My understanding is the small particles can basically cover the surface area of the fibers and block their electric field. Here’s an image from one of the linked studies showing filters that are (a) clean, (b) after one test, and (c) having absorbed 2 grams /​ m^2 of smoke and having its efficiency drop from 92% to 33%.
Efficiency can decrease too, especially when there are lots of very small particles like smoke. See this reddit thread: https://​​www.reddit.com/​​r/​​crboxes/​​comments/​​1fznar2/​​comment/​​lr2j404/​​.
My understanding is the small particles can basically cover the surface area of the fibers and block their electric field. Here’s an image from one of the linked studies showing filters that are (a) clean, (b) after one test, and (c) having absorbed 2 grams /​ m^2 of smoke and having its efficiency drop from 92% to 33%.