On the other hand, think for a moment of fume cupboards in chemical laboratories, or better, laminar-flow microbiological work hoods. Given that the work to be protected is at the bottom of the contained space, horizontal airflow above it is protective if the size/mass/velocity of contaminating particles is such as to prevent them from falling to the bottom of the space within the time taken for the air's transit of the space.
Still air demands total filtration/cleanliness, or the work can be contaminated by the effects of gravity on contaminating particles. Laminar flow allows for some contamination to be endured if the mathematics of risk are correctly evaluated. In conjunction with filtration of the inlet air, it can work a treat.
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