Apparently welding fumes are particles less than 0.3 micron,
and even mig produces this invisible metal smoke which can be toxic,
depending on the parent metal being welded.
Miller sells some high-tech "self-cleaning" filters (cost about
$600) not include the fan and ductwork.
But if you put that filter in series with the filters that are already there for the laser cutter, then the the overall flowrate is going to go down, so maybe it should be a separate stream.