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Hair
strength
For a healthy individual with no hair diseases, hair fiber is very
strong with tensile strength around 1.6 x10-9 N / m2 (times ten
to the power minus nine newtons per meter square of hair cross section
diameter). That makes hair about as strong as copper wire of the
same diameter. So as you can see hair is incredibly strong. It also
has elastic properties. It can stretch up to 20% of its original
length before breaking when it is dry and when it is wet it may
stretch up to 50% before breaking.
So how does hair break if it is so strong? It depends on the speed
at which the pulling force is applied to the hair. If you pull on
hair slowly and steadily with gradual increasing force the hair
will resist for quite some time before breaking. But if you apply
sudden force and yank on the hair it will break. The tensile strength
of the hair is reduced, the hair has no time to respond and stretch,
which distributes the force over the length of the hair fiber, and
the hair just snaps. So despite the apparent strength of hair, it
will break if you brush or comb too vigorously because this applies
a sudden force to the hair.
Can
you hang by your hair
 The
short answer is, yes I have seen circus artistes hang by
their hair and there is a long history in the circus world of
these feats of hair strength. The primary limiting factor in whether
someone
can hang by their hair is the strength of their neck rather than
the strength of their hair.
For a healthy individual with no hair diseases, hair fiber is very
strong so the key weak spot is the strength by which the hair fiber
is anchored in the hair follicle in the skin. Not so much research
has been done on this, but there are one or two research articles
that examine the force required to pluck hair fibers. They mostly
refer to body hair, but one assumes the anchoring strength for scalp
hair is similar.
The average anchoring strengths for normal anagen (growing) and
telogen (resting) chest hair is 71g and 70g respectively with variation
up to +/- 16g (For people with hair diseases such as loose anagen
syndrome the anchoring strength drops to 14g). Between 80-90% of
scalp hair is in anagen at any one time and virtually all the rest
are in telogen.
Taking 70g anchoring strength per hair to make calculations easy,
I assume we can just multiply the weight by the number of hairs
by which an individual is lifted. Red heads have about 80,000
total scalp hair follicles, blondes 120,000 and dark haired
individuals
typically around 100,000. So you could hang between 5600kg and
8400kg from
one head of hair (but it might break your neck to try). In reality,
the maximum weight would be somewhat less as not all hair fiber
is perfect. Some hair fibers will have weathering defects which
will make them more liable to break under strain. The weight
will
not be uniformly distributed to every hair fiber and not all hair
follicles will contain a hair fiber, etc. However, the overall
anchoring
strength is more than enough to lift a person by the hair - and
in principle several other people besides.
One example of the anchoring strength of hair is from "scalping"
accidents. Sometimes people get their hair caught in machinery.
The sudden yanking motion from the caught hair doesn't always
pull the hair fiber out - rather the entire skin can get pulled
off! Depending
on how bad the scalping is, the skin and hair follicles can be
reattached by plastic surgeons, but this is not always possible
and skin grafts are needed. So make sure to tie your hair back
when using machinery!
Hair
strength references
- Chapman DM. The anchoring strengths of various
chest hair root types. Clin Exp Derm 1992; 17: 421-423.
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