Two tunnel ventilated sow gestation buildings (78 m × 15 m), each holding approximately 645 sows, were monitored for emissions of ammonia (NH 3), carbon dioxide (CO 2), hydrogen sulfide (H 2S), odor, and particulate matter 10 μ or less (PM 10) for a large multi-state research project that collected data for 15 months. In order to measure emissions, ventilation rates from each barn were periodically measured using a portable Fan Assessment Numeration System (FANS) unit, that measured airflow capacity of the six (5 - 122 cm and 1 - 91 cm diameter blade fans) exhaust fans at one end of each barn. During the first routine airflow measurements of the large (122 cm diameter) belt-driven fans it was determined that when the fan drive belts were slightly loose that belt slippage occurred and the fan airflow delivery was reduced by 30 to 60% of the fan airflow delivery when the belts were properly tightened. This large reduction in airflow delivery represents a significant reduction in the total ventilation rate for these sow facilities since a large majority of the airflow capacity was provided by these belt-driven fans. Belt-driven fans are commonly used to provide most of the airflow capacity in tunnel ventilated buildings in the U.S. and elsewhere in the world. Belt slippage and dirty shutters can result in serious under ventilation of the barns, providing inadequate environmental control for the animals housed in these buildings.