This is Hanzo of the Salamander. When he was a child he had the poison gland of a salamander implanted that lived in the river in his village after it died. They gave him the ability to kill someone simply by breathing on them. Because of this he had to wear a respirator mask to prevent himself from accidentally killing his comrades.
When engaged in battle with weaker opponents he'd simply just take his mask off and kill them with his poison breath.

However when fighting more skilled opponents he'd refrain from taking his mask off since they could exploit his ability by rupturing his poison sack which would cause his poison to leak out into the air in it's gaseous state. Even though he built up an immunity to it breathing in enough would still cause momentary neurological impairment and paralysis leaving him wide open to an attack.

Here's the problem wouldn't the fact he has his mask on be irrelevant since all that poison sloshing around in his abdominal cavity would lead to septic shock regardless of his immunity?