Hmmm. It depends.
Pretty likely that a somewhat modern box fan would have a shaded pole AC motor. There are no brushes and thus no arcing from the motor. That would be for every user of such a fan in an application like this to verify. Carefully! On the other hand, a lot of electric hand drills, dremels, vacuum cleaners, etc. use an AC universal motor with brushes and are notorious "arc'ers." Many times you can even smell the ozone while they are running!
The issue with the switch arcing is also very real. Chances are that the switch is in a snap-together plastic housing. Not sure I'd trust that to exclude an explosive atmosphere.
I get the risks. A guy I grew up with became a professional painter. He was redoing a bathroom, working on the cabinets and apparently spraying a lacquer material. His boss came in to see how it was going. Unfortunately, he was smoking a cigarette. The bathroom blew up and both were very badly burned. Lots of surgeries and hospital time...and disfigured for life. Whatever it takes to avoid a situation like that--do it!
Be safe....