Several ventilation system designs are in common use, and very little is known about their relative performance in achieving an acceptable livestock building air quality. A ventilation system may be characterized quantitatively from the efficiency of the air exchange process and the effectiveness of the contaminant removal process. The theoretical framework is summarized and applied in an intervention study of upward vs. downward air flow in a pig house. The contaminants considered were NH3, CO2, dust, and excessive heat. It was concluded that the air exchange efficiency of upward air flow was superior to downward air flow. For some contaminants (CO2, excessive heat, dust) the ventilation effectiveness of upward air flow was superior to downward air flow.