I had a mushroom fan and no ridge vent. You could see the dust accumulation around switchplates and sockets where air was being drawn between the interior walls up into the attic by the negative pressure.
We were having our roof redone, so we added a full length ridge vent. I also had them blank out where the fan was. Good riddance. Proper passive ventilation beats those stupid fans all the way up the hallway and back.
Ridge vent retrofitting is pretty easy, and I would've done it if our roof wasn't so near replacement:
https://homeguides.sfgate.com/add-ro...oof-43671.html
Sam