Is it true that masks only work to prevent germs/diseases from exiting one’s mouth/nose and not from entering one’s mouth/nose? If so, why?
In: Other
Masks reduce the outflow and inflow of airborne pathogens. They are, however, more effective at reducing outflow (at least your normal masks, not the special completely sealing masks).
The reason for this is simple. The mask does not fully seal on your face, there are little regions on its edges that still allow for some air to enter around and not through the mask. When you inhale, the air rushes into your lungs and takes the path of least resistance (or flows preferentially via it, but passes through many paths), and that path would be the small openings around the mask. That means if there are airborne pathogens all around you then chances are some will go with the air not going through the actual mask (hence not filtered).
However, when you carry the pathogen, then your breath contains it, and the exhalation via the mask, whether the air passes through or around the mask, ends up flowing sideways from your face, and whatever flows through the mask is filtered. This means the radius around you where the pathogen is spread is smaller, so it reduces the risk of infecting others.
Keep in mind its more complex than that, because some pathogens may spread only via coughing, when large aerosols are sprayed out and those easily adhere to surfaces (so a mask would make them adhere to your face and the inside of the mask), it’s not like single viral particles go flying around in the air on their own (thought that happens, depending on the pathogen and its load in your body). There is also the concept of humidity to consider. Your exhaled breath has a lot of humidity in it, so the inside of the mask would be wet, and you can also consider that for the filteration process in addition to the layer number, layer thickness, filteration hole radius in the mask and so on.
Edit: English
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