That's a good lesson in data science (the author is one). He has all the numbers right but did he forget some things ?
I didn't see anything about re-usability of the masks (no one has an endless stash of mask with a fresh one for every use)? What about using them right when putting them on/off and while wearing them (masks are efficient only when worn correctly, or not?)?
VTT-research has tested different mask fabrics and made simulations of the spread
With mask: https://youtu.be/blpit-6A7Yo (red is dry particles, blue is droplets)
Without mask: https://youtu.be/HsGJlANzeq8
Tests re-affirm the existing conclusion that non-medical mask-wearing helps from spreading the disease but is not very effective in protect the wearer.
The best available masks are made of microfiber and can filter up to 70% of particles smaller than 2.5 micrometers. Conventional garment fabrics filter 20-45% of particles smaller than 0.65-3.3 microns in size, while FFP2 shields 96% and FFP3 99% of particles up to 0.35 micrometers. This means that a user of fabric masks will receive several times the dose of viruses under exposed conditions compared to a user of professional respirators.
From conventional fabrics microfiber mask is best, second is polyester, cotton is the worst. One should wash them in hot water (90 celsius) after each use or throw them away.
source (in Finnish):
https://www.vttresearch.com/sites/default/files/2020-05/Medi...
https://www.vttresearch.com/fi/uutiset-ja-tarinat/kangasmask...