If the mask fits: Facial dimensions and mask performance

Fit factor (FF) is a generally accepted metric to assess the facial fit of Respiratory Protective Devices (RPD); however, we question the utility of a numerical output. In this study, we aimed to examine the relationship between facial features and FF and explore the implications and validity of current fit test standards. A single accredited fit tester provided a test on a relatively large population of 'real world' RPD users, using the same model of RPD (3M 8835 + ). A total of 262 candidate test participants presented for face fit testing by their employers from 18 organisations across the UK, achieving an overall pass rate of > 90%. Compared to face length and width, we identified jaw width to have a statistically significant negative relationship with FF (rho = -0.28; p < 0.001). Given our findings, we present two contrasting scenarios based on the interpretation of fit test results: (1) if absolute FF scores are of value, jaw width should be given due consideration in mask design, otherwise (2) the presentation of FF scores with pass test results should be reconsidered. Whilst it is encouraging that a large proportion of our study sample achieved a passing FF score, we would welcome reconsideration of the use of FF scores in light of our proposed scenarios.