user-1: Which Stack Exchange would befit How exactly do respirators seal out pathogens?? 
I'm uncertain why moderator Bryan Krause closed this question on pathogens as off-topic. It can be asked for any surgical mask, not just 'construction of respirators'.

user9943: The question is on the engineering of a medical device and that's not our cup of tea, so Bryan rightfully closed it, responding to other votes from our community. 
However, the question is well researched and of too good a quality to be just buried and forgotten. So the mod team re-opened it for you and transferred it over to our medical experts on Medical Sciences SE.  
Thanks for bringing this up in meta.

Update
MedSci rejected the Q too, based on 

A doctor doesn't need to know a single thing about how respirators work

Which I found an interesting note....
Anyway, I brought it too Engineering Meta.SE and let's see what they have to say about this.  

Update
Engineering.SE accepted the question on its main site

-1: <p>Which Stack Exchange would befit <a href="https://biology.stackexchange.com/q/89917/4466">How exactly do respirators seal out pathogens?</a>? </p>

<p>I'm uncertain why moderator Bryan Krause closed this question on pathogens as off-topic. It can be asked for any surgical mask, not just 'construction of respirators'.</p>
 9943: <p>The question is on the engineering of a medical device and that's not our cup of tea, so Bryan rightfully closed it, responding to other votes from our community. </p>

<p>However, the question is well researched and of too good a quality to be just buried and forgotten. So the mod team re-opened it for you and <a href="https://medicalsciences.stackexchange.com/questions/20926/how-exactly-do-respirators-seal-out-pathogens">transferred</a> it over to our medical experts on <a href="https://medicalsciences.stackexchange.com/">Medical Sciences SE</a>.  </p>

<p>Thanks for bringing this up in meta.</p>

<p><br>
<strong>Update</strong><br>
MedSci rejected the Q too, based on </p>

<blockquote>
  <p>A doctor doesn't need to know a single thing about how respirators work</p>
</blockquote>

<p>Which I found an interesting note....</p>

<p>Anyway, I brought it too <a href="https://engineering.meta.stackexchange.com/questions/614/is-this-on-topic-here">Engineering Meta.SE</a> and let's see what they have to say about this.  </p>

<p><br>
<strong>Update</strong><br>
Engineering.SE <a href="https://engineering.stackexchange.com/questions/32993/how-exactly-do-respirators-seal-out-pathogens">accepted the question on its main site</a></p>
