Sickbert-Bennett et al measured the fitted filtration efficiency (FFE) of face mask combinations with one female and two male volunteers during a series of movements defined by the Occupational Safety and Health Administration. The authors found that the usage of a medical procedure mask underneath a cloth mask led to the best FFE of the combinations tested. On average, double-masking increased the FFE from 55% to 66%, with two procedural masks not increasing the FFE and a cloth mask worn over a procedural mask increasing the FFE to 77%. FFE is likely improved by double-masking because the fit is improved (similar to the tight fit of an N95) and the leaks are minimized.