A respirator, such as an N95 respirator, is a respiratory protective device designed to achieve a very close facial fit and very efficient filtration of small airborne particles. N95 respirators are capable of filtering out all types of small and large particles protecting you from exposure to biological aerosols including viruses and bacteria.
On the other hand, Surgical N95 respirators are commonly used in healthcare settings and are a subset of N95 respirators. A Surgical N95 respirator (also referred as a medical respirator) is recommended only for use by healthcare personnel who need protection from both airborne and fluid hazards such as splashes, sprays, etc.
The N95 designation means that when subjected to careful testing, the respirator blocks at least 95% of very small (up to 0.3 micron) particles. However, coronaviruses are even smaller in size (0.065–0.125 micron in diameter) and contain a single-stranded RNA as a nucleic material.
Therefore, the virus is smaller than the filtering pores on the N95 respirator, so you may wonder that the N95 respirator doesn’t work (can’t sieve out coronaviruses), right? Not exactly!
To understand this concept first we need to know few basic principles pertaining to the behavior of viruses. “There is never a naked virus floating in the air or released by people”. The COVID-19 particle is indeed around 0.1 micron in size, but it is always enveloped by something larger. For example when a COVID infected person breaths, talks, coughs, etc., the exhaled viruses tend to attach to small droplets or aerosols (consisting of water, mucus protein and other biological materials from inside the infected person) which are all larger than 1 micron. Therefore, activities such as breathing, talking and sneezing generate particles around 1 micron in size, which can be efficiently collected by N95 respirator filters.
Although N95 respirator filter is 0.3 micron size, it works even better for particles smaller than the 0.3 micron threshold. This is because the N95 respirator filter does not only work by sieving out larger particles but also employ other mechanisms.
However counterintuitive it may sound, scientists have valid explanations for this. Firstly according to the principle of “Brownian Motion”, particles smaller than 0.3 microns move in an erratic or zig-zag kind of motion increasing the possibility of them getting trapped by the filtering fibers. Secondly due to the electrostatic attraction generated by the N95 respirator itself, particles smaller than 0.3 microns are drawn to the charged fiber and get stuck there, instead of just passing through the filtering pores. When the electrostatic charges are dissipated during extended usage and storage, the capability of stopping virus-sized particles (around 0.1 micron) diminishes. This is why repeated use of N95 respirator is not recommended.