SRD:
Elemental Type: An elemental is a being composed of one of the four classical elements: air, earth, fire, or water.
Features: An elemental has the following features.
—8-sided Hit Dice.
—Base attack bonus equal to 3/4 total Hit Dice (as cleric).
—Good saves depend on the element: Fortitude (earth, water) or Reflex (air, fire).
—Skill points equal to (2 + Int modifier, minimum 1) per Hit Die, with quadruple skill points for the first Hit Die.
Traits: An elemental possesses the following traits (unless otherwise noted in a creature’s entry).
—Darkvision out to 60 feet.
—Immunity to poison, sleep effects, paralysis, and stunning.
—Not subject to critical hits or flanking.
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Unlike most other living creatures, an elemental does not have a dual nature—its soul and body form one unit. When an elemental is slain, no soul is set loose. Spells that restore souls to their bodies, such as raise dead, reincarnate, and resurrection, don’t work on an elemental. It takes a different magical effect, such as limited wish, wish, miracle, or true resurrection, to restore it to life.
—Proficient with natural weapons only, unless generally humanoid in form, in which case proficient with all simple weapons and any weapons mentioned in its entry.
—Proficient with whatever type of armor (light, medium, or heavy) that it is described as wearing, as well as all lighter types. Elementals not indicated as wearing armor are not proficient with armor. Elementals are proficient with shields if they are proficient with any form of armor.
—Elementals do not eat, sleep, or breathe
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Emphasis mine.
Still though, horrid wilting 'evaporates moisture', it doesnt necissarily say water however (although that is what would be assumed would be evaportated. The fire elemental himself is not very well explained, at least in the srd. Perhaps it is actually a spongy, fiery substance and removing whatever fiery liquid that allows him to move as he does breaks him up causing damage?
Take it and run with it however you like of course

Earth elementals you evaporate the acid that holds them together (or sublimation?), air elementals you dry them out carrying away the bonds of their form, fire elementals some strange fluid part of their substance is evaporated away in a fashion that heat never could.