American Industrial Hygiene Conference and Expo, May 21-26, 2005, Anaheim, California. Fairfax, VA: American Industrial Hygiene Association, 2005 May; :65-66
In November 2001, the United States Postal Service (USPS) asked the National Institute for Occupational Safety and Health (NIOSH) for technical assistance in evaluating new USPS ventilation and filtration systems (VFSs). These prototype VFSs were developed by outside vendors and had been installed on critical mail processing equipment in response to the 2001 bioterrorist attacks, when the USPS unknowingly processed letters laden with B. anthracis spores. In response to the USPS request for assistance, NIOSH investigators evaluated the overall filtration efficiencies of these VFSs at three USPS Processing and Distribution Centers. The new VFS units included high-efficiency particulate air (HEPA) filters and were required by USPS contract specifications to provide an overall filtration efficiency of 99.97% or better. Because no testing standards or procedures existed to meet the USPS requirements, the USPS evaluation involved a modification of an American Society of Agricultural Engineers (ASAE) methodology used to test total filtration efficiency in agricultural tractor cab enclosures. Optical particle counters, that were carefully calibrated and matched as a set, were placed upstream and downstream of the filter banks within the VFS housing. This modified sampling strategy proved effective for monitoring the filtration component of VFS performance. The results clearly showed that this testing method is capable of monitoring overall filtration efficiencies of 99.97% and higher. The method was also effective as a tool to distinguish between filtration units performing to the high USPS performance criteria and those needing repair, redesign, or replacement. Further, the modified methodology used for the USPS study is readily adaptable to any workplace wishing to evaluate air filtration systems, including high-efficiency systems.