Why does dust keep falling even in an enclosed space?

Question

Why does dust keep falling even in an enclosed space?

In: 1

Answer 1

The only way to prevent dust is with a negative pressure air system in a container environment.
Basically, a semiconductor clean room.

Answer 2

In a truly enclosed and hermetically sealed space like the inside of a pressure vessel there is no dust.

In a room with the door closed there’s still a lot of air transfer around the door and windows, and the building material itself is slowly disintegrating.