Masks in the hands of untrained non medical personal can be very useful for robbing banks, but are worse than useless for protecting anyone from a virus.
The gold standard for claims about the effectiveness or otherwise of anything medical is the randomized clinical trial. In 2014, and so well before the current insanity hit, a number of people with real Ph'ds in closely related specializations published a careful review of ten such trials: Wong VW, Cowling BJ, Aiello AE. Hand hygiene and risk of influenza virus infections in the community: a systematic review and meta-analysis. In May of 2020 a larger team including the original author reported revisiting the issue to include four more in the CDC's Journal of Emerging Infectious Diseases as: Nonpharmaceutical Measures for Pandemic Influenza in Nonhealthcare Settings-Personal Protective and Environmental Measures.
The key sentence from the summary for that report is:
"Although mechanistic studies support the potential effect of hand hygiene or face masks, evidence from 14 randomized controlled trials of these measures did not support a substantial effect on transmission of laboratory-confirmed influenza."
That's the science: 14 randomized clinial trials, all done before today's double goodthink became mandatory, and no evidence that masks used in non medical settings reduce viral infections.
The key reason masks are useless in non clinical settings is the combination of misuse with reuse.
The N95 mask is called an "N95" because, used properly, it retains about 95% of particles in the 250-350 nanometer range getting pushed into the barrier in either direction. Many of the particles carrying viral material fall into this range, but the virus itself will typically be less than a tenth of that - so on its scale an N95 mask looks like a mosquito net.
That might seem to make the typical consumer mask useless, but it's actually worse than that. What makes a mask effective in medical usage is a combination of fit, breathing protocol, and mask disposal. Pick up a low cost mask somewhere and it won't fit tightly; no one will tell you to breath through, and not around, the mask; and you'll probably wear it more than once.
To feel the effect hold your hand in front of your mouth as if it were a mask, breathe, and feel the air moving between your fingers and around the edges. On inhalation these air flows form thousands of mini tornados much the way water does in a flushing toilet. That action causes at least some of the stuff viral particles ride on to attach themselves to the fibers in the mask. Notice too that your hand now feels a little wet? that's why you don't reuse a mask.
So what happens when you exhale? Find a walk-in freezer and check out the fogging pattern your breath leaves in the air while wearing, and not wearing, a mask and you'll see: you blow some of those particles off the mask and into the air around you. Worse, as the aerosols held by the mask lose water over time, they become easier to dislodge, gain hang time during which to wait for another victim, and become more likely to pass through the next mask.
There's a reason medical professionals like dentists wear masks: but it's mainly for protection against bacteria, not viruses; they know how to use an N95, and they dispose of the mask as soon as they are done with a patient. That works for their needs, but absent proper fitting, good breath discipline, much tighter weaves, and burn bags for immediate disposal after one use, today's cloth masks aren't just useless against a virus, they're very effective tools for spreading it.
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Oops! missed something important
As things turn out a year or more later.. the data now show quite clearly that masks are counter-productive: i.e. that people who wear masks have more positive tests than people who do not.
There is a mechanistic explanation:
1 - one of the many ways the human body responds to a viral attack via the lungs or throat is to expel viral particles and so reduce the extent of the infection - often to the point that the immune response evolves faster than the virus replicates and so defeats it.
2 - but masks capture exhaled viral particles - and then return them during the inhalation sequence. That defeats a key element in the combined viral response and helps the virus overwhelm the immune system.
3 - aerosols lose volume while free floating but gain volume when inhaled and then exhaled. As a result aerosols small enough to pass through the mask on the way in, are often too large to pass through the same mask on the way out. Masks can function, therefore, as something of a one way gate: letting viral particles in, but preventing their escape.
Bottom line? mask mandates help the virus replicate.