Air must reach the back of the room and return. DMR limits air speed over the bed to 15 cm per second and asks for about 6 air changes per hour from venting to the end of the second flush, dropping to 4 later, with fresh air filtered to 2 micrometres.
That only happens if ducts, returns and doors stay clear. Rows pushed against a return, sheets hanging over a duct, or crates stacked in the corridor all change the map of the room.