MERV Filter Models for Aerobiological Applications.” Air

Recent concerns about bioterrorism and existing concerns about indoor air quality have raised interest in technologies that can remove indoor biological contaminants. Chief among these air cleaning technologies is filtration, and the ability of filters to remove microorganisms can be better understood thanks to the new standard for testing air filters, ASHRAE 52.2-1999. This ASHRAE standard provides a methodology for testing filter performance that includes measuring filter performance in the 0.3-10.0 micron size range and assigning a MERV rating to these filters. This size range includes all spores and most bacteria but it is necessary to know how well these filters will remove airborne microorganisms smaller than 0.3 microns, which includes all viruses and the smaller bacteria. Either additional test results or mathematical modeling can be used to determine or estimate the removal rates of microorganisms below the test range of MERV filters. In this article a modified classical model of filtration is described and used to generate filter performance curves that can be fit to MERV data in the 0.3-10.0 micron size range and that can be extended down to the size range of viruses. Coupled with the summary of logmean diameters of airborne microorganism included here, these models will enable estimation of filtration rates for viruses and bacteria, including those that might be used as bioterrorist weapons. The Classical Filter Model The following equation defines overall filter efficiency (E) for any particle size and set of conditions