Letter to the Editor

The recent commentary by Chang et al¹ raised questions about the validity of prohibiting N95 and other face masks with exhalation valves when the goal is SARS-CoV-2 source control from asymptomatic but contagious individuals.

Regardless of any other considerations, an inescapable fact about unfiltered exhaust valves such as the “Cool Flow Valve” found on 3M vented N95 masks (see Fig 1) is that during every exhalation, the mask has a big hole in the middle of it.

Figure 1.

The main practical difference between a simple 1-inch hole in the front of a mask and an unfiltered exhalation valve is that the plastic cover of the valve deflects the stream downward; perhaps better than shooting it right into someone else's face, but still a small consolation if the stream is full of virus-laden aerosol.

While it is true, as the authors referenced, that at least one study has reported similar droplet release from valved N95 respirators and some cloth masks under specific modeled conditions,² there are other compelling demonstrations of the unimpeded airflow through a valve as would be expected for such a large hole in the mask. For example, see the impressive videos published recently by Staymates³ and publicly accessible at the National Institutes of Standards and Technology website.⁴

The CDC and NIOSH released a study in December 2020 that measured outward particle flow through several models of valved N95-style respirators in an experimental mechanical system.⁵ Notably, this study can be easily misinterpreted, because while some of the valved models demonstrated a high outward penetration rate as would be expected, other models showed a very low outward penetration rate, giving the impression that these valved masks would indeed protect a bystander from the wearer. However, the discussion section reveals a crucial point (emphasis is mine): “NIOSH researchers inferred that the valves on some models may have remained mostly closed; this inference is based on low penetration findings such as measurements of <1% that would not have been possible with an open exhalation valve.”

I agree with the authors’ concerns that discarding such masks is unwise given shortages of PPE, but there is a simple solution that we have been advocating to the public: Tape over the valve. A piece of tape securely placed over the front and folded over the 4 sides of the plastic valve housing in gift-wrap fashion should withstand the outward pressure. A piece of tape over the inner side of the valve is more secure, but bystanders cannot tell that it is there, so still adding tape on the outside is a useful visual cue.