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A "respirator with a rating of P100, has been tested to be proof against oil, and tested to filter 99.97% of all particles 0.3 microns in diameter or larger.'

  1. As you know, an atom is 99.9999% empty space. So even if a respirator's edge is touching your face, why can't pathogens cross and sneak past the face seal? See the green arrows in my pictures below.

  2. To wit, how can we guarantee that the gap between your face and a respirator's edge is smaller than any virus or pathogen?

N95, P100: What do all these mask numbers mean and how do I know it's keeping me safe? - SFGate

If the mask isn’t sealed around your face, you’re just breathing through the edges, and it doesn’t filter much at all.  Men with beards cannot get the full benefit of a mask, and many men have looked at shaving it off for the sake of their lungs.

3M writes in this handy but rather complex guide: “Do not use tight-fitting respirators or loose-fitting facepieces with beards or other facial hair or conditions that prevent direct contact between the face and the edge of the respirator.”

The left is a 3M P100 respirator.

enter image description here

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  • $\begingroup$ The air flow out of the nose is directional - it is always downwards and forwards. Since the mask goes over the chin and is above the nasal ridge and the direction of flow of air would be along the filters. As you rightly pointed out the air molecules are small enough to easily slip into the filter of the mask and they follow the path where the energy expenditure is least. Thus most of the air breathed would get exchanged through the filter rather than the gap between the face and the mask $\endgroup$
    – One Face
    Feb 10 '20 at 15:25
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The mentioned P100 particulate filter approximately corresponds to a P3 filter according to European standards.

  • A half mask with a P3 filter has a nominal protection factor of 48.
  • A full face mask with a P3 filter has a nominal protection factor of 1000.

This difference already illustrates that the protection is not only determined by the quality of the filter but also by the face seal leakage.

Note that these nominal protection factors are tested in a laboratory. The values should not be used for the assessment of a real workplace.

There are many reasons for not using nominal protection factors for assessing the likely protection at the workplace. The laboratory tests do not represent the activities covered in workplace situations; the laboratory tests only involve a small number of individuals, who cannot represent a significant portion of the population of respiratory protective device wearers in the workplace; the individuals selected for the test panel are likely to be well-trained and are familiar with the test procedures; the standards allow for disallowing people who do not pass the initial screening test needed before the total inward leakage test is carried out.
– European Standard EN 529 – Respiratory protective devices – Recommendations for selection, use, care and maintenance – Guidance document (2005)

For a realistic assessment, it is better to use so-called assigned protection factors, which depend on applicable national regulatory requirements. Typically, these values correspond to the level of respiratory protection that can realistically be expected to be achieved in the workplace by 95 % of adequately trained and supervised wearers using a properly functioning and correctly fitted respiratory protective device.

  • A half mask with a P3 filter has an assigned protection factor of 30 (Germany) or 20 (UK).
  • A full face mask with a P3 filter has an assigned protection factor of 400 (Germany) or 40 (UK).

If better protection factors are needed, you should use a power assisted filtering devices, which has the advantage that the pressure inside the facepiece may remain above the ambient air pressure.

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    $\begingroup$ This does not answer the question of how pathogens are sealed out. $\endgroup$
    – Fred
    Feb 2 '20 at 18:02

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