Abstract: An inexpensive, bi-directional, low flow resistance air filter for medical ventilation equipment and the like includes a disposable shell possessing a cylindrical conduit terminating in an exponentially curved radially outwardly flaring surface. The exponentially curved construction minimizes dead space and hence also flow resistance Radially and axially spaced stepped annular surfaces connected by short axially extending cylindrical sidewall portions at a distal end of the shell function as coupling and sealing surfaces. A ring of adhesive applied adjacent a peripheral edge of an electret filter disk mounts the filter disk internally within the shell. An elastic retaining band detachably secures an equipment connection adapter to the shell, such that a technician may easily and quickly replace the shell and filter.