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- How do we balance the good and bad
effects of sunlight on human health?
- How strong is the evidence that
UV-B radiation causes skin cancer in humans?
- Should one have all moles removed
to decrease the risk of skin cancer?
- Do sunglasses protect against
cataracts?
- Is the UV amount one receives as
a child important even in later years?
- Are hair-covered animals at any
risk?
- Will penguins be affected by the
ozone hole?
- Is UV-B radiation a factor in the
decline of frogs and other amphibians?
- Does water effectively shield aquatic
organisms from UV exposure?
- What will be the effects of an
increased UV-B radiation on crop and forest yields?
- Can plants protect themselves against
increased UV-B?
- Is the increase in UV-B radiation
caused by ozone depletion equivalent to that incurred by moving several
hundred kilometres towards the equator?
- Can organisms adjust to a changed
UV environment?
- Does ozone depletion pose any danger
in the tropics?
- Do we need to worry about relatively
small increases in UV-B due to ozone depletion, when natural variability
is so much larger?
- Does one get higher UV exposures
at higher elevations?
- Does air pollution protect one
from UV-B radiation?
- Can changes in cloudiness cause
larger UV changes than ozone depletion?
- Are the risks of ultraviolet (UV)
exposure at the beach less on a cloudy day?
- Will sunscreens protect one from
harmful effects of increased UV-B radiation?
- Will getting a suntan help prevent
skin cancer?
- Is tanning with UV lamps safer
than with sunlight?
- Has the benefit of the Montreal
Protocol been worth the cost?
Health Effects
- 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, aging 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 disorders.
- 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.
- 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.
- 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.
Duration of Exposure to UV-B Radiation
- 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 and melanoma).
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.
Are Animals at Risk?
- 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.
- 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.
- 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.
Aquatic Life
- 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.
Terrestrial Plant Life
- 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 natural forests.
- 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.
Location-specific Issues
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
Clear Skies vs. Cloud Cover
- 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.
- 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.
Sunbathing
- 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.
- 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.
- 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.
Economic Consequences
- 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.
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