Make up air replaces air removed by ventilation. If there are contaminants in adjacent spaces, tight seals between spaces will prevent transfer of the contaminants between spaces. The presence of positive pressure in a contaminated space will spread contaminated air to adjacent spaces. Outdoor air quality standards limit the dumping of contaminants. As a result, it is common to clean contaminants from, the exhausted air.
Air moves through an enclosure called gravity chamber, which has a large cross sectional area.
When air enters the chamber it s velocity is slower than the entrance velocity. Due to the gravity , the heavy particles fall to the bottom of the chamber. Gravity chambers are low cost. Particles smaller than 40 m m in diameter pass through the chamber and are not collected. Because the air changes direction too quickly, particulates have more momentum and cannot make quick turns.