If you must know, it's neutron damage. It's from the reactor wall.
3 seconds sound clip from the Iron Man 2 movie soundboard.
You can hear this line at 00:48:46.584 in the Blu-ray version of the movie.
Quote context
[...]
- Are you listening to me? Are you okay?
- Hey, man. Hey, hey! You all right?
- Yeah, I should get to my desk.
- Yeah.
- Is that supposed to be smoking?
- If you must know, it's neutron damage. It's from the reactor wall.
- And how about the high-tech crossword puzzle on your neck?
- You wanna do this whole lone gunslinger act and it's unnecessary. You don't have to do this alone.
- You know, I wish I could believe that. I really do.
[...]