COVID-19 Day 20: How N95 masks are like a mosh pit

In my previous article, I wrote about a study questioning the superiority of N95 respirators over surgical masks. In today’s Deep Dive we’re going to look into what “N95” actually means and why it’s important in infectious disease. And what that moshpit GIF has to do with any of this.

N95 is a designation of the National Institute for Occupational Safety and Health (NIOSH). The ‘N’ stands for a material that is NOT oil-repellent or oil-proof. The ’95’ identifies the respirator as being able to block least 95% of particles 0.3 microns or larger. Respirators can have other classifications like R99, P100, etc. depending on filtering level and vulnerability to oil-based fumes.

So, if the CDC insists on the highest standards of protection, why settle for a mask that only filters out 95% of particles. Why not 99% (N99 respirators) or 100% (N100 respirators)? Because the higher filtering respirators restrict airflow and make it harder for the wearer to breathe. Wearing a mask that’s hard to breathe through, increases fatigue, greatly reduces a health care worker’s performance, cognitive ability, and health.

But an N95 isn’t really a trade-off in safety for practicality. A 95% filter doesn’t mean that it lets 5% of viruses get through. In actuality, an N95 filter catches closer to 99% of them.

Huh? Why don’t they call it an N99 mask then?

This is where things get weird because things get small. The 2019-nCoV virus (that causes COVID-19) is about 0.1 microns in size. That’s smaller than the 0.3 particles that N95 masks are rated to block?

Now, you may remember from high-school science class, there’s an effect called Brownian Motion: the random motion of individual atoms causing microscopic particles to move, even in a sealed container. And you thought you’d never need that info ever in life, right? Yay, science.

Particles 0.1 microns and smaller are subject to Brownian motion and travel in a turbulent, zig-zag path, causing them to end up getting caught by filter fibers just as easily as larger particles. In much the same way that jumpy little guy hyped up on Redbull takes up more room in a mosh pit than his fat buddy who’s standing around, and is more likely to bump into the security guard. A study by NASA documented this “Diffusion” effect (Fig A.) in HEPA filters.

NASA diagram of Brownian motion causing diffusion filtering

NASA diagram of Brownian motion causing diffusion filtering

It is thought that COVID-19 infections are primarily spread when viral particles are expelled with droplets of our breath when we cough, sneeze, or just talk. Micro-droplets range from 0.1-1.0 microns in size. Sneezed or coughed droplets are even to larger. Through diffusion and direct interception N95 masks can “effectively” capture over 99% of particles smaller than 0.3 microns even though their Specs say otherwise.

I wonder how much a dogmatic preoccupation with technical specifications has colored CDC guidelines on mask usage. Perfection is the enemy of the good (enough). This adage is no more relevant than today. If 95% rating was good enough to work, could that principle be applied to the shortage of Personal Protective Equipment for health care workers treating COVID-19 patients? Oy!



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