Can someone explain why wearing a mask won't help a

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3. It will "help" but not by much

Without a very well sealed mask, a lot of the air you breath comes through the path of least resistance, which is through the sides and gaps. Any nearby droplets just float and follow the flow of air through those same gaps.

Some of the air does flow through the fabric of the mask, and thus will stop those droplets, which is why it's a bit of help.

Still, the outside of the mask is then contaminated, so you have to be careful not to keep adjusting your mask as you then contaminate your hand and then potentially spread the virus around your face and so on.