Baristatic filters are a devices which remove pollutants from a planet's atmosphere. They function by projecting negatively-charged ions into the stratosphere, which then transfer their charge to atmospheric pollutants. The thus-charged pollutants are then collected by the positively-charged intake vents of the baristatic filter. A consequence of this system is that an ionic current is set up in the planet's atmosphere, which on a large enough scale can interfere with the function of transporters. A thousand baristatic filters were installed on Tagra IV in an effort to combat their pollution problem. (TNG: "True Q")