Displaced air, not spillage
Every kilogram of product entering a bag pushes out a volume of air roughly equal to its own bulk volume. On a fast powder line that is a continuous outward flow at the bag mouth for the whole fill, and it carries the finest fraction with it.
A hood above the spout only captures what has already escaped. Extraction has to be connected to the bag mouth and sized for the displaced flow, or the bag stays under positive pressure and the dust leaves anyway.
Four sources worth separating
Induced air from the free fall between the feeder and the bag mouth.
Displacement air from product entering the bag — usually the largest single source on powders.
Residual product at the spout when it withdraws, which drops or gets blown off.
Leakage through the bag fabric or the seams, which no amount of spout extraction will fix.
How to tell them apart on site
Watch the bag mouth during the fill. Dust curling out steadily from the mouth means the extraction is undersized for the displaced air. Dust puffing only at spout withdrawal points at the spout itself. A fine haze forming on the bag surface points at the fabric.
Each of those has a different fix, and only the first is solved by adding extraction volume.
Sealing is half the answer
An inflatable or clamped spout seal turns an open bag mouth into a closed transfer point. Once the mouth is closed, extraction only has to handle the displaced air and the small gap losses — a far smaller number than an open mouth.
Where to look next
The bag mouth as a continuous outlet is easiest to see on a powder line: automatic powder bagging line (I).
What happens to the bag after the spout — closing, checking, conveying — is under conveying and inspection, and the products that make the most dust are listed in application areas.