Not sure how to simplify this:
Let's say a mask blocks 90% outgoing and 50% incoming.
For math's sake, let's say a person exhales 100 particles. If they aren't wearing a mask and you aren't, chances of you getting Covid are high.
Now, if that person is wearing a mask, that means only 10 particles get out... your chances of getting Covid are pretty low.
Even better, if you're wearing a mask too, only 5 of those 10 particles may get to you... even way lower.
So the highest efficacy is when an infected person (who can be non-symptomatic) is wearing a mask because that blocks the most particles from getting into the air. Combine that with social distancing and that will greatly reduce the spread.
That's why science says wearing a mask protects others because you may be infected but asymptomatic so a face cover reduces the viral load... that's how community spread happens.
What I don't understand is how this became political.