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United
Nations Environment Programme |
Frequently Asked Questions
What is ozone and where is it in the atmosphere?
Ozone is a gas that is naturally present in our atmosphere. Each ozone molecule
contains three atoms of oxygen and is denoted chemically as O3. Ozone is found
primarily in two regions of the atmosphere. About 10% of atmospheric ozone is
in the troposphere, the region closest to Earth (from the surface to about 10-16
kilometers (6-10 miles)). The remaining ozone (90%) resides in the stratosphere,
primarily between the top of the troposphere and about 50 kilometers (31 miles)
altitude. The large amount of ozone in the stratosphere is often referred to
as the “ozone layer.”
How is ozone formed in the atmosphere?
Ozone is formed throughout the atmosphere in multistep chemical processes that
require sunlight. In the stratosphere, the process begins with the breaking
apart of an oxygen molecule (O2 ) by ultraviolet radiation from the Sun. In
the lower atmosphere (troposphere), ozone is formed in a different set of chemical
reactions involving hydrocarbons and nitrogen-containing gases.
Is there an ozone hole over the Arctic?
Significant reductions in ozone content in the stratosphere above the Arctic
have been observed during the late winter and early spring (January-March) in
6 of the last 9 years. However, these reductions, typically 20-25%, are much
smaller than those observed currently each spring over the Antarctic (the ozone
hole).
The difference between ozone content in the two polar regions (see figure below)
is caused by the dissimilar weather patterns. The Antarctic continent is a very
large land mass surrounded by oceans. This symmetrical condition produces very
low stratospheric temperatures within a meteorologically isolated region, the
so-called polar vortex, which extends from about 65°S to the pole. The cold
temperatures lead in turn to the formation of clouds, known as polar stratospheric
clouds. These clouds provide chemical changes that promote production of chemically
active chlorine and bromine that rapidly destroy ozone. The conditions that
maintain elevated levels of chemically active chlorine and bromine persist into
September and October in Antarctica, when sunlight returns over the region to
initiate ozone depletion.
The winter meteorological conditions in the Northern Hemisphere, just like in
the Southern Hemisphere, lead to the formation of an isolated region bounded
by strong winds, in which the temperature is also cold enough for polar stratospheric
clouds to form. However, the geographic symmetry about the North Pole is less
than about the South Pole. As a result, large-scale weather systems disturb
the wind flow, making it less stable over the Arctic region than over the Antarctic
continent. These disturbances prevent the temperature in the Arctic stratosphere
from being as cold as in the Antarctic stratosphere, and fewer polar stratospheric
clouds are therefore formed. Nevertheless, chemically active chlorine and bromine
compounds are also formed over the Arctic, as they are over Antarctica, from
reactions at the surface of the clouds. But the cold conditions rarely persist
into March, when sufficient sunlight is available to initiate large ozone depletion.
In recent years, there has been a string of unusually cold winters in the Arctic,
compared with those in the preceding 30 years. The cold and persistent conditions
have led to enhanced ozone depletion, since the atmospheric concentrations of
ozone-depleting gases have also been relatively large during these years. However,
the cause of the observed change in meteorological conditions is not yet understood.
Such conditions might persist over the coming years, further enhancing ozone
depletion. But it is also possible that, in the next few years, they could revert
to conditions characteristic of a decade ago. In the latter case, chemical ozone
depletion in the Arctic would be expected to diminish.
Therefore, although there has been significant ozone depletion in the Arctic
in recent years, it is difficult to predict what may lie ahead, because the
future climate of the Arctic stratosphere cannot be predicted with confidence.
When did the Antarctic ozone hole first appear?
The Springtime Antarctic ozone hole is a new phenomenon that appeared in the
early 1980s.
The observed average amount of ozone during September, October, and November
over the British Antarctic Survey station at Halley, Antarctica, first revealed
notable decreases in the early 1980s, compared with the preceding data obtained
starting in 1957. The ozone hole is formed each year when there is a sharp decline
(currently up to 60%) in the total ozone over most of Antarctica for a period
of about three months (September-November) during spring in the Southern Hemisphere.
Late-summer (January-March) ozone amounts show no such sharp decline in the
1980s and 1990s. Observations from three other stations in Antarctica and from
satellite-based instruments reveal similar decreases in springtime amount of
ozone overhead. Balloon-borne ozone instruments show dramatic changes in the
way ozone is distributed with altitude. As the figure below from the Syowa site
shows, almost all of the ozone is now depleted at some altitudes as the ozone
hole forms each springtime, compared to the normal ozone profile that existed
before 1980. As explained in an earlier question (page *), the ozone hole has
been shown to result from destruction of stratospheric ozone by gases containing
chlorine and bromine, whose sources are mainly human-produced halocarbon gases.
Before the stratosphere was affected by human-produced chlorine and bromine,
the naturally occurring springtime ozone levels over Antarctica were about 30-40%
lower than springtime ozone levels over the Arctic. This natural difference
between Antarctic and Arctic conditions was first observed in the late 1950s
by Dobson. It stems from the exceptionally cold temperatures and different winter
wind patterns within the Antarctic stratosphere as compared with the Arctic.
This is not at all the same phenomenon as the marked downward trend in total
ozone in recent years.
Changes in stratospheric meteorology cannot explain the ozone hole. Measurements
show that wintertime Antarctic stratospheric temperatures of past decades had
not changed prior to the development of the ozone hole each September. Ground,
aircraft, and satellite measurements have provided, in contrast, clear evidence
of the importance of the chemistry of chlorine and bromine originating from
human-made, compounds in depleting Antarctic ozone in recent years.
Why has an ozone hole appeared over Antarctica
when CFCs and Halons are released mainly in the Northern Hemisphere?
The Earth's atmosphere is continuously stirred over the globe by winds. As a
result, ozone-depleting gases get mixed throughout the atmosphere, including
Antarctica, regardless of where they are emitted. The special meteorological
conditions in Antarctica cause these gases to be more effective there in depleting
ozone compared to anywhere else.
Human emissions of chlorofluorocarbons (CFCs) and halons (bromine-containing
gases) have occurred mainly in the Northern Hemisphere. About 90% have been
released in the latitudes corresponding to Europe, Russia, Japan, and North
America. Gases such as CFCs and halons, which are insoluble in water and relatively
unreactive, are mixed within a year or two throughout the lower atmosphere.
The CFCs and halons in this well-mixed air rise from the lower atmosphere into
the stratosphere mainly in tropical latitudes. Winds then move this air poleward
- both north and south - from the tropics, so that air throughout the global
stratosphere contains nearly equal amounts of chlorine and bromine.
In the Southern Hemisphere, the South Pole is part of a very large land mass
(Antarctica) that is completely surrounded by ocean. This symmetry is reflected
in the meteorological conditions that allow the formation in winter of a very
cold region in the stratosphere over the Antarctic continent, isolated by a
band of strong winds circulating around the edge of that region. The very low
stratospheric temperatures lead to the formation of clouds (polar stratospheric
clouds) that are responsible for chemical changes that promote production of
chemically active chlorine and bromine. This chlorine and bromine activation
then leads to rapid ozone loss when sunlight returns to Antarctica in September
and October of each year, which then results in the Antarctic ozone hole. As
the figure below depicts, the magnitude of the ozone loss has generally grown
through the 1980s as the amount of human-produced ozone-depleting compounds
has grown in the atmosphere.
Similar conditions do not exist over the Arctic. The wintertime temperatures
in the Arctic stratosphere are not persistently low for as many weeks as over
Antarctica, which results in correspondingly less ozone depletion in the Arctic
(see the next question).
Does most of the Chlorine in the
stratosphere come from human or natural sources?
Most of the chlorine in the stratosphere is there as a result of human activities,
as the figure below illustrates.
Many compounds containing chlorine are released at the ground. Those that dissolve in water cannot reach stratospheric altitudes in significant amounts because they are "washed out" of the atmosphere in rain or snow. For example, large quantities of chlorine are released from evaporated ocean spray as sea salt (sodium chloride) particles. However, because sea salt dissolves in water, this chlorine is taken up quickly in clouds or in ice, snow, or rain droplets and does not reach the stratosphere. Another ground-level source of chlorine is from its use in swimming pools and as household bleach. When released, this chlorine is rapidly converted to forms that dissolve in water and therefore are removed from the lower atmosphere. Such chlorine never reaches the stratosphere in significant amounts. Volcanoes can emit large quantities of hydrogen chloride, but this gas is rapidly converted to hydrochloric acid, which dissolves in rain water, ice, and snow and does not reach the stratosphere. Even in explosive volcanic plumes that rise high in the atmosphere, nearly all of the hydrogen chloride is removed by precipitation before reaching stratospheric altitudes. Finally, although the exhaust from the Space Shuttle and from some rockets does inject some chlorine directly into the stratosphere, the quantities are very small (less than 1% of the annual input from halocarbons in the present stratosphere).
In contrast, the major ozone-depleting human-produced halocarbons - such as chlorofluorocarbons (CFCs) and carbon tetrachloride (CCl4) - are not soluble in water, do not react with snow or other natural surfaces, and are not broken down chemically in the lower atmosphere. Therefore, these and other human-produced substances containing chlorine do reach the stratosphere.
Several pieces of evidence combine to establish human-produced halocarbons as the primary source of stratospheric chlorine. First, measurements have shown that the chlorinated species that rise to the stratosphere are primarily manufactured compounds [mainly CFCs, carbon tetrachloride, methyl chloroform, and the hydrochlorofluorocarbon (HCFC) substitutes for CFCs], together with small amounts of hydrochloric acid (HCl) and methyl chloride (CH3Cl), which are partly natural in origin. Second, researchers have measured nearly all known gases containing chlorine in the stratosphere. They have found that the emissions of the human-produced halocarbons, plus the much smaller contribution from natural sources, could account for all of the stratospheric chlorine. Third, the increase in total stratospheric chlorine measured between 1980 and 1998 corresponds to the known increases in concentrations of human-produced halocarbons during that time.
Why do we care about atmospheric
ozone?
Ozone in the stratosphere absorbs some of the Sun’s biologically harmful
ultraviolet radiation. Because of this beneficial role, stratospheric ozone
is considered “good ozone.” In contrast, ozone at Earth’s
surface that is formed from pollutants is considered “bad ozone”
because it can be harmful to humans and plant and animal life. Some ozone occurs
naturally in the lower atmosphere where it is beneficial because ozone helps
remove pollutants from the atmosphere.
Is total ozone uniform over the globe?
No, the total amount of ozone above the surface of Earth varies with location
on time scales that range from daily to seasonal. The variations are caused
by stratospheric winds and the chemical production and destruction of ozone.
Total ozone I s generally lowest at the equator and highest near the poles because
of the seasonal wind patterns in the stratosphere.
How is ozone measured in the atmosphere?
The amount of ozone in the atmosphere is measured by instruments on the ground
and carried aloft in balloons, aircraft, and satellites. Some measurements involve
drawing air into an instrument that contains a system for detecting ozone. Other
measurements are based on ozone’s unique absorption of light in the atmosphere.
In that case, sunlight or laser light is carefully measured after passing through
a portion of the atmosphere containing ozone.
Can natural changes such as the
sun's output and volcanic eruptions be responsible for the observed changes
in ozone? Although there are natural forces that cause fluctuations
in ozone amounts, there is no evidence that natural changes are contributing
significantly to the observed long-term trend of decreasing ozone.
The formation of stratospheric ozone is initiated by ultraviolet (UV) light
coming from the Sun. As a result, the Sun's output affects the rate at which
ozone is produced. The Sun's energy release (both as UV light and as charged
particles such as electrons and protons) does vary, especially over the well-known
11-year sunspot cycle. Observations over several solar cycles (since the 1960s)
show that total global ozone levels vary by 1-2% from the maximum to the minimum
of a typical cycle. However, changes in the Sun's output cannot be responsible
for the observed long term changes in ozone, because the ozone downward trends
are much larger than 1-2%. As the figure below shows, since 1978 the Sun's energy
output has gone through maximum values in about 1980 and 1991 and minimum values
in about 1985 and 1996. It is now increasing again toward its next maximum around
the year 2002. However, the trend in ozone was downward throughout that time.
The ozone trends presented in this and previous international scientific assessments
have been obtained by evaluating the long-term changes in ozone after accounting
for the solar influence (as has been done in the figure below).
Major, explosive volcanic eruptions can inject material directly into the ozone
layer. Observations and model calculations show that volcanic particles cannot
on their own deplete ozone. It is only the interaction of human-produced chlorine
with particle surfaces that enhances ozone depletion in today's atmosphere.
Specifically, laboratory measurements and observations in the atmosphere have
shown that chemical reactions on and within the surface of volcanic particles
injected into the lower stratosphere lead to enhanced ozone destruction by increasing
the concentration of chemically active forms of chlorine that arise from the
human-produced compounds like the chlorofluorocarbons (CFCs). The eruptions
of Mt. Agung (1963), Mt. Fuego (1974), El Chichón (1982) and particularly
Mt. Pinatubo (1991) are examples. The eruption of Mt. Pinatubo resulted in a
30- to 40-fold increase in the total surface area of particles available for
enhancing chemical reactions. The effect of such natural events on the ozone
layer is then dependent on the concentration of chlorine-containing molecules
and particles available in the stratosphere, in a manner similar to polar stratospheric
clouds. Because the particles are removed from the stratosphere in 2 to 5 years,
the effect on ozone is only temporary, and such episodes cannot account for
observed long-term changes. Observations and calculations indicate that the
record-low ozone levels observed in 1992-1993 reflect the importance of the
relatively large number of particles produced by the Mt. Pinatubo eruption,
coupled with the relatively higher amount of human-produced stratospheric chlorine
in the 1990s compared to that at times of earlier volcanic eruptions.
How can Chlorofluorocarbons (CFCs) get to
the stratosphere if they're heavier than air? CFCs reach the stratosphere
because the Earth's atmosphere is always in motion and mixes the chemicals added
into it.
CFC molecules are indeed several times heavier than air. Nevertheless, thousands
of measurements from balloons, aircraft, and satellites demonstrate that the
CFCs are actually present in the stratosphere. This is because winds and other
air motions mix the atmosphere to altitudes far above the top of the stratosphere
much faster than molecules can settle according to their weight. Gases such
as CFCs that do not dissolve in water and that are relatively unreactive in
the lower atmosphere are mixed relatively quickly and therefore reach the stratosphere
regardless of their weight.
Measured changes in concentration of constituents versus altitude teaches us
more about the fate of compounds in the atmosphere. For example, the two gases
carbon tetrafluoride (CF4, produced mainly as a by-product of the manufacture
of aluminum) and CFC-11 (CCl3F, used in a variety of human activities) are both
heavier than air.
Carbon tetrafluoride is completely unreactive at altitudes up to at least 50
kilometers in the atmosphere. Measurements show it to be nearly uniformly distributed
throughout the atmosphere, as illustrated in the figure below. There have been
measurements over the past two decades of several other completely unreactive
gases, both lighter than air (neon) and heavier than air (argon and krypton),
that show that they also mix upward through the stratosphere regardless of their
weight.
CFC-11 is unreactive in the lower atmosphere and is similarly uniformly mixed
there, as shown in the figure. However, the abundance of CFC-11 decreases as
the gas reaches higher altitudes, because it is broken down by high-energy solar
ultraviolet radiation. Chlorine released from this breakdown of CFC-11 and other
CFCs remains in the stratosphere for several years, where every chlorine atom
destroys many thousands of molecules of ozone.
What is the evidence that stratospheric ozone
is destroyed by Chlorine and Bromine? Numerous laboratory investigations
and analyses of worldwide measurements made in the stratosphere have demonstrated
that chlorine- and bromine-containing chemicals destroy ozone molecules.
Research studies in the laboratory show that chlorine (Cl) reacts very rapidly
with ozone. They also show that the reactive chemical chlorine monoxide (ClO)
formed in that reaction can undergo further processes that regenerate the original
chlorine, allowing the sequence to be repeated very many times (a chain reaction).
Similar reactions also take place between bromine and ozone.
But do these ozone-destroying reactions occur in the "real world"?
All the accumulated scientific experience demonstrates that the same chemical
reactions do take place in nature. Many other reactions (including those of
other chemical species) are often also taking place simultaneously in the stratosphere.
This makes the connections among the changes difficult to untangle. Nevertheless,
whenever chlorine (or bromine) and ozone are found together in the stratosphere,
the ozone-destroying reactions are taking place.
Sometimes a small number of chemical reactions are so dominant in the natural
circumstance that the connections are almost as clear as in laboratory experiments.
Such a situation occurs in the Antarctic stratosphere during the springtime
formation of the ozone hole. Independent measurements made by instruments from
the ground and from balloons, aircraft, and satellites have provided a detailed
understanding of the chemical reactions in the Antarctic stratosphere.
Large areas reach temperatures so low (less than -80°C, or -112°F) that
stratospheric clouds form, which is a rare occurrence, except during the polar
winters. These polar stratospheric clouds allow chemical reactions that transform
chlorine species from those that do not cause ozone depletion into those that
do. Among the latter is chlorine monoxide, which initiates ozone destruction
in the presence of sunlight. The amount of reactive chlorine in such regions
is therefore much higher than that observed in the middle latitudes, which leads
to much faster chemical ozone destruction. The chemical reactions occurring
in the presence of these clouds are now well understood from studies under laboratory
conditions that mimic those found naturally in the atmosphere.
Scientists have repeatedly observed a large number of chemical species over
Antarctica since 1986. Among the chemicals measured were ozone and chlorine
monoxide, which is the reactive chemical identified in the laboratory as one
of the participants in the ozone-destroying chain reactions. The satellite maps
shown in the figure below relate the accumulation of chlorine monoxide observed
over Antarctica and the subsequent ozone depletion that occurs rapidly in a
few days over very similar areas.
Similar reactions involving chlorine and bromine have also been shown to occur
during winter and spring in the Arctic polar regions, which leads to some chemical
depletion of ozone in that region. Because the Arctic is not usually as persistently
cold as the Antarctic, fewer stratospheric clouds form, and therefore there
is less ozone depletion in the Arctic, which is the subject of a later question.
What are the principal steps in stratospheric
ozone depletion caused by human activities?
What emissions from human activities lead to ozone depletion?
What are the reactive halogen gases that destroy
stratospheric ozone?
What are the chlorine and bromine reactions that
destroy stratospheric ozone?
Why has an “ozone hole” appeared over Antarctica
when ozone-depleting gases are present throughout the stratosphere?
What are the principal steps
in stratospheric ozone depletion caused by human activities?
The initial step in the depletion of stratospheric ozone by human activities
is the emission of ozone-depleting gases containing chlorine and bromine at
Earth’s surface. Most of these gases accumulate in the lower atmosphere
because they are unreactive and do not dissolve readily in rain or snow. Eventually,
the emitted gases are transported to the stratosphere where they are converted
to more reactive gases containing chlorine and bromine. These more reactive
gases then participate in reactions that destroy ozone. Finally, when air returns
to the lower atmosphere, these reactive chlorine and bromine gases are removed
from Earth’s atmosphere by rain and snow.
What emissions from human activities lead to
ozone depletion?
Certain industrial processes and consumer products result in the atmospheric
emission of “halogen source gases.” These gases contain chlorine
and bromine atoms, which are known to be harmful to the ozone layer. For example,
the chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), once used
in almost all refrigeration and air conditioning systems, eventually reach the
stratosphere where they are broken apart to release ozone-depleting chlorine
atoms. Other examples of human-produced ozone-depleting gases are the “halons,”
which are used in fire extinguishers and which contain ozone-depleting bromine
atoms. The production and consumption of all principal halogen source gases
by human activities are regulated worldwide under the Montreal Protocol.
What are the reactive halogen gases that
destroy stratospheric ozone?
Emissions from human activities and natural processes are large sources of chlorine-
and bromine-containing gases for the stratosphere. When exposed to ultraviolet
radiation from the Sun, these halogen source gases are converted to more reactive
gases also containing chlorine and bromine. Important examples of the reactive
gases that destroy stratospheric ozone are chlorine monoxide (ClO) and bromine
monoxide (BrO). These and other reactive gases participate in “catalytic”
reaction cycles that efficiently destroy ozone. Volcanoes can emit some chlorine-containing
gases, but these gases are ones that readily dissolve in rainwater and ice and
are usually “washed out” of the atmosphere before they can reach
the stratosphere.
What are the chlorine and bromine reactions
that destroy stratospheric ozone?
Reactive gases containing chlorine and bromine destroy stratospheric ozone in
“catalytic” cycles made up of two or more separate reactions. As
a result, a single chlorine or bromine atom can destroy many hundreds of ozone
molecules before it reacts with another gas, breaking the cycle. In this way,
a small amount of reactive chlorine or bromine has a large impact on the ozone
layer. Special ozone destruction reactions occur in polar regions because the
reactive gas chlorine monoxide reaches very high levels there in the winter/spring
season.
Why has an “ozone hole” appeared
over Antarctica when ozone-depleting gases are present throughout the stratosphere?
Ozone-depleting gases are present throughout the stratospheric ozone layer because
they are transported great distances by atmospheric air motions. The severe
depletion of the Antarctic ozone layer known as the “ozone hole”
forms because of the special weather conditions that exist there and nowhere
else on the globe. The very cold temperatures of the Antarctic stratosphere
create ice clouds called polar stratospheric clouds (PSCs). Special reactions
that occur on PSCs and the relative isolation of polar stratospheric air allow
chlorine and bromine reactions to produce the ozone hole in Antarctic springtime.
How severe is the depletion of the Antarctic
ozone layer?
Is there depletion of the Arctic ozone layer?
How large is the depletion of the global ozone layer?
Do changes in the sun and volcanic eruptions affect
the ozone layer?
How
severe is the depletion of the Antarctic ozone layer?
Severe depletion of the Antarctic ozone layer was first observed in the early
1980s. Antarctic ozone depletion is seasonal, occurring primarily in late winter
and spring (August-November). Peak depletion occurs in October when ozone is
often completely destroyed over a range of altitudes, reducing overhead total
ozone by as much as two-thirds at some locations. This severe depletion creates
the “ozone hole” in ../images of Antarctic total ozone made from
space. In most years the maximum area of the ozone hole usually exceeds the
size of the Antarctic continent.
Is there depletion of the Arctic ozone
layer?
Yes, significant depletion of the Arctic ozone layer now occurs in some years
in the late winter/spring period (January-April). However, the maximum depletion
is generally less severe than that observed in the Antarctic and is more variable
from year to year. A large and recurrent “ozone hole,” as found
in the Antarctic stratosphere, does not occur in the Arctic.
How large is the depletion of the global
ozone layer?
The ozone layer has been depleted gradually since 1980 and now is about an average
of 3% lower over the globe. The depletion, which exceeds the natural variations
of the ozone layer, is very small near the equator and increases with latitude
toward the poles. The large average depletion in polar regions is primarily
a result of the late winter/spring ozone destruction that occurs there annually.
Do changes in the sun and volcanic eruptions
affect the ozone layer?
Yes, factors such as changes in solar radiation, as well as the formation of
stratospheric particles after volcanic eruptions, do influence the ozone layer.
However, neither factor can explain the average decreases observed in global
total ozone over the last two decades. If large volcanic eruptions occur in
the coming decades, ozone depletion will increase for several years after the
eruption.
CONTROLLING OZONE-DEPLETING GASES
Are there regulations on production of ozone-depleting
gases?
Has the Montreal Protocol been successful in reducing
ozone-depleting gases in the atmosphere?
Are there regulations on production
of ozone-depleting gases?
Yes, the production of ozone-depleting gases is regulated under a 1987 international
agreement known as the “Montreal Protocol on Substances that Deplete the
Ozone Layer” and its subsequent Amendment and Adjustments. The Protocol,
now ratified by over 180 nations, establishes legally binding controls on the
national production and consumption of ozone-depleting gases. Production and
consumption of all principal halogen-containing gases by developed and developing
nations will be significantly reduced or phased out before the middle of the
21st century.
Has the Montreal Protocol been successful
in reducing ozone-depleting gases in the atmosphere?
Yes, as a result of the Montreal Protocol, the total abundance of ozone-depleting
gases in the atmosphere has begun to decrease in recent years. If the nations
of the world continue to follow the provisions of the Montreal Protocol, the
decrease will continue throughout the 21st century. Some individual gases such
as halons and hydrochlorofluorocarbons (HCFCs) are still increasing in the atmosphere,
but will begin to decrease in the next decades if compliance with the Protocol
continues. By mid-century, the effective abundance of ozone-depleting gases
should fall to values present before the Antarctic “ozone hole”
began to form in the early 1980s.
IMPLICATIONS OF OZONE DEPLETION
Does ozone depletion cause climate change?
How severe is the ozone depletion now?
Is the ozone layer expected to recover?
Does depletion of the ozone layer increase
ground-level ultraviolet radiation?
Is depletion of the ozone layer the principal cause
of climate change?
Is the depletion of the ozone layer leading
to an increase in ground-level Ultraviolet radiation?
Does ozone depletion cause climate change?
Ozone depletion and climate change are linked in a number of ways, but ozone
depletion is not a major cause of climate change.
Atmospheric ozone has two effects on the temperature balance of the Earth. It
absorbs solar ultraviolet radiation, which heats the stratosphere. It also absorbs
infrared radiation emitted by the Earth's surface, effectively trapping heat
in the troposphere. Therefore, the climate impact of changes in ozone concentrations
varies with the altitude at which these ozone changes occur. The major ozone
losses that have been observed in the lower stratosphere due to the human-produced
chlorine- and bromine-containing gases have a cooling effect on the Earth's
surface. On the other hand, the ozone increases that are estimated to have occurred
in the troposphere because of surface-pollution gases have a warming effect
on the Earth's surface, thereby contributing to the "greenhouse" effect.
In comparison to the effects of changes in other atmospheric gases, the effects
of both of these ozone changes are difficult to calculate accurately. In the
figure below, the upper ranges of possible effects for the ozone changes are
indicated by the open bars, and the lower ranges are indicated by the solid
bars.
As shown in the figure, increases in carbon dioxide is the major contributor
to climate change. Carbon dioxide concentrations are increasing in the atmosphere
primarily as the result of the burning of coal, oil, and natural gas for energy
and transportation. The atmospheric abundance of carbon dioxide is currently
about 30% above what it was 150 years ago. The relative impacts on climate of
various other "greenhouse" gases are also shown on the figure.
There is an additional factor that indirectly links ozone depletion to climate
change; namely, many of the same gases that are causing ozone depletion are
also contributing to climate change. These gases, such as the chlorofluorocarbons
(CFCs), are green-house gases, absorbing some of the infrared radiation emitted
by the Earth's surface, thereby effectively heating the Earth's surface.
Conversely, changes in the climate of the Earth could affect the behavior of
the ozone layer, because ozone is influenced by changes in the meteorological
conditions and by changes in the atmospheric composition that could result from
climate change. The major issue is that the stratosphere will most probably
cool in response to climate change, therefore preserving over a longer time
period the conditions that promote chlorine-caused ozone depletion in the lower
stratosphere, particularly in polar regions. At present, the amplitude and extent
of such a cooling, and therefore the delay in the recovery of the ozone layer,
still have to be assessed.
How severe is the ozone depletion now?
Stratospheric ozone depletion, caused by increasing concentrations of human-produced
chemicals, has increased since the 1980s. The springtime loss in Antarctica
is the largest depletion. Currently, in non-polar regions, the ozone layer has
been depleted up to several percent compared with that of two decades ago.
As the figure below indicates, the magnitude of ozone depletion varies between
the regions of the Earth. For example, there has been little or no ozone depletion
in the tropics (about 20 degrees north and south of the equator). The magnitude
of the depletion also depends on the season. From 1979 to 1997, the observed
losses in the amount of ozone overhead have totaled about 5-6% for northern
midlatitudes in winter and spring, about 3% for northern midlatitudes in summer
and fall, and about 5% year round for southern midlatitudes. Since the early
1980s, the ozone hole has formed over Antarctica during every Southern Hemisphere
spring (September to November), in which up to 60% of the total ozone is depleted.
Since the early 1990s, ozone depletion has also been observed over the Arctic,
with the ozone loss from January through late March typically being typically
20-25% in most of the recent years. All of these decreases are larger than known
long-term natural variations.
The large increase in atmospheric concentrations of human-made chlorine and
bromine compounds is responsible for the formation of the Antarctic ozone hole.
Furthermore, the overwhelming weight of evidence indicates that it also plays
a major role in the ozone depletion in the Arctic and at midlatitudes.
In addition to these long-term changes, transient effects have also been observed
in the stratospheric ozone layer following major volcanic eruptions such as
Mt. Pinatubo in 1991. During 1992 and 1993, ozone in many locations dropped
to record low values. For example, springtime depletions exceeded 20% in some
populated northern midlatitude regions, and the levels in the Antarctic ozone
hole fell to the lowest values ever recorded. These unusually large, but short-term,
ozone decreases of 1992 and 1993 are believed to be related in part to the large
amounts of volcanic particles injected into stratosphere, which temporarily
increased the ozone depletion caused by human-produced chlorine and bromine
compounds, much as polar stratospheric clouds increase these chemicals' effect
on ozone depletion in polar regions. Because these particles settle out of the
stratosphere within a few years, the ozone concentrations have largely returned
to the depleted levels consistent with the downward trend observed before the
Mt. Pinatubo eruption. Should a similar eruption occur in the coming decade,
ozone losses of the same magnitude might be expected, because the chlorine levels
in the stratosphere will still be high.
Is the ozone layer expected to recover?
The ozone depletion caused by human-produced chlorine and bromine compounds
is expected to gradually disappear by about the middle of the 21st century as
these compounds are slowly removed from the stratosphere by natural processes.
This environmental achievement is due to the landmark international agreement
to control the production and use of ozone-depleting substances. Full compliance
would be required to achieve this expected recovery.
In 1987, the recognition of the potential for chlorine and bromine to destroy
stratospheric ozone led to the Montreal Protocol on Substances that Deplete
the Ozone Layer, as part of the 1985 Vienna Convention for the Protection of
the Ozone Layer, to reduce the global production of ozone-depleting substances.
Subsequently, global observations of significant ozone depletion have prompted
Amendment to strengthen the treaty. The 1990 London Amendment calls for a ban
on the production of the most damaging ozone-depleting substances by 2000 in
developed countries and 2010 in developing countries. The 1992 Copenhagen Amendment
changed the date of the ban to 1996 in developed countries. Further restrictions
on ozone-depleting substances have been agreed upon in Vienna (1995) and Montreal
(1997).
The figure below shows past and projected stratospheric abundances of chlorine
and bromine without the Protocol, under the Protocol's original provisions,
and under its subsequent agreements. Without the Montreal Protocol and its Amendment,
continuing use of chlorofluorocarbons (CFCs) and other ozone-depleting substances
would have increased the stratospheric abundances of chlorine and bromine tenfold
by the mid-2050s compared with the 1980 amounts. Such high chlorine and bromine
abundances would have caused very large ozone losses, which would have been
far larger than the depletion observed at present.
In contrast, under the current international agreements that are now reducing
the human-caused emissions of ozone-depleting gases, the net tropospheric concentrations
of chlorine- and bromine-containing compounds started to decrease in 1995. Because
3 to 6 years are required for the mixing from the troposphere to the stratosphere,
the stratospheric abundances of chlorine are starting to reach a constant level
and will slowly decline thereafter. With full compliance, the international
agreements will eventually eliminate most of the emissions of the major ozone-depleting
gases. All other things being constant, the ozone layer would be expected to
return to a normal state during the middle of the next century. This slow recovery,
as compared with the relatively rapid onset of the ozone depletion due to CFC
and bromine-containing halon emissions, is related primarily to the time required
for natural processes to eliminate the CFCs and halons from the atmosphere.
Most of the CFCs and halons have atmospheric residence times of about 50 to
several hundred years.
However, the future state of the ozone layer depends on more factors than just
the stratospheric concentrations of human-produced chlorine and bromine. It
will also be affected to some extent by the changing atmospheric abundances
of several other human-influenced constituents, such as methane, nitrous oxide,
and sulfate particles, as well as by the changing climate of the Earth. As a
result, the ozone layer is unlikely to be identical to the ozone layer that
existed prior to the 1980s. Nevertheless, the discovery and characterization
of the issue of ozone depletion from chlorine and bromine compound and a full
global compliance with the international regulations on their emissions will
have eliminated what would have been, as the figure illustrates, a major deterioration
of the Earth's protective ultraviolet shield.
Does depletion of the ozone layer
increase ground-level ultraviolet radiation?
Yes, ultraviolet radiation at Earth’s surface increases as the amount
of overhead total ozone decreases, because ozone absorbs ultraviolet radiation
from the Sun. Measurements by ground-based instruments and estimates made using
satellite data have confirmed that surface ultraviolet radiation has increased
in regions where ozone depletion is observed.
Is depletion of the ozone layer the principal
cause of climate change?
No, ozone depletion itself is not the principal cause of climate change. However,
because ozone is a greenhouse gas, ozone changes and climate change are linked
in important ways. Stratospheric ozone depletion and increases in global tropospheric
ozone that have occurred in recent decades both contribute to climate change.
These contributions to climate change are significant but small compared with
the total contribution from all other greenhouse gases. Ozone and climate change
are indirectly linked because ozone-depleting gases, such as the chlorofluorocarbons
(CFCs), hydrochlorofluorocarbons (HCFCs), and halons, also contribute to climate
change.
Is the depletion of the ozone layer
leading to an increase in ground-level Ultraviolet radiation?
The depletion of the ozone layer leads, on the average, to an increase in ground-level
ultraviolet radiation, because ozone is an effective absorber of ultra-violet
radiation.
The Sun emits radiation over a wide range of energies, with about 2% in the
form of high-energy, ultraviolet (UV) radiation. Some of this UV radiation (UV-B)
is especially effective in causing damage to living beings, for example, sunburn,
skin cancer, and eye damage to humans. The amount of solar UV radiation received
at any particular location on the Earth's surface depends upon the position
of the Sun above the horizon, the amount of ozone in the atmosphere, and local
cloudiness and pollution. Scientists agree that, in the absence of changes in
clouds or pollution, decreases in atmospheric ozone lead to increases in ground-level
UV radiation.
The largest decreases in ozone during the past 15 years have been observed over
Antarctica, especially during each September and October when the ozone hole
forms. During the last several years, simultaneous measurements of UV radiation
and total ozone have been made at several Antarctic stations. In the late spring,
the biologically damaging ultraviolet radiation in parts of the Antarctic continent
can exceed that in San Diego, California, where the Sun is much higher above
the horizon.
In areas where smaller ozone depletion has been observed, UV-B increases are
more difficult to detect. In particular, detection of trends in UV-B radiation
associated with ozone decreases can be further complicated by changes in cloudiness,
by local pollution, and by difficulties in keeping the detection instrument
in precisely the same condition over many years. Prior to the late 1980s, instruments
with the necessary accuracy and stability for measurement of small long-term
trends in ground-level UV-B were not available. Therefore, the data from urban
locations with older, less-specialized instruments provide much less reliable
information, especially since simultaneous measurements of changes in cloudiness
or local pollution are not available. When high-quality measurements have been
made in other areas far from major cities and their associated air pollution,
decreases in ozone have regularly been accompanied by increases in UV-B. This
is shown in the figure below, where clear-sky measurements performed at six
different stations demonstrate that ozone decreases lead to increased UV-B radiation
at the surface in amounts that are in good agreement with that expected from
calculations (the "model" curve).
Has the benefit of the Montreal Protocol been worth
the cost?
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.
STRATOSPHERIC OZONE IN THE FUTURE
How will recovery of the ozone layer be detected?
When is the ozone layer expected to recover?
How will recovery of the ozone layer be
detected?
Scientists expect to detect the recovery of the ozone layer with careful comparisons
of the latest ozone measurements with past values. Changes in total overhead
ozone at various locations and in the extent and severity of the Antarctic “ozone
hole” will be important factors in gauging ozone recovery. Natural variations
in ozone amounts will limit how soon recovery can be detected with future ozone
measurements.
When is the ozone layer expected to
recover?
The ozone layer is expected to recover by the middle of the 21st century, assuming
global compliance with the Montreal Protocol. Chlorine- and bromine-containing
gases that cause ozone depletion will decrease in the coming decades under the
provisions of the Protocol. However, volcanic eruptions in the next decades
could delay ozone recovery by several years and the influence of climate change
could accelerate or delay ozone recovery.
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?
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 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?
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 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?
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.
Does water effectively shield aquatic organisms
from UV exposure?
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.
IMPACT ON TERRESTRIAL PLANT LIFE
What will be the effects of an increased UV-B radiation
on crop and forest yields?
Can plants protect themselves against increased UV-B?
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.
Is the increase in UV-B radiation caused by ozone depletion
equivalent to that incurred by moving
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?
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.
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.
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?
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.
