Thursday, December 15, 2022

Yup, Masks Really Work. I'm not Sure Why This is So Hard to Grasp!

I've been inundated lately with pictures from the sides of mask boxes that say something like this:

I could return volley with this, from the side of my 3M Aura N95s:

Instead I went for a lesson in logic, specifically, the fallacy of accident:

And here's a great thread from Dr. Jeff Gilchrist explaining the benefits of masks, again, for the benefit of the many many people out there who really don't think they do anything to prevent the spread of viruses:

How respirators actually work and which type is best?

I see many claims masks don't work. This thread will explain how they block viruses much smaller than the size of the holes in the filter and why you can still smell nasty things while wearing effective masks.

One of the issues is that there are different types of masks that are designed for different purposes. Cloth masks and the common blue earloop procedure masks were never designed to protect people from airborne pathogens that hang in the air for extended periods of time. 

Covid and other respiratory viruses are transmitted via aerosols breathed, talked, coughed, sung, and sneezed into the air from an infected person. Someone can enter a poorly ventilated room after the infected person has left and still breathe in enough aerosols to become infected themselves. You can learn more about transmission and multiple methods to protect yourself here. Some masks, more commonly known as respirators, are designed to stop aerosols and other particulate matter and are used not only in medical settings but also industrial, commercial, and construction sectors to protect against many different airborne hazards. These are usually certified by some regulatory body with designations like N95 or N99 (NIOSH) in the USA, and CA-N95/N99/N100 (CSA Z94.4.1) in Canada. 

The material these certified respirators are made from will be able to filter more than 95% of particles of a certain size, and often more than 98%/99%, but that is only for air that passes through the filter. The most important part is how well the respirator fits on your face. If the fit is poor, and there are gaps for air to go around, none of that air will be filtered. 

This image (above) has been used as an example of how masks couldn't possibly work since the size of a single Covid-19 virus (virion) is 0.3 micron, smaller than a light dust particle. Many don't understand how viruses travel through the air and how the physics of masks actually work. First, viruses don't travel on their own through the air; they catch a ride inside larger aerosols which also contain water, mucins from the lining of the lungs, deep lung fluid, and surfactants to make up the complex blob you see in the image (at right). It is these aerosols that the respirator blocks and filters, and therefore stops the virus particles at the same time.  

But even with aerosols, some are still smaller than the holes within the N95 respirator material, so how can they effectively filter out such small aerosols? @minutephysics created an amazing 6 minute animation showing the astounding physics of N95 masks. N95 respirators are actually very good at blocking both the largest and smallest particles, while medium size particles are the hardest to block. It's not the fibres themselves, but because the size of the particles are so small, everything is sticky at a microscopic scale. The van der Waals forces between molecules is more than enough to hold very small things in place. N95 respirators use several tricks to get particles to touch their fibres:

  1. Capture by inertial impaction
  2. Capture by diffusion
  3. Capture by interception
  4. Capture by electrostatic attraction
With multiple layers of fibres, particles larger than 1 micron typically travel in a straight line so are almost guaranteed to hit a fibre and stick. Particles smaller than 0.1 micron are so light that collisions with air molecules bounce them around so they move in a random zigzag pattern (Brownian motion) making it extremely likely the particle will bump into a fibre and get stuck. Particles in between those sizes (around 0.3 microns) don't travel in straight lines and also don't zigzag randomly but get carried along with the air as it flows around fibres and likely past fibres so can possibly sneak by respirators even with multiple layers. So N95s use a final trick of capturing particles of all sizes using an electric field where even neutral particles will still be attracted (as you can see from neutrally charged styrofoam sticking to a cat with static charged fur.) 

N95s don't rely on static electricity; their fibres work like permanent magnets but for electricity called electrets. You can electrotize a piece of plastic to give it a permanent electric field which allows the fibres in N95s to capture ten times more particles than regular fibres. Respirators can block almost 100% of the smallest and largest particles, and if more than 95% of the tricky medium size particles can be filtered out, the respirator is rated N95. If it is 99%, then it's rated N99, and 99.97% is N100. You many have also seen R95 or P100 ratings where N represents filtering of non-oily particles, R is somewhat resistant to oil, and P represents strongly resistant to oily particles.

Remember that even if the material in an N95 can filter >95-99% of particles, they need to go through the mask to work, so fit is super important, which Henry Reich highlights in the caveats part of that video above. 

If N95s can block all these tiny particles, how come you can still smell some nasty things like farts while wearing a fit tested N95 respirator, and does this mean the respirator isn't working? Thankfully, @Wikisteff has done some awesome calculations to explain why you can still smell through respirators:

"The smell of farts comes from hydrogen sulfide (34 daltons), methanethiol (48 daltons), dimethyl sulfide (62 daltons), indole (117 daltons), and skatole (131 daltons). A SARS-CoV-2 virion masses 6 billion daltons, and an aerosol droplet 160 billion."

A dalton (Da) or unified atomic mass unit (u) is commonly used in physics and chemistry to express the mass of atomic-scale objects such as atoms, molecules, and elementary particles. As @Wikisteff points out, the smell of a fart comes from multiple molecules that range between 34 Da and 131 Da, while a single Covid-19 virion is 6 billion Da, and an aerosol droplet 160 billion Da, both monstrous sizes in comparison. The atomic mass of these molecules you can smell are more than a billion times less than the aerosols that Covid-19 virions catch a ride in. That's why molecules like oxygen (16 Da) can get in through respirators to you, and exhaled carbon dioxide (44 Da) can get out so you don't suffocate. It is estimated that a Covid-19 infected person carries between 1 billion and 100 billion virions during peak infection.

It's nice to know science and engineering have provided respirators that use the laws of physics to help protect people from all those virions doing damage to your body. Now that you know how respirators work to capture very tiny particles, is it better to get a P100 that can filter more particles than an N95? Aaron Collins created a video looking at the options. While a P100 respirator can filter more and protect against both oil and non-oil based particles compared to an N95, they are designed differently, and, as a result, also are typically less breathable, so there are trade-offs. It's much better to wear a great fitting N95 respirator than a less well-fitting P100. If you have a proper fit factor of 100 (pass a fit test), an N95 ends up with a worn filtration efficiency of 98.3%, and N99 is 98.8% and P100 at 99%. 

The better the fit, the higher the filtration efficiency, and the worse the fit, the less well the respirator works in real life with some air going around the gaps and not being filtered at all. As an example, Collins aka @masknerd created a scenario of a 1 hour meeting where ventilation is 2ACH (air changes per hour) in an average size conference room (150 cubed feet) with 8 people and a community Covid rate of 12.5%, so at least one person in the meeting is infected. If nobody is wearing a respirator except for you, what would happen with different masks and respirators at different fit/face leakage values? 

These are just high level rough estimates, but if people were just talking, those without a mask have a 37% change of becoming infected. With a cloth mask, even if you seal it well and tape it to your face, because the filtration ability is low, your risk only drops from 37% down to 22%. "While cloth masks are better than nothing, they are closer to nothing than they are to better." Looking at N95s, if you have a poor fit with 30% leakage, the risk of infection drops from 37% without one to 16% with - much better than nothing. Even without passing a quantitative fit test, leakage of 5% drops that even further to 3.4%. If you compare the N95 at 3.4% to N99 at 3.2% and P100 at 3.1%, you can see there is very little difference between those respirator types, so it may not be worth giving up extra breathability and additional cost if all of those types fit the same.  


If you have the opportunity to do an actual fit test and can find an N95 respirator has a fit factor of 100 (so 1% face leakage), the risk drops from 37% without a mask to 1.1% with a well-fitting N95. A great fitting N95 is three times better than the slightly better filtration efficiency of an N99/P100 with a slightly worse fit (5% face leakage) and many times better than one with even worse fit.

Besides disposable respirators, there are also reusable elastomeric respirators that have replaceable often longer-lasting filters. Some elastomeric respirators are much more breathable than any N95 I've tried, become cheaper to use fairly quickly, and the soft material (e.g. silicone) allows more peopel to pass a fit test. 

Are you looking to buy a respirator, elastomeric, or even a CO2 monitor? The @DonateMask store uses 100% of money they raise to send free N95-equivalent masks and rapid tests to peopel in need across Canada. They also have affordable sample packs so you can more easily try to find masks that fit you and your family. You can also request masks and rapid tests or donate to their charity.

I should also add that these physics principles also apply to non-mask filters as well. @JimRosenthal4, the "R" in Corsi-Rosenthal (CR) Boxes points out that the simple straining/sieve method only accounts for about 1% of particles being removed.

Get it?? So wear an N95 whenever you possible can when not at home especially through the break and the weeks following. If you look at the data from my region, and compare December 7th in the past three years, it's clear that our hospitalization rates are closer to pre-break in 2020, than 2021. Our rates haven't gone back down to a reasonable place since last July. 

It's hard to read, so...
  • On Dec. 7, 2020, we were averaging 34 hospitalizations per day and then had a full lockdown to control the spread from December 26 (have to let people do their Christmas shopping!) to into February. 
  • On Dec. 7, 2021, we were averaging 11 hospitalizations per day, so we got a little over-confident and extended the school break by just two days into January, and then we ended up averaging 144 hospitalization per day by January 24th! 
  • This year, on Dec. 7, 2022, we were at 31 hospitalizations per day, three times higher than last year, and we don't have HEPA filters in all classrooms yet, and people are fighting against this crazy notion that we maybe just politely ask people to please wear a mask if they can, and there's no way we'll have another lockdown. 
I'm very concerned what the end of January will look like - and extremely concerned for so many children. This year, it's little ones filling the hospitals. Please wear an N95. 

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