I’m not a doctor; I don’t play one on TV. My observation is that it is almost impossible to position my face diaper such that my eyeglasses do not fog with each breath I exhale. If I am contagious, that vapor contains the pathogen along with water vapor. I thusly conclude that the mask is utterly ineffective in stopping the virus from spreading.
Recall the scenes from China from January/February of people in full hazmat suits.
Bare arms, no gloves and a simple bandage over the nose and mouth aren’t going to be any credible defense against THAT.
The people in the lab wore even heavier gear.