____________________
and
____________________
,
two
well
-
known
organochlorinated
compounds
,
have
been
detected
in
the
____________________
of
animals
that
live
in
the
Arctic
or
Antarctic
all
year
round
.
These
molecules
are
ingested
at
the
____________________
of
the
food
____________________
and
gradually
make
their
way
____________________
,
until
they
are
found
at
elevated
concentrations
in
the
largest
____________________
,
such
as
polar
bears
and
humans
.
Biologists
are
now
beginning
to
record
the
health
effects
on
individuals
at
the
upper
end
of
the
food
chain
,
with
cases
of
low
fertility
,
immune
deficiency
,
disruptions
to
the
endocrine
system
,
____________________
mutations
being
seen
.
These
effects
may
endanger
the
____________________
of
some
polar
species
Among
the
main
threats
currently
hovering
over
the
poles
,
the
long
-
distance
transport
of
contaminants
from
____________________
or
agricultural
areas
to
the
polar
regions
is
of
particular
concern
.
In
the
Arctic
,
various
ice
____________________
samples
extracted
from
the
Greenland
Ice
Sheet
show
that
the
effects
of
industrial
pollution
have
already
been
felt
for
about
a
century
.
Today
,
particles
of
soot
,
nitrogen
and
sulphur
dioxide
(
____________________
rain
)
,
____________________
(
PCBs
,
DDT
,
etc
.
)
,
heavy
metals
,
____________________
isotopes
,
etc
.
can
be
detected
in
measurable
quantities
in
the
air
,
snow
,
sediment
and
water
of
the
polar
regions
.
This
is
particularly
true
in
the
Arctic
,
where
many
____________________
from
the
northern
hemisphere
are
collected
(
also
as
the
result
of
river
runoff
)
.
Of
all
the
many
pollutants
released
into
the
environment
by
human
activity
,
____________________
organic
pollutants
(
POPs
)
are
among
the
most
dangerous
.
POP
with
the
worst
reputation
is
DDT
.
DDT's
stability
,
persistence
and
widespread
use
,
especially
in
____________________
countries
,
have
all
contributed
to
the
fact
that
residues
of
the
compound
are
now
found
as
far
afield
as
the
polar
environment
,
for
example
,
or
in
polar
bears
in
the
Arctic
.
The
harmful
effects
on
the
eggshells
of
birds
,
was
highlighted
clearly
by
scientists
in
the
1970s
.
This
led
to
strict
restrictions
on
the
use
of
DDT
and
ultimately
to
its
almost
total
ban
.
Despite
this
,
the
presence
of
DDT
can
still
be
detected
in
____________________
all
over
the
world
.
____________________
(
polychlorinated
biphenyls
)
,
also
POPs
,
are
used
to
manufacture
transformers
,
condensers
,
plastics
and
paint
additives
.
PCBs
include
about
____________________
different
products
,
all
highly
toxic
even
in
low
doses
.
Studies
have
demonstrated
their
sterilising
immunosuppressant
effects
on
many
species
in
the
animal
kingdom
including
seals
and
____________________
in
the
polar
regions
.
Because
they
are
highly
stable
,
POPs
persist
in
the
environment
for
several
____________________
or
even
decades
.
This
allows
them
to
circulate
across
the
entire
globe
via
a
process
called
the
?
grasshopper
effect
?
-
an
often
seasonal
phenomenon
in
which
successive
evaporations
and
deposits
enable
transportation
following
atmospheric
currents
.
In
the
Arctic
,
for
example
,
pollutants
come
essentially
from
two
sources
:
the
east
and
west
coasts
of
North
____________________
in
winter
(
big
cities
and
industrials
regions
)
and
from
central
Europe
in
____________________
(
industry
and
power
stations
of
the
former
Soviet
Bloc
,
among
others
)
.
The
ice
ecosystems
at
both
the
North
and
South
Poles
have
been
particularly
affected
by
POPs
that
find
their
way
into
the
food
chain
through
land
plants
and
marine
plankton
and
filtering
organisms
that
absorb
the
nutrients
in
the
water
column
.
They
are
then
eaten
by
fish
,
accumulating
in
their
adipose
tissue
(
fat
)
.
POPs
are
extremely
persistent
and
highly
lipophilic
,
so
that
they
end
up
becoming
concentrated
in
the
adipose
tissue
of
animals
higher
up
the
food
chain
,
where
they
can
reach
levels
up
to
several
tens
of
thousands
of
times
higher
than
in
the
surrounding
environment
.
POPs
climb
up
the
food
chain
from
fish
to
birds
and
finally
reach
the
large
marine
mammals
(
as
well
as
humans
)
at
the
top
of
the
food
chain
,
at
increasing
concentrations
.
The
traditional
____________________
of
Arctic
peoples
is
based
on
hunting
and
____________________
,
which
means
that
the
accumulation
of
POPs
and
____________________
in
the
food
chain
has
an
effect
on
human
health
.
Due
to
their
migration
,
birds
also
spread
POPs
that
they
have
ingested
throughout
the
entire
land
-
based
____________________
,
sometimes
several
thousand
kilometres
from
their
original
source
.
Pollution
in
the
Arctic
is
of
particular
concern
at
the
top
of
the
food
chain
,
where
heavy
metals
and
other
pollutants
are
found
in
polar
bears
,
belugas
,
orcas
,
etc
.
Scientists
from
the
environmental
protection
organisation
WWF
have
shown
that
____________________
(
killer
whales
)
are
now
the
most
polluted
predator
in
the
____________________
.
