In the movie, all the people who have been injected with Extremis are killed by some sort of explosion. But what is not clear is what kind and what is the threshold?

We have seen that most of them can pass through fire, and Pepper also survived falling into a giant explosion/fire. Killian isn't killed when Mach 42 explodes, but is later killed with an explosion. Is it sufficient to raise the temperature to certain levels that only powerful explosions acheive, or is it necessary to blast the person into bits?

In short, what's required to kill them once and for all?

  • 3
    "How do they Terminate extremis subjects?" With extreme prejudice.
    – Jeff
    May 12 '13 at 14:23

It honestly seems like there's only two ways to do it - overdose them (making them detonate spectacularly) or cause massive trauma to their heart. This would seem to make 'sense' (for a comic-based movie) - repairing the trauma and/or rebuilding a limb takes resources. If your heart isn't beating (like because it's GONE) those resources can't be delivered to the site of the trauma. Most likely, the heart (if the damage isn't too great) can be repaired, allowing for the restoration of the rest of the body.

When Iron Man blows a hole clean through one, he stays down - that hole went straight through where his heart would be.

When the woman in Tennessee is killed, it's by a massive explosion which almost certainly had enough concussive force to pulp her internal organs.

Later, both Pepper and Killian survive explosions. Pepper falls into a large explosion, and Killian is at the center of a suit detonation.

What Pepper fell into, though, was more firestorm than explosion by the time she reached it - she would have passed quickly through the worst of the heat, and into the area below it, where the heat would be less intense (though still fatal under normal circumstances). There was no massive concussive force of the explosion, only from the fall. Normal humans, depending on how they land, can survive a 200-foot fall (though it's rare). Given that Extremis could let someone walk essentially unscathed through fire, it's easy to see how Pepper could survive this.

Killian is more troublesome - he's at the center of the detonation of the Mark 42. The explosion probably should have killed him (I suspect the true reason he survived was plot armor). In-Universe, the explosion was likely intended to be a self-destruct, not an execution tool. It would have been focused on destroying the suit, though easily strong enough to kill any normal person inside. Thus, it's likely that the self-destruct used the dispersed power plants in the armor (which let pieces fly independently) to create explosions designed to destroy the tech. It would make sense for the focal point of these to be internal, focused outward. This is the only thing I could see that would have given Killian a chance to survive - most of the concussive force was directly outwards, not inwards. It still hurt like a son of a bitch, I bet.

  • 4
    Killian also seems to have a more... advanced(?) version/mutation of Extremis. He is shown to be able to breathe fire (or at least face-melting heat beams) whereas none of the other Extremis test subjects were capable of the same. It isn't inconceivable that he might also have greater durability or faster healing than the others.
    – Steam
    May 13 '13 at 14:16
  • "This isn't even my final form!" - Killian, pre-iron man suit detonation.
    – Jeff
    May 21 '13 at 12:48

When Happy is trailing an AIM agent to the Chinese theater, the agent gives an Extremis-injeted soldier a briefcase that looks to be filled with additional Extremis injections. Based on the conversation the two had, it sounds like the soldier "couldn't regulate", so when he overdosed he exploded.

Overall, IMO the movie isn't very consistent regarding what it takes to kill someone injected with Extremis. The Mark 42 shot a hole through the center of the chest of one agent on Air Force 1, while the female agent trailing Tony in Tennessee appeared to be mostly intact after the explosion in the kitchen yet she stayed dead.

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