures in the retina. Too much UV results in short-term "snow
blindness", but the effects of prolonged, repeated exposure are
not known.  People living in naturally high UV environments
such as Bolivia or Tibet do have a high incidence of cataracts,
and overall cataracts are more frequently seen at lower latitudes.
[Tevini]


8. Are sheep going blind in Chile?

If they are, it's not because of ozone depletion.

 For a short period each year, the edge of the ozone hole passes
over Tierra del Fuego, at the southern end of the South American
continent. This has led to a flurry of reports of medical damage
to humans and livestock. Dermatologists claim that they are seeing
more patients with sun-related conditions, nursery owners report
damage to plants, a sailor says that his yacht's dacron sails have
become brittle, and a rancher declares that 50 of his sheep,
grazing at high altitudes, suffer "temporary cataracts" in the
spring. (_Newsweek_, 9 December 1991, p. 43; NY Times, 27 July
1991, p. C4; 27 March 1992, p. A7).

 These claims are hard to believe. At such a high latitude,
springtime UV-B is naturally very low and the temporary increase
due to ozone depletion still results in a UV fluence that is well
below that found at lower latitudes. Moreover, the climate of
Patagonia is notoriously cold and wet. (There is actually more of
a problem in the summer, after the hole breaks up and ozone-poor
air drifts north. The ozone depletion is smaller, but the
background UV intensity is much higher.) There may well be effects
on _local_ species, adapted to low UV levels, but even these are
not expected to appear so soon. It was only in 1987 that the hole
grew large enough to give rise to significant UV increases
in southern Chile, and cataracts and malignant melanomas take many
years to develop. To be sure, people do get sunburns and
skin cancer even in Alaska and northern Europe, and all
else being equal one expects on purely statistical grounds such
cases to increase, from a small number to a slightly larger number.
All else is definitely not equal, however - the residents are now
intensely aware of the hazards of UV radiation and are likely to
protect themselves better. I suspect that the increase in
sun-related skin problems noted by the dermatologists comes about
because more people are taking such cases to their doctors.

As for the blind sheep, a group at Johns Hopkins has investigated
this and ascribes it to a local infection ("pink eye"). [Pearce]

 This is _not_ meant to dismiss UV-B increases in Patagonia as
insignificant. Damage to local plants, for example, may well emerge
in the long term, as the ozone hole is expected to last for 50
years or more. The biological consequences of UV radiation are real,
but often very subtle; I personally find it hard to believe that
such effects are showing up so soon, and in such a dramatic fashion.
Ozone depletion is a real problem, but this particular story is a red
herring.


9. What effects does increased UV have upon plant life?

 Generally harmful, but hard to quantify. Many experiments have
studied the response of plants to UV-B radiation, either by
irradiating the plants directly or by filtering out some of the UV
in a low-latitude environment where it is naturally high. The
artificial UV sources do not have the same spectrum as solar
radiation, however, while the filtering experiments do not
necessarily isolate all of the variables, even when climate
and humidity are controlled by growing the plants in a
greenhouse.

 Out of some 200 agricultural plants tested, more than half show
sensitivity to UV-B increases. The measured effects vary markedly
from one species to another; some adapt very readily while others
are seriously damaged. Even within species there are marked differences;
for example, one soybean variety showed a 25% growth reduction under a
simulated ozone depletion of 16%, whereas another variety showed no
significant yield reduction. The general sense seems to be that
ozone depletion amounting to 10% or more could seriously affect
agriculture. Smaller depletions could have a severe impact on local
ecosystems, but very little is known about this at present.

 I have not investigated the literature on this in detail, not
being a biologist. Interested readers should consult [Tevini and
Teramura] or the book by [Tevini] and the references therein.
If any botanist out there would like to write a summary for
this FAQ, please let me know.


10. What effects does increased UV have on marine life?

 Again, generally harmful but hard to quantify. Seawater is
surprisingly transparent to UV-B. In clear waters radiation at 315
nm is attenuated by only 14% per meter depth. [Jerlov]. Many marine
creatures live in surface waters, and they have evolved a variety
of methods to cope with UV. Some simply swim to lower depths, some
develop protective coatings, some work at night to repair the
damage done during the day. These natural mechanisms however, are
often triggered by _visible_ light intensities, in which case they
do not protect against an increase in the _ratio_ of UV to visible
light. Also, if a photosynthesizing organism protects itself by
staying at lower depths, it will get less visible light and produce
less oxygen. An increase in UV-B can thus affect an ecosystem
without necessarily killing off individual organisms.

 Many experiments have been carried out to determine the
response of various marine creatures to UV radiation; as with land
plants the effects vary a great deal from one species to another,
and it is difficult to draw general conclusions at this stage. We
can infer that organisms that live in tropical waters are safe,
since there is little or no ozone depletion there, and that
organisms that are capable of living in the tropics are probably
safe from large depletions at high latitudes since UV intensities
at high latitudes are always low. (One must be a little careful
with the second inference if the organism's natural defenses are
stimulated by visible light.) The problems arise with organisms
that have adapted to the naturally low UV levels of polar regions.

 In this case, we have a natural laboratory for studying UV
effects:  the Antarctic Ozone hole. (Part III of the FAQ discusses
the hole in detail.) The outer parts of the hole extend far out
into the ocean,  beyond the pack ice, and these waters get
springtime UV-B doses equal to or greater than what is
seen in a normal antarctic summer. [Frederick and Alberts] [Smith
et al.]. The UV in shallow surface waters is effectively even
higher, because the sea ice is more transparent in spring than in
summer. There has been speculation that this UV could cause a
population collapse in the marine phytoplankton, the microscopic
plants that comprise the base of the food chain.

 To my knowledge, only one field study has been published so far.
[Smith et al.]. These workers measured the photosynthetic
productivity of the phytoplankton in the "marginal ice zone" (MIZ),
the layer of relatively  fresh meltwater that lies over saltier
deep water.  Since the outer boundary of the ozone hole is
relatively sharp and fluctuates from day to day, they were able to
compare photosynthesis inside and outside the hole, and to
correlate photosynthetic yield with shipboard UV measurements.
They concluded that the UV-B increase brought about an overall
decrease of 6-12% in phytoplankton productivity. Since the "hole"
lasts for about 10-12 weeks, this corresponds to an overall decrease
of 2-4% for the year. The natural variability in phytoplankton
productivity from year to year is estimated to be about + or - 25%,
so the _immediate_ effects of the ozone hole, while real, are far
from catastrophic. To quote from [Smith et al.]:  "Our estimated
loss of 7 x 10^12 g of carbon per year is about three orders
of magnitude smaller than estimates of _global_ phytoplankton
production and thus is not likely to be significant in this
context. On the other hand, we find that the O3-induced loss to a
natural community of phytoplankton in the MIZ is measurable and the
subsequent ecological consequences of the magnitude and timing of
this early spring loss remain to be determined."  It appears, then,
that overall loss in productivity is not large - yet. (The
cumulative effects on the marine community are not known. The ozone
hole first became large enough to expose marine life to large UV
increases in 1987, and [Smith et al.] carried out their survey in
1990.) Ecological consequences - the displacement of UV-sensitive
species by UV-tolerant ones - are likely to be more important than
a decline in overall productivity reduction, although they are
poorly understood at present.

_____________________________________________________________________

 REFERENCES FOR PART IV

A remark on references: they are neither representative nor
comprehensive. There are _hundreds_ of people working on these
problems. For the most part I have limited myself to papers that
are (1) widely available (if possible, _Science_ or _Nature_ rather
than archival journals such as _J. Geophys. Res._) and (2) directly
related to the "frequently asked questions". Readers who want to
see "who did what" should consult the review articles listed below,
or, if they can get them, the WMO reports which are extensively
documented.


Introductory Reading:

[Graedel and Crutzen] T. E. Graedel and P. J. Crutzen,
_Atmospheric Change: an Earth System Perspective_, Freeman, NY
1993.

[Rowland 1989] F. S. Rowland, "Chlorofluorocarbons and the
depletion of stratospheric ozone", _American Scientist_ _77_, 36,
1989.

[Zurer] P. S. Zurer, "Ozone Depletion's Recurring Surprises
Challenge Atmospheric Scientists", _Chemical and Engineering News_,
24 May 1993,  pp. 9-18.

----------------------------
Books and Review Articles:

[Rowland 1991] F. S. Rowland, "Stratospheric Ozone Depletion",
_Ann. Rev. Phys. Chem._ _42_, 731, 1991.

[Tevini] M. Tevini, editor: "UV-B Radiation and Ozone Depletion:
Effects on humans, animals, plants, microorganisms, and materials"
Lewis Publishers, Boca Raton, 1993.

[Wayne] R. P. Wayne, _Chemistry of Atmospheres_, 2nd.  Ed.,
 Oxford, 1991.

[WMO 1988] World Meteorological Organization,
_Report of the International Ozone Trends Panel_,
 Global Ozone Research and Monitoring Project - Report #18.

[WMO 1989] World Meteorological Organization,
_Scientific Assessment of Stratospheric Ozone: 1989_
 Global Ozone Research and Monitoring Project - Report #20.

[WMO 1991] World Meteorological Organization,
_Scientific Assessment of Ozone Depletion: 1991_
 Global Ozone Research and Monitoring Project - Report #25.

-----------------------------------
More Specialized:

[Blumthaler and Ambach] M. Blumthaler and W. Ambach, "Indication of
 increasing solar ultraviolet-B radiation flux in alpine regions",
 _Science_ _248_, 206, 1990.

[Bruehl and Crutzen]  C. Bruehl and P. Crutzen, "On the
disproportionate role of tropospheric ozone as a filter against
solar UV-B radiation",_Geophys. Res. Lett._ _16_, 703, 1989.

[Frederick and Alberts] J.E. Frederick and A. Alberts, "Prolonged
 enhancement in surface ultraviolet radiation during the Antarctic
 spring of 1990", _Geophys. Res. Lett._ _18_, 1869, 1991.

[Frederick et al. 1993] J.E. Frederick, P.F. Soulen, S.B. Diaz,
I. Smolskaia, C.R. Booth, T. Lucas, and D. Neuschuler,
"Solar Ultraviolet Irradiance Observed from Southern Argentina:
 September 1990 to March 1991", J. Geophys. Res. _98_, 8891, 1993.

[Grant] W. Grant, "Global stratospheric ozone and UV-B radiation",
  _Science_ _242_, 1111, 1988. (a comment on [Scotto et al.])

[Jerlov] N.G. Jerlov, "Ultraviolet Radiation in the Sea",
_Nature_ _166_, 112, 1950.

[Kerr and McElroy] J. B. Kerr and C. T. McElroy, "Evidence for Large
 Upward Trends of Ultraviolet-B Radiation Linked to Ozone Depletion",
 _Science_ _262_, 1032, 1993.

[Liu et al.] S.C. Liu, S.A. McKeen, and S. Madronich, "Effect of
 anthropogenic aerosols on biologically active ultraviolet
radiation", _Geophys. Res. Lett._  _18_,  2265, 1991.

[Madronich 1992] S. Madronich, "Implications of recent total
atmospheric ozone measurements for biologically active ultraviolet
radiation reaching the earth's surface",
_Geophys. Res. Lett. _19_, 37, 1992.

[Madronich 1993] S. Madronich, in [Tevini], above.

[Madronich and de Gruiji] S. Madronich and F. R. de Gruiji,
"Skin Cancer and UV radiation", _Nature_ _366_, 23, 1993.

[Pearce] F. Pearce, "Ozone hole 'innocent' of Chile's ills",
 _New Scientist_ #1887, 7, 21 Aug. 1993.

[Roach] M. Roach, "Sun Struck", _Health_, May/June 1992, p. 41.
 (See especially the sidebar by Steven Finch on p. 50).

[Scotto et al.] J. Scotto, G. Cotton, F. Urbach, D. Berger, and T.
Fears, "Biologically effective ultraviolet radiation: surface
measurements in the U.S.",  _Science_ _239_, 762, 1988.

[Seckmeyer and McKenzie] G. Seckmeyer and R. L. McKenzie,
"Increased  ultraviolet radiation in New Zealand (45 degrees S)
relative to Germany (48 degrees N.)", _Nature_ _359_, 135, 1992.

[Setlow et al.] R. B. Setlow, E. Grist, K. Thompson and
A. D. Woodhead, "Wavelengths effective in induction of Malignant
Melanoma", PNAS _90_, 6666, 1993.

[Skolnick] A. Skolnick,  "Is ozone loss to blame for melanoma
upsurge?" JAMA, _265_, 3218, June 26 1991.

[Smith et al.] R. Smith, B. Prezelin, K. Baker, R. Bidigare, N.
Boucher, T. Coley, D. Karentz, S. MacIntyre, H. Matlick, D.
Menzies, M. Ondrusek, Z. Wan, and K. Waters, "Ozone depletion:
Ultraviolet radiation and phytoplankton biology in antarctic
waters", _Science_ _255_, 952, 1992.

[Tevini and Teramura] M. Tevini and A. H. Teramura, "UV-B effects
on terrestrial plants", _Photochemistry and Photobiology_, _50_,
479, 1989. (This issue contains a number of other papers dealing
with biological effects of UV-B radiation.)