The advantage of our NSF®-certified VSSD  and BSSM style hoods is that the make-up air (MUA), which is air channeled directly from outside the building, goes directly into the hood where it is needed and not on top of the cook’s head, which is where external make-up air plenums typically blow.  Having the air ducted directly into the hood provides a more efficient, economical , and comfortable system overall.