A passive air filter is usually a type of HEPA (High Efficiency Particulate Absorption) filter that is positioned in the aerosol delivery line of an inhalation exposure system. It is used on negative pressure system to allow passive or negative pressure flow to enter the system. The filtered air acts to dilute and dry the aerosol. A passive air filter also acts as a pressure buffer and enables steady maintenance of chamber pressure and easy, rapid adjustments. In positive pressure systems it acts to scrub particulates out of the air stream before it exits the system.
