So, I did a little experiment this morning. I filled a nasal spray container with a dilute solution of saline and fluorescein and inhaled the solution with a couple of shots in each nostril going down into the respiratory tract. (Except for a couple of coughs after inhaling I have not had any coughing or respiratory irritation except for the normal throat-clearing and the occasional irritation I get from eating certain foods.) I thoroughly my hands and forearms all the way to the elbow, and the face and neck and then dried them with a linen dish towel. I then proceeded to tie a poly-cotton shemagh around my face and neck, fully covering up to the bridge of the nose an all the way doing to the shoulders with a double-double layer across my nose and mouth, making a quasi-seal below the eyes and enclosing the entire neck area all the way down to my shirt. The shemagh was washed relatively recently in Woolite and air-dried so there are no phosphor compounds on it. I then went for a ~20 minute walk around my neighborhood with an insulated coffee bottle in my left hand (more to prevent using that hand to touch anything) and my right hand mostly in my pocket or tucked into my shirt Napoleon-style. I
After returning from the walk I went into an enclosed unlit room, and used a UV flashlight to inspect myself and the shemagh. The shemagh was moist to the touch around the nose and mouth area, and under UV light was completely saturated with exhaled fluorescein in a large patch that essentially covered the whole face. Only the material on the back side of the mask was relatively clear (probably contaminated when I removed it). My face, including my forehead, area under the chin, and front of the neck was pretty uniformly covered in fluorescein. My right hand, which I used occasionally to adjust the mask as it started to loose and slip, was covered in the fingers and palm, and even the back had a large enough amount to be seen. My left hand, which held the coffee bottle nearly the entire time (I put it down once to check my phone) and never touched my face or came closer than 12 inches, had detectable levels of fluorescein on the back, but not on the front where it was wrapped around the coffee bottle.
While this is not a scientific test with any type of control (i.e. someone walking around with no mask) and the shemagh has an intentionally coarse weave so it is not the best filter material, this does lead me to conclude that improvised non-respirator masks are essentially useless for preventing the spread of fine mist or aerosols from the respiratory tract. The shemagh face covering did not even prevent exhaled droplets or aerosols from covering the hand that was not touching my face, and because of how saturated the fabric became in even a short period anything that touches it, like a hand to adjust it, becomes thoroughly contaminated as well. I suspect normal, non-respirator surgical masks are not much better because of the gapping they have in the side (to permit breath in and out) and because they do not cover below the chin where larger droplets may settle on the face and neck. Wearing masks is a social cue that you are taking this epidemic seriously and that you presumably expect others to do the same, but as a method of preventing the spread of the virus, they are all but useless.
Stranger