Content area
Full text
ABSTRACT
The filtration of airborne particulates has been studied extensively and removal efficiencies can be adequately predicted from theory or from catalog data. The filtration of airborne microorganisms, however, has not been specifically addressed by theory and has seen limited empirical study. This paper addresses the variety of factors that may cause microbial filtration efficiency to deviate from predicted values based on particulate size alone. A model is developed to incorporate those factors likely to have significant impact, namely, aspect ratios and lognormal size distributions. This model is then challenged with a database of known airborne pathogens and allergens for which these parameters have been established. Results suggest existing filtration models are accurate within reason for the prediction of filtration efficiencies of airborne bacteria and spores, provided logmean diameters are used. Implications for the use of filtration in health care facilities are discussed.
INTRODUCTION
Airborne microorganisms present a challenge to engineering control of indoor air quality (IAQ) in hospital, commercial, and residential buildings. Filtration of particulates has been studied extensively, but the filtration of microorganisms remains one of the least understood applications. This paper addresses two interrelated aspects of microbial filtration necessary for accurate prediction, filter modeling and microbial modeling.
A classic mathematical model of filtration forms the basis of this methodology. This filter model incorporates multiple fiber diameters. The general mathematical models are fit to a broad range of empirical data for five grades of filters, the HEPA 99.97% and ASHRAE 90%, 80%, 60%, and 40% filters. The HEPA is often called an absolute filter and sees applications in hospital operating rooms, TB isolation rooms, and pharmaceutical clean rooms. The other filters are all termed high-efficiency filters and are used in a wide range of hospital, health care, and commercial applications.
Microbes differ from particulate matter in several respects, such as density, the presence of hydrophobic capsules or slime layers, and in having flagella that enable motility. The factors that may cause filtration efficiency to differ from predictions based on particle size alone are reviewed.
Most microbes are spherical or ovoid and are adequately described by diameters. Some rod-like bacteria have large aspect ratios that can impact filtration efficiency. An empirical relation is used to correct the effective diameter of these microbes....