Lots of places are now recommending or requiring masks, and I understand they serve a dual purpose - protect the wearer if the wearer is not infected, and protect the people around them if the wearer is infected.
But many types of masks only filter one way. You breathe in through the filter, and exhale through a one-way valve with no filter. That includes N95 masks, as well as the HEPA masks that some pollen allergy sufferers use. Are these filters still effective for protecting other people? Or by wearing those, am I complying with the letter but not the intent of the rule? Should we only be wearing masks without the one-way valves?
I think “masks” fitted with exhaust valves are in fact respirators. Respirators are intended to protect the wearer, not bystanders - and respirators are not masks (PDF), so using a respirator in lieu of a mask would not comply with the letter of the rule.
As the flyer I linked to shows, there are some respirators that don’t have valves in them. ISTM these would comply with the intent of the rule (in that they would also protect bystanders), but not the letter. In fact, a valveless respirator seems like it would protect bystanders better than a mask, since it seals better against the wearer’s face.
Part of the reason for the mask is if you cough or sneeze many of the droplets would be caught by the fabric, though you are right about the one way valve, but again better than nothing.
At my fire department we were given a memo to use full breathing apparatus (SCBA) when going to a area of vulnerable people, for our protection and theirs. I asked how does that help them, since the exhaliation just comes out of a one way valve. After some long silence from those in the know, the answer is that’s the best we have at this time. Now we got filter attachments for that, but not at first.
I’m not seeing that on the flyer, and I don’t understand what you mean. The exhaled air has to go somewhere, where is it going in a valveless respirator? Is it leaking out around the sides? If so, how can it be a benefit that it seals better against the wearer’s face?
When there’s a one-way valve with no filter, I would still have thought that there’s a significant effect in slowing the velocity of exhaled breath so that it doesn’t project so far. Is that not the case, is it really just like breathing out completely unobstructed?
In the valveless respirator shown on the flyer I linked to, the user inhales and exhales through the filter material; there doesn’t need to be any leakage around the edges.
The exhaled air goes out through the filter, just like the inhaled air comes in through the filter. If the person’s not infected, the wearer is protected from any infection in the outer air. If the person is infected, then other people are protected, because the filter works both ways.
And the one-way valve is going to increse the velocity of exhaled breath so that it projects further than if the person weren’t wearing a mask at all, for the same reason that partially covering the end of a garden hose makes the water a more forceful stream: putting the same volume over the same time througn a smaller opening speeds it up and makes it more forceful.
I really question this. Most one way valves I’ve worn have a larger cross section than both my nostrils combined. I have never noticed any back pressure in them which is what would be needed to increase velocity.