Two masks can be worse than one: N95 respirator failure caused by an overlying face mask.

We have demonstrated the effect of covering an N95 filtering facepiece respirator (FFR) with an overlying face mask. In total, 100 participants successfully completed quantitative fit testing wearing a 3M 1870+ FFR. Among them, 13 (13%; 95% CI, 7%-22%) failed subsequent fit testing when simultaneously wearing a Halyard 47117 procedural mask over the FFR.