I like it! The leaky bag would work until an anti-vaxxer driven mad(der) by low blood oxygen levels in the brain bumps into you, squeezing the leaky bag flat and pushing viruses in through the holes, some of which may remain to be inhaled when the bag is inflated again by the fan (I think you may be underestimating the power required by the fan to push through the filter), therefore a bag covered in very small valves to keep air from entering via anywhere other than the fan inlet, might be wiser?On the contrary , the fan sucks air into a leaky bag via the filter. Since the bag is now slightly above room pressure it will stop potentially virus laden air seeping in a 12v fan running at half voltage will provide about 40 cfm into a static head of a few mm of water pressure. And drawing less than a watt. So yes it needs a buffer volume ..which can be the hose. Not saying that a sub aqua mask is a bad idea, or demand valves in the mouth but the pressure differential in an atmospheric pressure zone is much less difficult than underwater
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