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I like the idea of elves having a distinct weakness against iron, to the point where they're intensely scared of it and can't stand the touch of it.

for reference

My question is: Assuming the Elves are mostly like humans, just how extreme can the weakness to iron realistically be? I know some people are allergic to steel (I am slightly), but would it ever be so bad that it's painful to the touch? Would such an extreme reaction prevent them from eating normal food that contains iron compounds? Could this be possible in a humanoid species without drastically changing their basic physiology?

Ideally, I want a race of elves who normally look like pointy-eared humans, are able to pass themselves off as human with a decent disguise, and can interbreed with humans, with the exception that for most of them, touching solid iron or steel physically hurts. How might this be possible and what would the ramifications be?

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    $\begingroup$ I've added the chemistry tag, because people watching that particular tag might have something to contribute. Please revert the edit if you feel that it's currently not what you're aiming for.. $\endgroup$ – BLT-Bub Oct 17 '19 at 21:01
  • $\begingroup$ @011358smell Ah, no that makes perfect sense. Thank you. $\endgroup$ – Maddock Emerson Oct 17 '19 at 21:04
  • $\begingroup$ It's usually related to magnetism $\endgroup$ – Separatrix Oct 17 '19 at 21:07
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The "interbreed" part is where it all falls down. Our physiology is critically dependent on iron (we're awash with iron compounds from conception on). Assuming that your hypothetical elves used a different blood chemistry (copper, paging Dr. Spock), they'd be a radically different species, even with similar morphology.

This would make interbreeding absolutely impossible.

Now, that aside, if you had a copper-based blood chemistry, like the horseshoe crab, then something like a contact allergy could be a very real condition endemic to this species. 85% of the human population is allergic to Urushiol, the molecule common to poison oak, poison ivy, and poison sumac, among other plants. It's not unreasonable for an even more common contact allergy to be characteristic to this species, and while that wouldn't result in immediate pain upon touching iron, would certainly result in a visible rash and extreme discomfort after contact.

So in place of testing someone with a horseshoe, they might be compelled to wear an iron chain for a few hours. If they can't stop scratching, you've got yourself an elf!

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    $\begingroup$ Elves are traditionally allergic to "cold" iron, i.e., wrought iron, 99.9% pure elemental iron, not iron compounds $\endgroup$ – nzaman Oct 18 '19 at 8:14
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You can go two ways.

One, they have a chemical aversion for iron. They might have slightly acidic skins, enough to quickly dissolve enough iron ions to get an allergic reaction. That would neatly explain the "burning" sensation. Basically, iron for them is what poison ivy is for humans. Using silk and leather gloves would allow a Sidhe to manipulate iron without problems.

Two, more esoteric - it's not iron they have problems with, it's ferromagnetism. Elven powers and their very biology are a (waves hands quickly) mixture of magnetic quantum effects, so they are most at ease in presence of diamagnetic materials - silver, copper, gold - and strongly repelled by ferromagnetic metals like iron, nickel, or cobalt. Iron is the only one they were familiar with. Also, not all kinds of iron are equal; heat-treated austenitic steel is only weakly magnetic and thus bearable, while cold-forged iron is the worst. One consequence of Method 2 is that Elves can't abide technological civilizations, with their electric distribution grids and haphazard magnetic fields thrown every which way.

Unfortunately, method 2 requires a biology so different from human norm that you couldn't get interbreeding with it for any price. Unless you exploited the nanomachine Deus ex Machina - Elves are normal humans with some genes tweaked and a compatibility with nanomachines. Unfortunately, those nanomachines can't work properly in a magnetic "choppy sea", as in near ferromagnetic iron or electronic components. In that case you're left with the not inconsiderable problem of explaining how these Elves and their nanomachine guests happened to come by.

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    $\begingroup$ Terry Pratchett's elves had a similar problem to method 2, in that they hated ferromagnetism, but it wasn't a matter of underlying physiology. They had a highly developed magnetic sense, similar to the one pigeons use to help navigate. They relied on this sense so much that anything that interfered with the local geomagnetic field caused them extreme anxiety bordering (iirc) on pain. The extreme reaction seems a bit of a stretch, but all it requires is an extra sense organ (or a set of organs under the skin?) - the rest of the body could be human. $\endgroup$ – Pastychomper thanks Monica Dec 9 '19 at 13:31
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Voltaic effect.

Many people have encountered this effect in the mouth. Taking a piece of aluminum (or possibly other metal) into the mouth produces an electrochemical reaction with silver-containing metal fillings. If the metal actually touches the filling this can produce a painful shock.

Dental Filling Discomforts Illustrate Electrochemical Potential of Metals

The first of the discomforts is known to anyone who has bitten a piece of aluminum foil, perhaps on an incompletely unwrapped piece of candy, in such a way that aluminum presses against a filling. The sharp pain which momentarily results is quite memorable. In effect, a voltaic cell has been created in the mouth. The aluminum (E° = -1.66 V) is the anode and the filling is the cathode. Saliva and gum tissue fluid act as the electrolyte. When the two metals touch, a short circuit results and a feeble current flows. The nerve of the tooth is more than sensitive enough to detect this current. Rapid polarization at the electrodes causes the pain to be brief. In the writer’s experience less active metals, such as nickel, iron, stainless steel, and copper, do not deliver a detectable sensation. A more sensitive individual, however, might be able to discern differences and, thus, construct an electrochemical series physiologically.

A similar phenomenon happens with elves and iron. Elves are always sticky with sweat, and their sweat contains large quantities of silver ion among others, accounting for their need to periodically gnaw on silver. Elfin skin is thin and their nerves are many, rendering them exquisitely sensitive to all sorts of stimuli good and bad. The electrovoltaic potential produced between their silver-laden thin skin and iron metal is considered bad. The discomfort is transient, but elves are also drama queen wusses and so make a huge deal about the sort of discomfort lesser races resign themselves to deal with many times a day.

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