Discussion

In this study, the application of a procedural face mask over an N95 FFR led to FFR fit-testing failure in 13% of participants. This empirical result is consistent with the theoretical physics and engineering model previously reported by the authors. ^{Reference Mueller, Karimi, Poterack, Seville and Tipton4} Additional mechanisms such as deformation of the N95 FFR by the overlying face mask could also contribute to N95 FFR failure. The combined empirical results from 2 previously published studies also predict a risk of N95 FFR failure with overlying face masks. Sinkule et al demonstrated increased airway pressures when an N95 FFR is covered by a surgical mask. ^{Reference Sinkule, Powell and Goss6} Nelson and Colton ^{Reference Nelson and Colton5} showed that increased airway pressures lead to air-purifying respirator leakage. Based on their similar seal configurations, these results suggest that pressure-driven leakage would also occur with N95 FFRs. ^{Reference Nelson and Colton5} The combined result of these studies support both our empirical findings and the previously reported theoretical model. ^{Reference Mueller, Karimi, Poterack, Seville and Tipton4}

In healthcare settings, specific N95 FFR models are fit tested to ensure an adequate N95 FFR seal. Events or use conditions that degrade the quality of that fit reduce or eliminate the expected protective effect of the FFR. This assertion is underscored by research demonstrating that most particle transmission to an N95 FFR user is through face-seal leakage rather than through the filter medium. ^{Reference Grinshpun, Haruta, Eninger, Reponen, McKay and Lee8}

Disposable N95 FFRs are designed, regulated, and marketed for single use. Manufacturers’ user instructions include statements such as “Discard after every use when used for surgical procedures,” ⁹ and “Fit testing must be performed while the test subject is wearing any applicable safety equipment that may be worn during actual respirator use which could interfere with respirator fit.” ¹⁰ The latter statement indicates that an individual’s N95 FFR fit-testing result is valid only for the actual test conditions. Thus, adding an overlying face mask later invalidates test results obtained when the overlying face mask was not in place.

Our study had several limitations. It was conducted at a single institution. A single type of fit-testing device was utilized and a single specific N95 FFR and procedural mask combination was tested. The results may not be applicable to other combinations of N95 FFRs and face masks. The binary pass–fail results of standard employee or occupational health testing are reported; actual measured leakage flow rates were not available. The reproducibility of standard occupational health quantitative fit testing was not verified by performing repeat testing without a covering face mask in place. The clinical impact of the measured increased failure rate is unknown.

In conclusion, this study demonstrated that covering N95 FFRs with a face mask can cause N95 FFR failure. Generic guidance on the use of N95 FFRs should consider the potential risk of increased failure when they are worn with an overlying face mask. Further research to verify the results with our and other FFR–face-mask combinations is needed.