extractor fan in a staff changing room

Careful extraction fan are similar to,but different from extraction fans (air exchange fans). Lets try to explain, an extraction fan function is to extract ,lets say water molecules (Steam) from the air,good. While those that are are air exchangers only exchange the air (not heavier pollutants) within the room over much much longer period of time, this being dependant upon that rooms give volume ect. People more often buy what they believe to be an extractor, who's designed function is only to exchange the air in a room, instead of a one that is good at extracting fumes ect. Not what you asked for but I Hope this helps. Nothing worse then spending money on some thing that's not designed for the task.

Depends on the size of the room what size fan you have to install. Why not put a non timed fan in with an occupancy detector? Loads of ways you can do it to be honest. Are you qualified in any sort of a trade? As your questions suggest you should be able to answer yourself! Just comes naturally when you know what you are doing.