Poison Gas allows Takeshi to generate a toxic fog that make people feel sick when they breath it. If Takeshi does not release it, then the poison gas begins to accumulate inside him.
The quirk does not stop generating the venomous gas, and although Takeshi can save up the poison inside his body to avoid poisoning innocent people, at some point he has to release it, because once it reaches a limit, his own quirk begins to negatively affect his health. After getting rid of all the poison, Takeshi feels much better.
Due to the danger of his Quirk, Takeshi has trouble using it in places where there are large crowds of people because many people would be affected.