Industrial air filtration (I use to sell and service dust collectors 50 years ago), and I wrote EPA air and waste water permits the last 30 years... Sand blasting, coatings and painting fields for one..and I got my engineering degrees 30 years ago...
It is also the dose that matters. One or 2 or 3 virus particles is not enough to cause an infection. One of major clues was the number of people in a church practice choir singing that all got Covid19 indoors in a church from one one asymptotic person, Wuhan strain early in 2020
And how dry the air is, and how dry and small the airborne particles are...
Masks can reduce the amount of exposure and distance they can travel... Quantity / dose matters...
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