Originally posted by h1a8
When was this stated or shown?
She's been felled via gas attacks before when shielding herself and others. She obviously needs oxygen to permeate the force fields when she's using them defensively, less she and others suffocate.
She can also make them airtight in order to suffocate others, but she uses that sparingly in order for gentle(ish) incapacitation, obviously, rather than outright murder.