Ok.
While the fan is running, damp air will all be expelled.
While not running, it's negligible, so no problem.
I must say though, that I wouldn't even bother with an inline. Just a normal extractor fan on the bathroom ceiling.
No matter though. Wire the fan into the ceiling light, so that it comes on with the light. Do remember to wire it to give an "override" after you switch the light off (adjustable.)