Stocks vs. flows. The idea is that masks reduce the flow into a reservoir (like inside air of a building), and not just direct transmission. In your example dou

Stocks vs. flows. The idea is that masks reduce the flow into a reservoir (like inside air of a building), and not just direct transmission. In your example double masking might reduce your personal intake from a large reservoir (refilled by unmasked people who happen to be sick), while everyone wearing a mask should reduce the size of the reservoir.