Passive vents, as the name suggests, passively interact with the
atmosphere. Basically think of them as an open pipe.
This makes them useful to vent a pipe network to a vacuum, or equalize
the atmospheres of two otherwise unconnected rooms. Being passive
however, they are just as capable of intaking air as they are outputting
air, with no way to control it. If you want to guarantee that air only
goes -out-, use an injector or a pump vent.
Cody Brittain