1) How do we balance the good and bad effects
of sunlight on human health? In general, moderate exposure to sunlight
in the course of everyday life is not detrimental. This basic exposure
evidently allows us to function normally, and it proves to be sufficient
to maintain an adequate level of vitamin D (in combination with our dietary
intake). While sunlight is important for physical health it also causes
various adverse health effects such as skin cancer, ageing of the skin,
eye disorders and suppression of the immune system. It is clear that excessive
UV exposure should be avoided to minimise the risk of development of such
2) How strong is the evidence that
UV-B radiation causes skin cancer in humans? The evidence is strong. The earliest experimental
evidence that UV-B radiation causes skin cancer was acquired with animals;
in humans there was a clear association between sun exposure and skin cancer,
but that did not point specifically to UV-B. In recent years the advancement
of molecular biology has provided us with analyses that produce direct
evidence that genetic alterations found in human skin carcinomas are indeed
caused by UV-B radiation.
3) Should one have all moles removed
to decrease the risk of skin cancer? No, there is no evidence to suggest that
removing all of the moles would reduce the risk of skin cancer. However,
it is important to be alert to atypical moles, especially those exhibiting
changes in appearance (in colour or at the edges), and to screen those
individuals that are known to run a high risk, either from a family history
of melanoma mortality or of atypical moles.
4) Do sunglasses protect against cataracts? Sunglasses that markedly reduce the UV-exposure
of the eyes will reduce UV damage, such as cataracts. The best protection
is achieved by a combination of UV-absorbing glasses and a shielding against
light coming into the eyes from the sides. However, some sunglasses may
not effectively block UV radiation and eye damage may occur.
of Exposure to UV-B Radiation
5) Is the UV amount one receives as
a child important even in later years? Yes. Children should not be overexposed
to UV radiation: sunbathing should be strongly discouraged. UV exposure,
and especially sunburns, in early life can substantially increase the skin
cancer risk later in life (especially the risk of basal cell carcinoma
Even if the risk is related to total accumulated
exposure, as appears to be the case for a part of the non-melanocytic skin
cancers (SCC), exposures early in life still may carry a greater risk.
There is a long lag time, typically of several decades, between exposure
and the development of a tumour. Therefore, early exposures have a greater
probability in resulting in a tumour.
Animals at Risk?
6) Are hair-covered animals
at any risk?
Yes. Skin cancer is found in almost all animals
that have been studied in the long-term, for example, cattle, goats, sheep,
cats, dogs, guinea pigs, rats, and mice. Direct effects of UV-B radiation
on body parts which are covered by thick hair are negligible. However,
even furred animals usually have exposed skin around mouth and nostrils,
and sometimes on some other parts of the body. These parts, unless they
are heavily pigmented, can be damaged by radiation.
7) Will penguins be affected by the
ozone hole? To our knowledge there are no studies
concerning UV-B effects on penguins. As their eyes are exposed to a lot
of UV due to the high reflectivity of snow and a marked enhancement during
the ozone hole, investigation into the impact on penguins is desirable.
The fact that penguins are visual predators, eating krill or fish in the
water column, would make any eye damage an important issue for survival.
8) Is UV-B radiation a factor in the
decline of frogs and other amphibians? Possibly. Amphibian populations are in
serious decline in many areas of the world, and scientists are seeking
explanations for this. Most amphibian population declines are probably
due to habitat destruction or habitat alteration. Some declines are probably
the result of natural population fluctuations. Other explanations for the
population declines, as well as the reductions in range of habitation,
include disease, pollution, atmospheric changes and introduced competitors
and predators. UV-B radiation is one agent that may act in conjunction
with other stresses to adversely affect amphibian populations. Field studies
in which embryos of frogs, toads, and salamanders were exposed to natural
sunlight or to sunlight with UV-B radiation removed have shown conflicting
results. Some studies resulted in increased embryonic mortality after UV-B
exposure, whereas others show that current levels of UV-B radiation are
not detrimental. Factors such as water depth, water colour, and the dissolved
organic content of the water at the sites of egg deposition effectively
reduce UV-B penetration through the water and reduce exposure to UV-B radiation
at all life history stages. Biotic factors, such as jelly capsules around
eggs, melanin pigmentation of eggs, and colour of larvae and metamorphosed
forms, further reduce the effects of UV-B exposure.
9) Does water effectively
shield aquatic organisms from UV exposure?
No. Pure water is quite transparent to UV
radiation; a beam of UV-B radiation must travel over one-half kilometre
through pure water in order to be completely absorbed. Natural waters do
contain UV-absorbing substances, such as dissolved organic matter, that
partly shields aquatic organisms from UV-B, but the degree of shielding
varies widely from one water body to another. In clear ocean and lake waters
ecologically-significant levels of UV-B can penetrate to several tens of
meters; in contrast, in turbid rivers and wetlands UV-B may be completely
absorbed within the top few decimetres. Most organisms in aquatic ecosystems,
such as phytoplankton, live in the illuminated euphotic zone close to the
water surface where exposure to UV-B can occur. In particular, UV-B radiation
may damage those organisms that live at the surface of the water during
their early life stages.
10) What will be the effects of an increased
UV-B radiation on crop and forest yields? There are some UV-B-sensitive varieties
of crops that experience reductions in yield. However, there are also UV-B-tolerant
varieties, providing the opportunity to breed and genetically engineer
UV-B tolerant varieties. For commercial forests, tree breeding and genetic
engineering may be used to improve UV-B tolerance. For unmanaged or natural
forests, these methods are not an option. While many forest tree species
appear to be UV-B tolerant, there is some evidence that UV-B effects, sometimes
detrimental, can slowly accumulate from year to year. If this finding is
a general phenomenon, this would be cause for concern since it would greatly
complicate breeding efforts in commercial forests and negatively affect
11) Can plants protect themselves against
increased UV-B? Yes, partly. Plants already have reasonable
UV shielding; for most plants only a small proportion of the UV-B radiation
striking a leaf actually penetrates very far into the inner tissues. Also,
when exposed to an enhanced UV-B level, many species of plants can increase
the UV-absorbing pigments in their tissues. Other adaptations include increased
thickness of leaves which reduces the proportion of inner tissues exposed
to UV-B radiation. Several repair mechanisms also exist in plants, as is
the case for other organisms. This includes repair systems for DNA damage
or oxidant injury. The net damage a plant experiences is the result of
the balance among damage, protection and repair processes. For many plants,
the net damage is negligible.
12) Is the increase in UV-B radiation caused
by ozone depletion equivalent to that incurred by moving several hundred
kilometres towards the equator? Yes, but this comparison does not nullify
the serious impact of an ozone depletion, as is sometimes suggested by
questions like this. The suggestion is based on a fallacy, namely, comparing
a personal risk perception with the effect on a population. An elevation
of say 10% in risk would not be noticeable for the person involved. For
a population it is quite different. With regard to skin cancer such an
increase could mean 100-200 extra cases a year per million people. This
would be an important public health effect. However, movements of entire
populations, or even ecosystems, do not usually occur in a human lifetime,
and the comparison is therefore inappropriate.
13) Can organisms adjust to a changed
UV environment? Yes, many organisms can respond physiologically
with changes such as development of UV screening compounds and additional
layers of protective tissues. However, there are genetic limitations to
the degree to which these physiological adjustments can take place for
each organism. Some can adjust more effectively than others. Over long
periods of time and several generations of populations, there is the possibility
that genetic adaptation can develop as well. However, in organisms with
moderately long life spans and small population sizes, the genetic adaptation
is likely to be very slow.
14) Does ozone depletion pose any danger
in the tropics? Probably not. Increases in UV-B radiation
are unlikely, since no significant trend in stratospheric ozone has been
observed in the tropics. However, viewing the biosphere as a unit, there
may be indirect effects of ozone depletion at other latitudes on tropical
ecosystems. If ozone were to be depleted in the tropics, this would constitute
a serious danger because of the naturally occurring high levels of UV-B
radiation due to the high solar angles and already relatively low normal
stratospheric ozone levels.
15) Do we need to worry about relatively
small increases in UV-B due to ozone depletion, when natural variability
is so much larger? Yes. The change in UV-B from ozone depletion
is systematically upward. The natural variability (e.g., from time of day,
or clouds) can be larger, but goes in both directions, up and down. While
the evidence for ozone depletion is very strong, there is little evidence
for long-term changes in cloud cover.
Many detrimental effects of UV-B are proportional
to the cumulative UV-B exposure. For example, skin cancer results from
the total exposure accumulated over many years under both sunny and cloudy
conditions. Any systematic increase in UV-B radiation will increase incidence
among a population (as well as individual risk) regardless of the natural
variability of the UV-B radiation.
16) Does one get higher UV exposures
at higher elevations? Yes. Higher elevations have less atmosphere
overhead, as evidenced by the thinner air and lower atmospheric pressure.
The increase in sun-burning UV radiation is typically about 5-10% for each
kilometre of elevation, the exact number depending on the specific wavelength,
solar angle, reflections, and other local conditions. Frequently, other
factors besides thickness of the atmosphere cause even larger differences
in UV radiation between elevations. Snow is more common at higher elevations,
and reflections from it can lead to very large increases in exposure.
Lower locations tend to have more haze
and more polluted atmosphere which can block some UV radiation.
17) Does air pollution protect one
from UV-B radiation? Yes, but at a high price. Air pollution
is generally undesirable due to the numerous other serious problems associated
with it, including respiratory illness, eye irritation, and damage to vegetation.
While most of the atmospheric ozone resides in the stratosphere, some ozone
is also made in the troposphere by the chemical interactions of pollutants
such as nitrogen oxides and hydrocarbons. This tropospheric ozone is a
component of the photochemical smog found in many polluted areas. Airborne
particles (smoke, dust, sulphate aerosols) can also block UV radiation,
but they can also increase the amount of scattered light (haze) and therefore
increase the UV exposure of side-facing surfaces (e.g., face, eyes).
No single value can be given for the amount
of UV-B reduction by pollution, because pollution events tend to be highly
variable and local. Comparisons of measurements made in industrialised
regions of the Northern Hemisphere (e.g., central Europe) and in very clean
locations at similar latitudes in the Southern Hemisphere (e.g., New Zealand)
suggest pollution-related UV-B reductions can be important.
Skies vs. Cloud Cover
18) Can changes in cloudiness cause larger
UV changes than ozone depletion? Long-term trends in cloud type and amount
are largely unknown due to the relatively short data record of comprehensive
cloud observations, and the high variability of clouds on inter-annual
and longer time scales. Some evidence exists showing that, at least over
the time span of satellite-based ozone measurements, changes in cloud cover
have been much less important than stratospheric ozone reductions in causing
surface UV changes.
19) Are the risks of ultraviolet (UV)
exposure at the beach less on a cloudy day? Not necessarily. The effect of clouds
on UV radiation is as varied as the clouds themselves. Fully overcast skies
lead to reductions in surface UV irradiance. On average, scattered or broken
clouds also cause reductions, but short-term or localised UV levels can
be larger than for cloud-free skies if direct sunlight is also present.
Clouds tend to randomise the directions of the incoming radiation (because
of scattering) so that a hat may provide less protection on a cloudy day
relative to a clear day.
Furthermore, people often change their
behaviour on cloudy days. If they spend more time out in the open, or forego
the use of sunscreen, they may end up with a very bad sunburn. In general,
less UV radiation is received per hour under an overcast sky than under
a clear sky, but extending one's stay at the beach may easily compensate
for this effect. A completely cloud-covered sky may still transmit substantial
amounts of UV-B radiation. In principle, any amount of UV-B radiation exposure
contributes to the skin cancer risk.
20) Will sunscreens
protect one from harmful effects of increased UV-B radiation? Not always. Sunscreens applied to human
skin limit the penetration of UV radiation into the skin, and thus sunburn
can be prevented. Sunscreens were primarily developed for this purpose.
The effectiveness of sunscreens in protecting against skin cancer and immune
suppressions is under debate. Any effectiveness in these respects may well
be lost if the sunscreen is used to stay out in the sunlight longer than
would be done without the sunscreen. It should also be kept in mind that
there are other ways to protect the skin. These include staying out of
the sunlight during the hours when the UV-B is maximal around solar noon,
seeking the shade, wearing clothes, and especially hats.
21) Will getting a suntan help prevent
skin cancer? No. There is no evidence that getting
a suntan will help prevent skin cancer. The UV exposure needed to acquire
the tan adds to the skin cancer risk. The fact that one is able to tan
well does, however, signify that the personal risk is lower (by a factor
of 2 to 3) than for people who do not tan. Naturally dark-skinned people
have a built-in protection of their skin against sunlight.
22) Is tanning with UV lamps safer
than with sunlight? No. The risks are approximately equal.
For some time it was hoped that UV lamps could be made safer by making
more use of long-wavelength (UV-A) radiation. That type of radiation is
much less carcinogenic than the shorter-wavelength UV-B radiation, but
one needs more UV-A than UV-B for acquiring a tan.
23) Has the benefit
of the Montreal Protocol been worth the cost? Yes. Several attempts have been made to
investigate the economic impacts of the problem of a depleted ozone layer.
Such attempts meet with many problems. There are good reasons for concern
for effects on humans, animals, plants and materials, but most of these
cannot be estimated in quantitative terms. Calculating the economic impact
of such effects is uncertain. Moreover, economic terms are applicable only
to some of the effects, such as the cost of medical treatments, and the
loss of production in fisheries and agriculture, and damage to materials;
but what is the cost equivalent of suffering, of a person becoming blind
or dying, or the loss of a rare plant or animal species?
In spite of all these difficulties, attempts
have been made. The most comprehensive example is a study initiated by
Environment Canada for the 10th anniversary of the Montreal Protocol on
Substances that Deplete the Ozone Layer. In this study, 'Global Costs and
Benefits of the Montreal Protocol' (1997), the costs were calculated for
all measures taken internationally to protect the ozone layer, such as
replacement of technologies using ozone-depleting substances. The benefits
are the total value of the damaging effects avoided in this way. The total
costs of the measures taken to protect the ozone layer were calculated
to be 235 billion US (1997) dollars. The effects avoided world-wide, though
far less quantifiable, were estimated to be almost twice that amount. This
latter estimate included only reduced damage to fisheries, agriculture
and materials. The cataracts and skin cancers, as well as the potential
associated fatalities avoided, were listed as additional benefits, and
not expressed in economic terms.