[CodeToad]

"but they are VERY useful in stopping you from giving the disease to others."

No, they are not.

[babble-on]

That’s a brilliant piece of science, but on the off chance you’re serious, it’s because a fart is a gas but the Covid travels in droplets of water that will be absorbed by the mask. It’s more like when you wet your pants the wet stays absorbed on the pants rather than all over the floor.

[Flaming Conservative]

“but they are VERY useful in stopping you from giving the disease to others.”

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“No, they are not.”

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I don’t know the size of odor molecules from passing gas, but I imagine they don’t have to actually penetrate the clothing to be detected.
As I have advised before, the best homemade mask would probably be made from the material in furnace filters that are certified to filter bacteria and viruses. If we didn’t already have some good masks purchased years ago, that’s what I would make some from.