having spent quite a number of years installing and commissioning fumecupboards and BSCs, the airflow required to achieve containment at even quite a small aperture size is large. Conversely the velocity required to allow good capture within a carbon filter is... very low.
eg. a schools portable fumehood requires a sash opening of perhaps only 250x600mm (say) so that a low face velocity of 0.3m/s give reasonable containment and time for the filter to adsorb. Vs a ducted fumecupboard that will be around 1m/s average face velocity to contain within a sash opening of 450x1200mm.
That's a lot of air...