This is a transferring of air in the pump bearing case as it changes temperature during the startup, run, and shut down cycles.
Aspiration allows moisture laden air into the bearing oil that can cause both hydrogen embrittlement and corrosion problems.
Aspiration also occurs in the bearing casing during pump operation when the warmer air vents out the casing vent and is replaced by the cooler, moisture laden air we find in many parts of the world.
Labyrinth seals are effective in removing moisture when the pump shaft is rotating, but many labyrinth designs are not effective when the pump stops.
Positive face seals for bearings, are a better choice.