U of North Carolina studies masks and ways to improve them

finds most masks around 26 to 36% effective filtering 0.3 micron droplets breathing in.

best masks perhaps 80% effective filtering 0.3 micron particles that are 2X the diameter of the 0.125 micron Covid-19 virus

Has a video on a fast simple tweak to double the filtration efficiency of a surgical mask.

https://medicalxpress.com/news/2020-12-mask-modifications-covid-.html

The Danish 6000 person clinical trial used simple surgical masks,
so the UNC study partially explains why
at best the Danes in the study wearing surgical masks
were only at best 15% better protected when breathing in,
or (2.1-1.8/2.1)