This mechanism of protection is referred to as the mucociliary escalator.
"I'm a doctor not an escalator," as Dr. McCoy attempts to helps a young a pregnant woman up a steep rocky landscape of a foreign planet while on an away mission.
Postural drainage is used to allow gravity to aid the mucociliary escalator.
During the final mission, at the escalator where there are 3 guards talking at the bottom of it...wait at the top of the escalator and one of the guards will be begin walking up to your level.
This "mucociliary escalator" is one of the principal defenses against lung infection.
The boys' perverse behavior of running up the down escalator brought attention from the security guards.