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Effect of 4 h of use on the performance of surgical masks (left) and respirators (right). Performance was measured by air permeability (Ap) (l/m2/s) (A, B); burst resistance (Br) (bar) (C, D); and pressure differential (Pd) (Δp) (E, F). *P < 0.05; **P < 0.01; ***P < 0.001. ns, not significant.
