Face coverings, N95 masks, respirators, and P99 masks—how do we make sense of this confusing alphabet soup?
Standard face coverings do not have a rating. They made from cloth or paper, and they usually resist fluids. But they leak. Air comes in around the sides–anyone who has worn a mask (and who hasn’t?) knows this. Nevertheless, mask usage is key to protecting you from exposure to SARS-CoV-2, the virus that causes COVID-19. However, there are more infectious organisms that require a rated mask for protection. One such organism is Mycobacterium tuberculosis, the organism that causes tuberculosis. Another such organisms may be the B.1.1.7 variant of SARS-CoV-2, the variant of concern, which has a transmission rate fifty percent greater than standard SARS-CoV-2.
Rated masks have a letter prefix, either N, R, or P, followed by a number, usually 95, 99 or 100. The letter refers to how resistant the mask is to oil, and the number refers to the percentage of airborne particles filtered by the mask.
| Mask Type | Airborne Particles Filtered | Resistance to Oil |
| N95 | At least 95% | No |
| N99 | At least 99% | No |
| N100 | At least 99.97% | No |
| R95 | At least 95% | Somewhat |
| P95 | At least 95% | Strong |
| P99 | At least 99% | Strong |
| P100 | At least 99.97% | Strong |
These masks are held snuggly around the face by elastic bands or straps around the head that eliminate the gaps between the skin and the mask. Note that “KN95” masks do not meet U.S. standards, and often have bands that loop over the ears and not over the head. Don’t let the word “respirator” confuse you: respirator is just another name for a rated mask. All the masks in the table above are also known as respirators. For protection against most respiratory organisms, an “N” mask is adequate since resistance to oil is not necessary.
So what level of protection is needed? Standard face coverings are adequate protection against SARS-CoV-2 exposure if worn properly by all individuals interacting at distances less than six feet, but an N95 mask is needed to provide adequate respiratory protection against tuberculosis. At 2 microns long, tuberculosis organisms are much larger than SARS-CoV-2 particles which measure 0.1 microns or less. Still, I believe that N95 masks also provide adequate protection against COVID, even when unmasked, infected individuals are nearby because of the experience of collectors at the test collection centers run by the laboratories where I work. These collectors use an N95 mask and a face shield for respiratory protection, and none has become infected at work despite collecting thousands of positive swabs.
How does an N95 mask work? In addition to forming a tight seal on the face around mouth and nose, N95 masks are made from material with pore sizes between 0.1 and 0.3 microns in diameter.
“But wait!” you might say, “You said that a SARS-CoV-2 virus particle is 0.1 microns or less. What keeps the virus from slipping through?” The spread of SARS-CoV-2 virus is on respiratory droplets. These droplets are 5-10 microns in diameter, much larger than the pore size of the masks, keeping most viral particles out of the nose of the mask wearer.
What about the coming B.1.1.7 variant of SARS-CoV-2? I want to be very clear: the FDA does not recommend the general use of an N95 or higher rated mask for COVID-19 prevention. Although N95 masks protect against SARS-CoV-2, there continues to be supply shortages of these masks. Therefore, the FDA recommends that their use be reserved for health care providers. However, as we discussed last time, there may soon be a time that this recommendation changes. It is possible that an N95 mask will be required to protect against the B.1.1.7 variant because of its higher transmission rate.
So what other new information is there about the SARS-CoV-2 variants? We will discuss the latest information about variants next time.
