Department of

Architectural Engineering

 


Filtration of Microorganisms

Three types of filters exist for use in ventilation systems, prefilters, HEPA (High Efficiency Particulate Air) filters and FilterULPA filters. A typical HEPA filter, such as the one shown at right will filter micron sized particles at Filterabout 95% efficiency. Some box or pleated type filters can be as thin as 2-4 inches, or as wide as 8-12 inches. The picture at the right shows a bag type HEPA filter, which can extend up to 24 inches. Bag type filters typically have a lower pressure drop than the pleated or box type HEPA. The picture below shows a typical installation with a bank of prefilters at the outside air inlet of a large air handling unit. These prefilters are typically between 70-90% efficient.

Prefilters and HEPAs, whether bag or box type, will filter particles down to below 1 micron in size, but with varying Filterefficiencies. Different filters have different pressure drop characteristics, which is a factor in energy and cost analysis. HEPA filters are comonly found in hospital isolation rooms, operating theaters, and Level 3 & 4 containment facilities, as well as in industrial clean rooms.

HEPA filters are typically rated as 99.97% effective in removing dust and particulate matter above 0.3 micron in size, based on DOP (diocytl phthalate) testing usually performed by the manufacturer. In theory, HEPA filters should be highly effective against bacteria and fairly effective against viruses, but real world installations do not always achieve perfomance limits measured in laboratories.

Air Filtration - Theory and Application

HEPA filters consist of fine fibers as illustrated in the diagram at the right. Materials vary, but generally these are made of synthetic fibrous materials. The principle of HEPA filtration is not to restrict the passage of particulate by the gap between fibers, but by altering the airflow streamlines. The airflow will slip around the fiber, but any higher-density bioaerosols or particulate matter will not change direction so rapidly and, as a result of their inertia, will tend to impact the fiber. Once attached, most particulates will not be re-entrained in the airstream.

In the diagram below, the airstream is depicted winding its way around a single fiber. The heavier particulates will either impact the fiber directly, or sometimes attach by close passage, due to static electrical attraction, or simply by physical attachment.

In this diagram the airstream is depicted winding its way around a single fiber.

The following diagram shows the effects of Brownian motion on particles approaching molecular dimensions. Viruses can be small enough to be dominated by Brownian motion as opposed to gravity or inertial forces.

Some early studies found HEPA filters could remove bacterial spores at 99.9999 % efficiency and viruses at 99.999% efficiency (Harstad 1969, Thorne 1960), but this was under ideal laboratory conditions. The Harstad study noted that manufacturer's quality control had the most significant effect on filter performance, and that even a single pinhole could seriously affect filter efficiency. Also, operating outside design conditions of airflow or humidity could multiply the amount of virus penetration.

An additional factor that can have a major impact on filter performance is the installation and maintenance of the filters. Poor tolerances in the fit of the filters to the frames can seriously degrade performance by bypassing unfiltered air. In applications that demand high performance levels, such as the nuclear industry and clean room technology, DOP testing is performed with in-place filters. The testing determines the presence of leaks in the filters or frames, mixing uniformity, and airflow, but does not determine actual filter efficiency (Ornberg 1978, US NRC Reg. Guide 1.52 & 1.140). It is assumed that if all these other conditions are met, filter efficiency will approach that obtained in the factory, or 99.97 % at 0.3 microns. Achieving all the requirements for acceptable operation often yields only borderline results.

No formal studies exist in which actual HEPA filter installations (for humans) have been put to the test with live viruses and bacteria, and therefore quantitative data on real-world efficiencies are unavailable. There have been reports of tuberculosis bacilli (1 - 5 micron rod-shaped bacteria) penetrating HEPA filters in treatment facilities. It is entirely possible that bacteria of this size may pass through HEPA filters due to the fact that they are dynamic living organisms that do not wish to remain attached to dry surfaces without nutrients.

Viruses can be much smaller than 0.3 micron and although HEPA filters can theoretically remove particles down to about 0.01 microns in size, their performance is nonlinear and the efficiency drops off sharply at this size. As has been pointed out by some biologists, the use of HEPA filters may provide evolutionary pressure for smaller microorganisms.

Office buildings, schools and other such facilities do not normally include HEPA filters in the ventilation system, although they often include pre-filters and filters of lower efficiencies. The addition of HEPA filters to standard building systems may have a significant effect on the reduction of airborne bacteria, viruses and fungi, as well as other particulates. The overall effectiveness of such an approach, and economic comparisons with other methods for controlling airborne pathogens, is currently being studied at Penn State through the use of computer models. The construction of a model HEPA filter bank, and testing of filtration efficiencies with live bacteria and viruses, is being planned for the Spring semester of 1997. Updates of progress and results will be reported here.

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