Normally a person's body would maintain aetheric balance on its own, analogous to the real-world process of homeostasis.

Alisaie: [...] Under normal circumstances, a person's aether naturally tends to equilibrium[...]
While a mage is more exposed to elemental aether, one probably need to do something extreme to disrupt this process, perhaps by channeling a large amount of aether though yourself, as White Mages and Black Mages do.