I was part of mask and respirator testing in the Air Force to support a new method of repairing runways after an attack. The main issue we were battling was the amount of respirable crystalline silica in our new repair material ... and OSHA recently added it to their standards list. Therefore, we need to find a solution. Our main contenders were: 1) surgical mask, 2) N95 respirator, and 3) USAF gas mask.

After lots of laboratory testing and field testing, we determined the N95 respirator was the least intrusive method to meet the OSHA requirements. Yes, the USAF gas mask would work but placed considerable undue stress on the Airmen performing the repair work on the runway.

DETAILS:
Respirators (ex. N95) are designed and engineered for distinctly different functions than surgical masks (and homemade cloth masks). And filters (respirators or surgical) do NOT act as sieves. Finally, the filter’s collection efficiency is a function of the size of the particles and is not dependent on whether they are bioaerosols or inert particles.

There is a particle size at which none of the “mechanical” collection mechanisms (interception, impaction, or diffusion) is most effective. This “most penetrating particle size” (MPPS) marks the best point at which to measure filter performance. Therefore, N95 respirators filter at least 95% of airborne particles and is not resistant to oil. (R is somewhat resistant to oil and P is strongly resistant to oil).

It is important to note that respirators must meet stringent certification tests established by NIOSH, whereas manufacturers of surgical masks get to choose from a variety of protocols (including ones that charge the particles) and only have to demonstrate their product is at least as good as a mask already on the market.
We looked at numerous studies comparing the performance of surgical masks using a standardized airflow, filter performance showed to be highly variable. Collection efficiency of surgical mask filters can range from less than 10% to nearly 90% for different manufacturers’ masks when measured using the test parameters respirators need for NIOSH certification. Therefore, surgical masks were out of the running.

N95 respirators were then compared to USAF gas masks. Both worked at filtering the silica, however, the USAF gas mask was bulkier, heavier, and more costly. Unfortunately, the most important aspect of a respirator’s performance is how well it fits the face and minimizes the degree of leakage around the facepiece. Therefore, we had to institute a fit test program for all engineers.

OSHA's action level for silica is 50 μg/m3 (micrograms of silica per cubic meter of air), averaged over an 8-hour day. While the size of silica varies significantly from .01 μm (micrometre or micron) to 100 μm in diameter, our median silica particle size was ~3 μm.
SARS-CoV-2 is significantly smaller than our median silicon particle size (0.1 μm in size), SARS-CoV-2 always bonds to something larger. It could be respiratory droplets (>5 μm) or aerosols (<5 μm).

Therefore, if you want to be effective in stopping particles, you need a N95 respirator and it needs to be fit tested to the individual and is only good for one time use.