Gas

Sleep Gas is an unnamed Quirk possessed by Mustard.

Description
This Quirk grants Mustard the ability to generate a poisonous, sleep-inducing gas, which he can control at will.

Mustard can release a great amount of toxic gas from his body. Those who breathe in the gas will be knocked out almost instantly. Prolonged inhalation of this gas can put victims into a coma. Mustard is also able to detect movement through the gas, which prevents him from being sneak attacked. In contrast, Mustard can use the gas' mistiness to hide from view and perform surprise attacks of his own.

Mustard is not immune to his own gas, implied by the fact that he wears a gas mask. This also means that anyone properly equipped with breathing apparatus can avoid the effects of his Quirk, if only temporarily. If Mustard gets knocked unconscious, the gas will dissipate.