The capture efficiency of these units depends a lot on surrounding air current patterns. If the area is turbulent, it will cut into the capture rate. For example, if the backdraft table is next to a garage door that is often open, it won’t work perfectly. This turbulence will disturb the suction and could prevent some of the weld fumes from being captured.
On the other hand, a backdraft table that is well placed will perform with a high capture rate. A table that is enclosed in some way could be expected to capture 90-100% of weld fumes.