This study evaluated two government facility ventilation systems. One was a metropolitan government office complex with a recirculation system where outside air was the makeup air; the other was a NIOSH facility that used 100% outside air with no recirculation. The methodology employed was a modified American Society of Agricultural Engineers standard (S525) for testing total enclosure filtration efficiency, in agricultural tractor cabs, with optical particle counters (OPC). The low-efficiency bag filters were tested when new and after being in the ventilation system for 3 months. The replacement medium-efficiency filters were evaluated for 6 months (the manufacturer's suggested change-out schedule). These eight-chamber, medium-efficiency filters had an increased filter surface area that resulted in increased airflow through the system. Unfortunately, these filters contained electrostatic filter media and lost filtration efficiency rapidly, which was subsequently confirmed in a 30-day study conducted to determine an appropriate change-out schedule for the eight-chamber bag filters. The study determined that less than 6 months' use was justified due to the reduced efficiency of the electrostatic filter media. The NIOSH facility's air handler #8 (100% outside air unit) was upgraded from electrostatic bag filters, which had a suggested 9-month change-out schedule, to V Bank mechanical, wet-laid, glass fiber filters. The results of a 3-year evaluation showed that the V Bank filters had better filter efficiency after 3 years of service than the electrostatic filters had at 9 months. Both studies employed matched OPC instruments to reduce instrument-to-instrument bias. The methodology is adaptable to monitoring the total efficiency of most air filtration systems, and results can help make decisions about upgrading filter performance.