Poison Gas allows Takeshi to generate a toxic fog that make people feel sick when they breathe it. If Takeshi does not release it, then the poison gas begins to accumulate inside him. The gas is seen to be very potent, though this is probably due to years of accumulation without release.
The Quirk does not stop generating the 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. The gas he produces is also known to be very flammable, as seen when Katsuki Bakugo used an explosion to burn out the gas.
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.
- Poison Gas is similar to Gas, a Quirk that also generates a toxic gas from the user's body that the user can be affected by, though Gas can be voluntarily used and doesn't need to be continuously stored up.
- My Hero Academia: Team-Up Missions Manga: Chapter 1.