Where does the "doubling" in the title come from? I see a 10-15% increase from the article. And only when the masks are used in a specific ordering: procedure m

Where does the "doubling" in the title come from? I see a 10-15% increase from the article. And only when the masks are used in a specific ordering: procedure mask on face, and cloth mask over procedure mask. > wearing a procedure mask under the cloth face covering produced marked improvements It should also be noted that there were 3 volunteers. > Limitations of this study are that we tested only 1 type of procedure mask and that 3 volunteers participated in the doubling evaluations