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09/09 ISSUE 58
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Seaworthy is produced by the Directorate of Navy Safety Systems in the interests of promoting
safety in the Navy. The contents do not necessarily reflect Ser vice policy and, unless stated
otherwise, should not be construed as orders, instructions or directives – KEEP NAVY SAFE .
September 17, 2009
EVERY year Australians die while
breath-hold diving – that is, diving
without life-support apparatus. Making
the activity even more risky is that vic-
tims often hyperventilate before diving,
thinking that, by taking a lot of rapid,
deep breaths, they will be able to hold
their breath for longer, dive deeper and
swim further underwater.
John Pennefather, a scientist at the
RAN Submarine and Underwater
Medicine Unit at HMAS Penguin, warns
against hyperventilating before breath-
hold diving, saying that, if you do it
alone, things can go terribly wrong.
“You may lose consciousness in
the water and, if this happens, you will
drown unless rescued,” he said.
“Hyperventilation is dangerous
because of the effect it has on our breath-
ing. Carbon dioxide (CO2) is our main
stimulus for breathing; when CO2 builds
to a triggering level, it signals that it’s
time to breathe.”
Mr Pennefather said hyperventilation
lowered the amount of CO2 in the blood
and fooled the body into believing it
didn’t need to breathe, even if the oxygen
supply was low.
“ The low oxygen level in the blood
stream causes loss of consciousness –
oxygen is needed for our brain cells to
work and the brain gets this oxygen from
its blood supply,” he said
“Exercise and diving increase the risks
from hyperventilation. Exercise increases
the rate oxygen is consumed, so the time
between the ‘I need a breath’ feeling and
unconsciousness is decreased.
“With diving, the pressure reduction
with a return to the surface disrupts the
oxygen supply. At depth, a diver may
have enough oxygen to survive, but
surfacing lowers the pressure and this
decreases the amount of oxygen avail-
able. Divers have been seen to lose con-
sciousness as they reach the surface.
Often they have also been struggling to
bring a big fish up to the surface while
spear fishing and this has caused a fatal
increase in the oxygen consumed.
“ Some of these people had a long
career as spear fishermen and it is also
believed that training decreases the
responses that made them breathe, mak-
ing them more likely to lose conscious-
Mr Pennefather said there was a group
of people who competed in breath-hold
diving events for sport.
“A study of entrants at an interna-
tional competition found that more than
two thirds of them had lost consciousness
underwater,” he said.
“ They then had to rely on escort
divers to rescue them. Readers might
think they are crazy to compete with this
level of intensity – it is the only sport
where, the more you train, the more like-
ly you are to die because of the effect
training has on your body.
“ Deaths will continue if we do not
increase awareness of the risk of hyper-
ventilation before breath-hold diving.
Most diving manuals have warnings
about it and professional aquatics organi-
sations caution against it.
“All parents, life guards, divers, spear-
fishermen etc., should discourage hyper-
ventilation and ensure that breath-hold
diving is closely supervised. Heed the
warning – don’t hyperventilate before
breath-hold diving. Come back alive.”
Mr Pennefather said that, if you need
a rule on how to avoid the problem, you
should borrow his: “I never take more
than three deep breaths before a snorkel
dive and have survived so far.”
can be dangerous
NOT RECOMMENDED: Breath-hold diving is dangerous particularly when
coupled with hyperventilation.
STATISTICALLY the likelihood
of a shark attack remains improb-
able in Australia – the presence of a
shark in an area of operations does
not necessarily result in a shark
It is impossible to accurately
calculate the likelihood of an attack
and, therefore, a subjective approach
is required to determine a likelihood
value that can be used in a hazard
risk assessment of an in-water activ-
A Shark Presence Index (SPI) is
a planning tool that has been devel-
oped to assist in the risk assessment
process and combines a number of
factors known to influence shark
Taking note of historical shark
attack data and the statistical likeli-
hood of shark attack, although sub-
jective, the SPI value can be equated
to a likelihood of shark attack for use
in a HRI calculation of activities that
require personnel to enter the water.
The SPI value is not based on
scientific evidence and must not be
taken as a guarantee that a shark
attack will not occur. The final
decision to begin an activity must
be based on a full appreciation of
the risk achieved through the robust
application of the hazard risk man-
agement process, and not simply
based on the likelihood of shark
attack determined by this tool.
The Shark Attack Likelihood Calculator
can be found on the DIVESAFE Website
A risk mitigation strategy for shark
attack is outlined in ABR 155 Chapter 9.
Shark attack likelihood calculator
PRESENCE: Doesn’t mean
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