Repost MPPS concept of filtration again.....
Some pax are still not getting this concept correct and saying that N95 masks cannot filter viruses. (same goes for HEPA)
https://multimedia.3m.com/mws/media/409903O/respiratory-protection-against-biohazards.pdf
Very large particles (< 100 μm) in slow moving airstreams may settle out due to gravity. However, most respirable particles are too small for this mechanism. Respirable particles above 0.6 μm in diameter are typically captured efficiently by interception and inertial impaction. Inertial impaction occurs when a particle cannot follow an air streamline around a fiber because of its inertia and instead impacts into the fiber. In the interception mechanism, the particle holds to the streamline, but that streamline will naturally bring the particle close enough to come in contact with the fiber.
In contrast, diffusion is typically very efficient for particles smaller than 0.1 μm. Random movements of air molecules collide with these very small particles and cause them to wander across streamlines until they come in contact with a fiber. Because of the various mechanisms by which particulate filtration occurs, the smallest particles are typically not the most difficult to filter. Most particulate filters have a region of lower filtration efficiency somewhere between 0.05-0.5 μm.
Particles in this range are large enough to be less effectively pushed around by diffusion, but small enough to be less effectively captured by interception or impaction. The most penetrating particle size (MPPS) will depend on the filter media, air flow, and
electrostatic charge on the particle. Filters that use electrostatic attraction may have a MPPS shifted to a slightly smaller size range.
Filtration efficiencies of six different commercially available US N95 filtering facepiece respirators as tested by 3M are shown in the left side of Figure 1. (Previous research has shown that for 3M products, European FFP2 respirators have equivalent or
better filtration efficiency in tests representative of health care environments.) Averaged filtration efficiencies are shown as a function of different sized sodium chloride particles at a flow rate of 85 liters per minute.
While there was variability between different samples of the same respirator model, and between different models, the MPPS included particles with a diameter between 0.04 and 0.1 μm. As seen in Figure 1, particles that are smaller or larger than the MPPS are captured with higher filtration efficiency. Filtration via diffusion (most noticeable for particles smaller than 0.1 μm) actually increases as particle size decreases. Other research has confirmed that filter efficiency increases with decreasing particle size, even for particles as small as 0.003 μm (much smaller than that of virus).
A size distribution from a sneeze is shown on the right side of Figure 1.23 It should be noted that most of the droplet nuclei are larger than the MPPS. In other words, droplet nuclei that may contain microorganisms will be filtered with high efficiency by these respirators.
Bonus