The 85% number comes from my interview with Monica Gandhi, a professor of medicine at UCSF. Recent research from Duke comparing the effectiveness of different masks also shows surgical masks and multi-ply cotton masks are very effective at filtering out particles, even when compared to N95s.

https://advances.sciencemag.org/content/6/36/eabd3083

https://coronavirus.medium.com/can-masks-function-as-a-crude-vaccine-696049cb06db