Poison Gas

Poison Gas is an Emitter Quirk with long range, carried by Takeshi Bushijima. It first appears in Chapter 1 of My Hero Academia: Team-Up Missions. The ability manufactures a toxic cloud that sickens anyone who breathes it in, and the unusual potency Takeshi displays is thought to come from years of storing the stuff rather than releasing it.

Production is constant and sits outside its owner's control. Takeshi can keep the toxin bottled inside himself instead of exposing bystanders, but that is only a delay. Once the amount crosses a limit, his own Quirk turns on him and his health suffers for it, and clearing everything out leaves him feeling far better. One other property matters in a fight: the fog ignites readily, which Katsuki Bakugo demonstrated by burning a cloud away with Explosion.

Takeshi Bushijima is the only known holder, and crowds are his central problem. Anywhere thick with people, venting becomes difficult because so many would be caught in it. The Quirk invites comparison with Gas, another power that pushes toxic fumes out of its user's body and can rebound on that user, though Gas answers to intent and carries no forced buildup.