I live in a prewar apartment building. At the top of the shower, on the wall where the water comes in, is a passive vent. There is no powered fan that is installed and no nearby electrical wiring to facilitate it.
When I open a window in my apartment, air clearly flows nicely through the vent - a piece of toilet paper can remain suspended to the vent grating.
However, with the window closed, no air flow occurs.
How can I fix this? My assumption is that air flow through the vent is possible but is occurring through a very narrow opening. When a window is open, air can flow one way through the vent and be replaced by air coming from the window. When the window is closed, the air vent is too clogged and thus too narrow to permit two way air flow and so no air flow occurs.