On whether it is safer to fasten a helmet or to leave the chin-strap loose.
[This question came from an experienced soldier now working in humanitarian demining.]
QUESTION: Question: does a solid fastened chinstrap (without any sort of special
release) always cause a neck injury in a blast?
ANSWER: No, but it probably depends on the size of the blast. Most mine blasts are very small compared to bomb blasts encountered in combat. In demining, there have been no
instances of neck injury recorded when wearing a fastened helmet in a
blast - but there have been only a few where a combat-helmet was worn and it
was not recorded whether it was fastened in most cases. There has been an instance where the chin-strap
on a MedEng plastic visor/helmet slipped and severely damaged a man's nose as the blast front passed. This kind of helmet/visor breaks in a blast more often than others - probably because it is held
rigidly in place by the chin strap. The breaking does not seem to cause
injury - it is usually (but not always) just the "wings" where the visor attaches to the helmet that
snap. In another accident a metal helmet buckle snapped and severely lacerated the wearer's neck as a combat-style helmet was torn away.
With small blasts in demining, I don't think it matters.
With all blasts, unless you are on top of the device, the fragments strike
before the blast wave (it's a tiny timespan, but long enough). An
unsecured visor stops the fragments of earth, mine casing, etc just as well
as a secured one. [You can see the fragment marks on the unsecured visor
face in tests and after accidents - but 5mm polycarbonate is often holed by
metal fragments, of course.] The blast wave passes after the fragments have
already been stopped and it usually removes the visor and headframe. The
low-pressure behind the expanding blast wave causes turbulence and the visor
often flies high - but it had already stopped the fragments. Dust is drawn
into the low-pressure area as the pressure equalises and the victim
generally gets dust in his eyes - nothing that cannot be washed out if the
visor was in place when the fragments associated with an AP blast mine struck.
The general rule with blast is - don't try to stop it. The forces are
immense. The HE in a large AP mine can lift a tank that bellies out on it (while not damaging it, of course). Let
the blast go - and let it take the visor with it. It seems obvious that this
is easier on the neck - but there is no proof of that in the accident
record. Whiplash does not occur because the blast wave expands as a dome and
hits the kneeling deminer low down fractionally before hitting his head -
his whole body is pushed up and back. If the deminer were lying down so that
the head was the only thing to take the blast wave, this would not be true
and severe whiplash should be anticipated.
According to one report from MedEng, combat helmets can "trap" the blast and increase the probability of severe hearing damage. I have no evidence of this in the accident record. The company's research has (as you might expect) reached conclusions that favour its own PPE designs in other cases, so I treat their claims with skepticism. (It seems obvious to me that commercial companies should not be expected to conduct objective research.) But it may be true that helmets that flare over the ears to allow normal hearing increase the risk of damage to the tympanic membranes.
For a deminer, being separated from his visor is not an issue after an
accident. The blast and fragment strike was a single event and he is not
going to pick up tools and work again immediately. For a soldier, being
separated from his helmet when all hell is still breaking loose around him
might be a different issue.
Based on my experience with small blasts and mine fragmentation (which I acknowledge is not entirely relevant), my belief is, if the helmet is meant to protect against shrapnel or bullets,
it might deflect them slightly easier if it could move away - but I would
want to keep it in place because more might be coming! If the soldier is
only going to be close to a single blast, he should unfasten the helmet and let it go
where it wants.
This opinion is empirically based, and subject to review as more evidence becomes available.
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