Whatever you wear over your mouth and nose, if it traps moisture, it reduces virus spread.
The water droplets you breathe out, they vary widely in size. If the smallest ones are likened to jet skis, the biggest ones are like aircraft carriers.
And every one of them is super huge to a covid-19 virion (virion = single virus particle). I mean, the virion is like an ant by comparison.
And these ants, the absolute best way for them to travel around the vast reaches of space between one human host and another...is stuck on the side of one of those water droplets. Either the jet ski sized ones, or the aircraft carriers.
From the virion's perspective, the advantage of the jet ski size is, they stay floating in the air a whole lot longer. Unfortunately, they also evaporate faster, leaving the virion without support quicker.
The advantage of the aircraft carrier is, it won't evaporate for hours. The disadvantage is, it gets pulled down to the ground much faster, doesn't float on air currents as well.
But the virion doesn't get to choose which it rides on, so we end up seeing virus particles on all the different sizes of water droplets. Almost all of them too small for a human to see with their naked eye. Even the aircraft carriers.
So now you're wearing what you call a t-shirt over your mouth and nose. Is it trapping a lot of that water vapor? Oh heck yes. You can tell for yourself: breathe in a mask, even the flimsiest mask, for 10-15 minutes. Then see if it's damp on the inside. Yep, sure is. That's virus protection.
Even as flimsy a mask as a t-shirt can screen a surprising percentage* of those water droplets out of the air you're breathing out. And do the same with the air you're breathing in, as well.
That's virus protection. Masks work. It's just absolutely, plain and simple common sense.
* Saw a scientific paper several months ago that concluded N95 masks, properly worn, are 95% effective at blocking covid-19 (largely because it blocks the water droplets on which most covid virions ride), and that even a single ply of cotton fabric, like a handkerchief, blocks roughly 50% (again, worn snug over both mouth and nose). That t-shirt is more effective than you think.