I am looking to add some storage space to my bathrooms by installing some wall-mounted, pre-built upper cabinets. The bathroom is arranged such that the only reasonable location for a cabinet would be directly behind/above the toilet. The bathroom's vent fan is directly above the toilet, and installing the cabinet here would mean that the vent fan would be pointing at the top of the cabinet.
I'm concerned that I might be inadvertently blocking the vent fan's airflow and reducing its effectiveness. How much clearance is required below a vent fan for it to operate normally? I haven't found any such numbers in product manuals or install instructions, could there be an relevant building code?
I don't know the particular model of my vent fans, but this is a US home around 10 years old. The bathrooms have 9-foot ceilings. The fans are mounted near-ish the walls, but have at least 30 inches clearance on the sides and 48 inches in the front.