Mach trim
The reason that "I" would prefer to have crews turn the fans on BEFORE entering the ramp area is as you mentioned that on Initial start up a "mist" of brake-dust which can be seen(carbon which isn't really healthy for any-one being of a cumulative nature)and the "gassing" from the Lithium grease when it reaches a "certain" temperature (invisible but smelt)(wheel bearings) that always seems to happen to me when setting up the fuel panel.Added to this any lubricants that have made their way (gravity with assistance from condensation)to the brake area (including landing gear grease,spilt strut fluid,corrosion inhibitors etc etc)which all add up to "interesting smells" being blown around.