Wearing of face coverings serves two purposes: reducing the concentration of ambient particles inhaled and reducing the emission of respiratory particles generated by the wearer. The efficiency of different face coverings depends on the material, design, and fit. Face coverings such as N95 respirators, when worn properly, are highly efficient at filtering ambient particles during inhalation. Some N95 respirators, as well as other face covering types, include a one-way valve to allow easier exhalation while still maintaining a high efficiency of filtration of inhaled ambient particles. The extent to which these valves decrease the efficiency of filtration of emitted respiratory particles is, however, not well established. Here, we show that different valved N95s exhibit highly variable filtration efficiencies for exhaled respiratory particles. As such, valved N95s may not provide reliable source control of respired particles and their use should be discouraged in situations in which such source control is needed.
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