Fit test for filtering facepieces: search for a low-cost, quantitative method

Conventional fit-testing methods use HEPA filter cartridges to distinguish face-seal leakage from filter penetration and thereby test a respirator's ability to face-seal. These methods cannot be applied to low-efficiency filtering facepieces that also protect the wearer through aerosol particle retention by the filter material. Therefore, the ability of a filtering facepiece to face-seal is of interest to the developer for improving the design and to the user for finding the conditions for optimal face-seal. A unique difference has been found between the combined aerosol penetration through filter medium and leak site at low versus high flow rate. This feature has been used to differentiate face-seal leakage from filter penetration. A "fit index" has been introduced as the most sensitive indicator of fit. When normalized by reference to the aerosol penetration through the filter material, this index displays a unified behavior irrespective of the filter material used. The fit index is determined by relating the measured total aerosol concentration at a high flow rate to that at a low flow rate. Tests during normal breathing of a human subject compare well with the data obtained during breath-holding experiments. Available instruments may be used for this test, but need to be modified for tests on humans.