I have a 45 year old tri-level home, the bottom floor bathroom doesn't have an air vent in it. How do I go about putting a air vent in there?

Run a 4 in round duct through in the ceiling, between the joists to the exterior wall. With standard construction, you should be able cut the opening for the ceiling fan and opening in the exterior wall and snake the duct through.

Codes now require all bath vents to go outside. You will need to access ceiling space, cut through the outside wall and install a solid or flex duct.