Question: Is inserting a PM2.5 filter into a cotton or silk mask’s filter pocket as protective as double masking?

Answer:  Yes, based on what we currently know, adding PM2.5 filters is as protective as double masking.  Returning to the Q&A of 1/26 on double masking, wearing a three-layer mask that has “outer layers consisting of a flexible, tightly woven fabric that can conform well to the face and a middle layer consisting of a non-woven high-efficiency filter material (e.g., vacuum bag material)” is highly effective.  Put in more straightforward terms– wearing a tightly woven mask that has 3 layers, including a filter in the middle, is highly effective.  And since you mentioned silk, I thought I’d add one interesting finding CDC reported, which is that silk may help repel moisture droplets, thereby reducing fabric wetting and increasing comfort.  And as a reminder, Figure1 shows CDC’s Do’s and Don’ts for masks.

Figure 1. CDC Mask Recommendations (from CDC)