[M T]

You're correct that viral particles do not float freely in the air.  They're always suspended in moisture droplets that are easily captured by exhalers' masks (or at least slowed down), if they're N95-grade. 

That's not quite true.  When expelled, as best we know, the virus are in moisture droplets.  But anything less than 100 microns in diameter (depending on humidity & temp.) dries out (the water evaporates) before hit falls to the floor.  The result is called a droplet nuclei, typically less than 5 microns in size and can stay aloft for hours.   Some studies

• The size and concentration of droplets generated by coughing in human subjects (My old posts should have links to more articles.)  
  
• On coughing and airborne droplet transmission to humans (a modeling study) 
  
• COVID-19. Transport of respiratory droplets in a microclimatologic urban scenario (modeling study)
  

While N95 & KN95 are spec'ed in terms of blocking 0.3 micron particles, that's about the size they are least effective at.  I believe that's why 0.3 microns was chosen for measuring effectiveness.  Smaller particles are captured by (typically) electrostatic attraction to the fibers in the mask.  From a 1998 study:

N95 respirators made by different companies were found to have different filtration efficiencies for the most penetrating particle size (0.1 to 0.3 micron), but all were at least 95% efficient at that size for NaCl particles. Above the most penetrating particle size the filtration efficiency increases with size; it reaches approximately 99.5% or higher at about 0.75 micron. Tests with bacteria of size and shape similar to Mycobacterium tuberculosis also showed filtration efficiencies of 99.5% or higher. Experimental data were used to calculate the aerosol mass concentrations inside the respirator when worn in representative work environments. The penetrated mass fractions, in the absence of face leakage, ranged from 0.02% for large particle distributions to 1.8% for submicrometer-size welding fumes.

I haven't kept up with recent studies on mask effectiveness.  The problem is not the filtration of the mask, but the wearer and how it fits them.  Not wearing a mask is obviously the biggest issue.  Surveys of usage usually depend upon self-reporting.  Once a person is actually wearing a mask, the next question is fit.  A bandana has basically no fit.  It might capture ballistic droplets, but other droplets will escape or be inhaled where the mask isn't held against the face.   Some earloop masks will allow particles to exit or enter toward the ears.  Especially the homemade masks without nose clips will tend to let particles enter/exit toward the eyes.   Even small openings dramatically decrease the effectiveness of a (K)N95 mask when the particles are uniform around the mask.

I really wish all the health agencies were collecting data on the people that get COVID.  Even self-reported mask usage or glasses/shield usage and other exposure information is more info than none.   California could have 800K data points instead of zero.

[M T]

You're correct that viral particles do not float freely in the air.  They're always suspended in moisture droplets that are easily captured by exhalers' masks (or at least slowed down), if they're N95-grade. 

That's not quite true.  When expelled, as best we know, the virus are in moisture droplets.  But anything less than 100 microns in diameter (depending on humidity & temp.) dries out (the water evaporates) before hit falls to the floor.  The result is called a droplet nuclei, typically less than 5 microns in size and can stay aloft for hours.   Some studies

• The size and concentration of droplets generated by coughing in human subjects (My old posts should have links to more articles.)  
  
• On coughing and airborne droplet transmission to humans (a modeling study) 
  
• COVID-19. Transport of respiratory droplets in a microclimatologic urban scenario (modeling study)
  

While N95 & KN95 are spec'ed in terms of blocking 0.3 micron particles, that's about the size they are least effective at.  I believe that's why 0.3 microns was chosen for measuring effectiveness.  Smaller particles are captured by (typically) electrostatic attraction to the fibers in the mask.  From a 1998 study:

N95 respirators made by different companies were found to have different filtration efficiencies for the most penetrating particle size (0.1 to 0.3 micron), but all were at least 95% efficient at that size for NaCl particles. Above the most penetrating particle size the filtration efficiency increases with size; it reaches approximately 99.5% or higher at about 0.75 micron. Tests with bacteria of size and shape similar to Mycobacterium tuberculosis also showed filtration efficiencies of 99.5% or higher. Experimental data were used to calculate the aerosol mass concentrations inside the respirator when worn in representative work environments. The penetrated mass fractions, in the absence of face leakage, ranged from 0.02% for large particle distributions to 1.8% for submicrometer-size welding fumes.

I haven't kept up with recent studies on mask effectiveness.  The problem is not the filtration of the mask, but the wearer and how it fits them.  Not wearing a mask is obviously the biggest issue.  Surveys of usage usually depend upon self-reporting.  Once a person is actually wearing a mask, the next question is fit.  A bandana has basically no fit.  It might capture ballistic droplets, but other droplets will escape or be inhaled where the mask isn't held against the face.   Some earloop masks will allow particles to exit or enter toward the ears.  Especially the homemade masks without nose clips will tend to let particles enter/exit toward the eyes.   Even small openings dramatically decrease the effectiveness of a (K)N95 mask when the particles are uniform around the mask.

I really wish all the health agencies were collecting data on the people that get COVID.  Even self-reported mask usage or glasses/shield usage and other exposure information is more info than none.   California could have 800K data points instead of zero.