The air purification system (also called an air filtration system) is a part of the life support system on starships in the 23rd century.
One of the many idiosyncratic details that Cervantes Quinn had noticed on the starship Dulcinea was how the air purification system had a curious rattle in the filter above the main corridor, just outside the cockpit entrance. (VAN - Declassified novella: The Stars Look Down)
In 2268, Montgomery Scott assigned a crew to repair the air filtration system aboard the bounty hunter ship Gezary. (TOS novel: Sanctuary)