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. 2/

COVID-19 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 ( https://twitter.com/jeffgilchrist/status/1574709291520593923 ). 3/

Some masks, more commonly known as respirators, are designed to stop aerosols and other particulate matter are used not only in medical settings but also industrial, commercial, and construction sectors to protect against many different airborne hazards. 4/

These are usually certified by some regulatory body with designations like N95 or N99 (NIOSH) in USA ( https://www.cdc.gov/niosh/npptl/topics/respirators/disp_part/default.html ), FFP2 or FFP3 (EN 149) in Europe ( https://icc-iso.org/index.php/en/certificates/58-maskat-en ), and CA-N95/N99/N100 (CSA Z94.4.1) in Canada ( https://www.csagroup.org/store/product/CSA%20Z94.4.1%3A21/ ) and will have headband straps instead of earloops for a better fit. 5/

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. 6/

I have seen this image used a lot 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. This is where people don't understand how viruses travel through the air and how the physics of masks actually work. 7/

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 ( https://twitter.com/jeffgilchrist/status/1467582905396432904 ). It is these aerosols that the respirator blocks and filters, and therefore stops the virus particles at the same time. 8/

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? Henry Reich (@minutephysics) created an amazing animation showing the astounding physics of N95 masks ( https://www.youtube.com/watch?v=eAdanPfQdCA ). 9/

N95 respirators are actually very good at blocking both the largest and smallest particles, while medium size particles are actually the hardest to block. The N95 doesn't stop particles like a sieve, but uses the notion that any airborne particles that touch a fibre in the respirator will stick and not become airborne again. 10/

It is 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. 11/

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

12/

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. 13/

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. 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). 14/

N95s don't rely on static electricty, 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 10x more particles than regular fibres. 15/

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, and if it is 99% then is rated N99 and 99.97% is N100 ( https://www.cdc.gov/niosh/npptl/topics/respirators/disp_part/default.html ). You may have also seen R95 or P100 ratings where N represents filtering of non-oily particles while R is somewhat resistant to oil and P represents strongly resistant to oily particles. 16/

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 @minutephysics highlights in the caveats part of his video ( https://youtu.be/eAdanPfQdCA?t=249 ). 17/

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 ( https://twitter.com/Wikisteff/status/1599187855003521024 ). 18/

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 ( https://en.wikipedia.org/wiki/Dalton_(unit) ). 19/

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 ( https://www.pnas.org/doi/10.1073/pnas.2024815118 ). 20/

The atomic mass of these molecules you can smell are more than a billion times lower than the aerosols that COVID-19 virions catch a ride in. That is why molecules like oxygen (16 Da) can get in through respirators to you and exhaled carbon dioxide (44 Da) can get out just fine so you don't suffocate. 21/