How #respirators actually work and which type is best?
I see many people claiming #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. 🧵 1/
This thread is long so a 1 page unrolled version can be found here: https://pingthread.com/thread/1602994049815908353
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/
@jeffgilchrist Does this mean that simply exposure to air (as opposed to usage) degrades those masks? (In the same way electrets made from some waxes degrade by being covered by charged dust particles from the air.)
