AIR FILTRATION OF MICROBIAL PARTICLES

Transcription

1 A review of the problem of cleaning air to remove microbial particles is presented with particular attention to the kinds of equipment available for this purpose. The potentialities and limitations of various filters and other devices are discussed. Recommendations are made for different situations. AIR FILTRATION OF MICROBIAL PARTICLES Herbert M. Decker, M.S.; Lee M. Buchanan, Goddard, B.S. B.S.; Lawrence B. Hall, M.S.; and Kenneth R. N RECENT years, the subjects of micro- I biological air pollution, air sanitation, and air cleaning equipment have gained importance. Considerable information is now available concerning the immediate and latent ill effects caused by inhalation and retention of foreign air-borne particles and bacteria. Present knowledge indicates that those particles approximately 1 to 5 microns in diameter, and those less than 0.2 tu in diameter, are most effective for penetration and retention in the deep pulmonary spaces. Larger particles bearing many organisms may infect open wounds. We suggest, therefore, that proportionate attention be given to the removal or inactivation of biological particles of all sizes from the air used in critical spaces. In general, most air cleaning equipment used to remove dust, mist, or fumes from the air will also remove some bacteria. This paper covers the efficiencies of various types of filters, electrostatic precipitators, air washers, and air scrubbers. In addition, air incineration and ultraviolet sterilization are touched upon. Since our experience has been widest in the use of air filtration as a method for removing bacteria from the air, this device receives more attention than the others. Few practical air filters depend upon screening or sieving action. Since the interstices of a screening-type filter are necessarily smaller than the smallest particle to be removed, the resistance to air flow is high. As the surface becomes covered with suspended material, resistance rises and ultimately air flow stops as all the interstices become plugged. All practical aerosol filters consist of randomly oriented fibers of various materials placed in such a manner that most of the open spaces or interstices are much larger than the diameter of the particles to be removed. The filtering action depends upon the particle coming in contact with and adhering to the fibers or collecting surface. There are several mechanisms that may cause suspended particles to impact on the fibers. They are (a) inertial effect, (b) diffusion, (c) electrostatic effect, (d) direct interception, and (e) deposition in accordance with Stoke's Law. Direct interception and deposition are of less effect in removing particles in filters. In considering biological air cleaning, filters will be divided into four categories according to their efficiency and use. These categories are (a) roughing filters, (b) medium-efficiency filters, (c) VOL. 53. NO. 12. A.J.P.H.

2 AIR FILTRATION OF MICROBIAL PARTICLES high-efficiency filters, and (d) ultrahighefficiency filters. Roughing filters are usually composed of materials such as loosely packed fibers of animal hair, synthetic fibers, or woven metal screens. Medium-efficiency filters may consist of compressed glass, asbestos, cellulose, or other types of fibrous media. High-efficiency filters chiefly contain glass fibers, a good grade of fiber paper, or asbestos fibers, with the diameter of the fiber usually ranging from 1 to 5 pt. Filters classified as ultrahigh-efficiency filters achieve maximum removal of small biological and radioactive particles. Ultrahigh-efficiency filters are used in the pharmaceutical, electronic, and spacecraft industries. The ultrahighefficiency filter was developed originally by the U. S. Army Chemical Corps for use in gas masks and in building filtration systems for removing bacteria and other particles not removed by charcoal filters. The original filter, unfortunately not fireproof, contained Bolivian or African Blue asbestos, esparto grass, and kraft fibers and was known as Chemical Corps Type 6 medium. Emphasis by the Atomic Energy Commission on fire resistance resulted in technological improvement. It is now possible to obtain commercial filters designed for fire-resistive (2500 F) and high-temperature (1,0000 F) operation. Fire-resistive fibers are constructed with glass or glassasbestos fiber paper or ceramic fiber medium. One of the fire-resistive glass media is made into filter paper without a binder. Frames, separators, and cements are incombustible, incorporate fire-suppressing chemicals, or have been impregnated to resist spread of fire. In addition to these qualities, filters can be obtained with resistance to chemicals and high humidity. Air washing is another method for cleaning air. It is chiefly used for removing dust and other particles, although in some instances it has been installed to remove bacteria. Spray towers, zig-zag baffles, metal screens, and glass capillary cells are features of some of these cleaning devices. The most efficient air washers are those in which the suspended matter is impinged on wet surfaces and then washed off. However, instances have been reported in which the bacterial content of the recirculated wash water has increased the contamination in the air. The increase is due to the re-aerosolization of the bacteria that accumulated in the wash water. Another device is the air scrubber. This device frequently is used in chemical processes to bring the gas into close association with the liquid. One of these particular devices has been used in ventilating systems to control humidity. These scrubbers employ a hygroscopic solution such as lithium chloride to control humidity. This arrangement appears to be an improvement over air washers using water, because there is no reaerosolization of bacteria. One of the popular air cleaners is the electrostatic precipitator. In this air cleaning method, air passes through a high-voltage field where the suspended particles are charged and collected on electrodes of opposite charge. Although electrostatic precipitators can remove a high percentage of bacteria and dust from air, they may not be as satisfactory as filters where a constant and dependable supply of clean air is required. In case of power failure, it would be possible for contaminated air to pass through the device, a condition that could not be tolerated in some situations. Automatic closures could prevent this occurrence but would increase the cost of the installation. Some units shut off for a time to allow cleaning; during this time there is no filtration. It has been our experience that the electrostatic precipitator will exhibit high collection efficiency only if it receives good maintenance service. Although removal of bacteria by filtration is satisfactory in most cases, DECEMBER,

3 there are some situations that require absolute certainty. For example, in research laboratories where infectious diseases are studied, high concentrations of pathogenic microbial aerosols may be created. Even though ultrahigh-efficiency filters are extremely satisfactory, if the concentration generated is extremely high (millions of organisms per cubic foot of air), passage of a few organisms undoubtedly will occur. Under such circumstances, air incineration is preferred. The organisms are not removed from the air, but are killed by heat. Ultraviolet (UV) air sterilizers have been reported effective for inactivating organisms in the air stream. However, tests of a relatively sophisticated UV air sterilizer designed for hospital use indicated a kill by UV light of only 10 to 12 per cent of the vegetative cells in the air stream and almost none of the spores. In addition, these sterilizers have the disadvantage that the UV lamps must be cleaned and tested periodically. Furthermore, UV light has limited penetrating ability, and those organisms protected by dust may not be killed. Therefore, UV treatment of air is probably more useful against droplet nuclei and of less value against dust-borne organisms. Maintenance requirements and operational monitoring are more severe and critical for UV installations than for electrostatic systems. It is recommended that considerable study be given to the availability of proper monitoring of UV energy output before UV lamps are installed as the sole method for routine elimination of air-borne bacteria. It is recommended that air from all the devices described, except possibly incinerators, be given a factor of safety by passing it through high-efficiency filters before delivery to critical areas. The data on the performance of filters, filter media, and other air cleaning devices sometimes vary considerably from the performance claimed by manufacturers, usually because of differences 1984 in the methods of evaluation. Performance of filters to be discussed in this paper was evaluated on the basis of biological test procedures developed by the authors. Most of the filter evaluations in the past have been based on physical test methods. The results of those methods at times do not correlate closely with the results from using viable bacterial particles. Several papers have been published by Decker and co-workers on Serratia indica and Bacillus subtilis var niger (Bacillus globigii) as organisms that may be used for evaluation of filters and filter systems. The selection of the organism for evaluation is left to the discretion of the investigator. It has been found that the most consistent results are obtained when a heat-shocked suspension of Bacillus subtilis spores is used. Spores are resistant to death in an air stream and can be collected with a filtering type of air sampler such as the cotton collector. Vegetative bacteria are much less resistant and die off rapidly in an air stream; hence, if a test organism such as S. indica is selected, a liquid impinger or agar-inpaction air sampler should be used. The system shown in Figure 1 is a typical test arrangement that permits accurate determination of the bacterial arrestance of filters or filter media prior to installation in an air filtration system. The test system is simple and can be set up quickly. The bacteria are nebulized into a chamber where the cloud of bacteria is mixed with more air. The aerosol is then drawn into the duct through the filter under evaluation at the rated face velocity and is then exhausted through a blower to the outside. Aerosol samples are taken before and after the filters. If the test filter is not of the ultrahigh-efficiency type, it may be advisable to place an ultrahighefficiency filter in the blower discharge to prevent contaminating the atmosphere with the test bacteria. VOL. 53. NO. 12. A.J.P.H.

4 AIR FILTRATION OF MICROBIAL PARTICLES Although many air cleaning devices can be evaluated in the laboratory, where conditions for such studies are favorable. in some instances an installed air cleaning device must be evaluated. If an entire air-handling system is to be evaluated, sampling points should be established at each end of the system. But if only one component is under test the system first should be examined to see whether a filter, washer, or other cleaning device is already in place between the sampler and the apparatus being evaluated. For example, an attempt to measure the efficiency of an air washer that was located behind a filter probably would be affected by the fact that the air approaching the washer was itself low in bacterial count. To test a cleaning device adequately, it must be challenged by air with relatively high bacterial concentration. An air cleaner may be tested in place under its normal operating conditions, including such factors as species of organisms, particle size, concentration, temperature, and humidity. The results will be representative of the situation studied. If fixed filter systems in occupied hospitals are to be evaluated with artificial microbial aerosols, a word of caution should be heeded. Although artificially produced aerosols of Serratia marcescens have been used in some occupied buildings, this procedure is not recommended, especially for occupied hospitals, because certain debilitated patients might be affected adversely. However, coli bacteriophage, which at times has been given in human therapy, is safe. For unoccupied hospitals, S. marcescens is an excellent test organism. B. subtilis var niger is more resistant and, therefore, provides a more rigorous test of the facility, but the long persistence of the spore may be undesirable because the presence of the spore may cause contamination of routine bacteriological analysis. Extensive experience with this test organism at the U. S. Army Biological Laboratories has shown that human inhalation of more than one million organisms is not harmful. The results of our tests are shown in Table 1. It is obvious that the particular filter selected determines the degree of air cleanliness. In selecting a hospital air-purification system, it is worth while FILTER UNIT Figure 1-System for Determining Bacterial Arrestance of Filters with Spores DECEMBER,

5 Table 1-Efficiency Range of Devices for Removing Biological Particles (1 to 5,&) from Air Cleaning Device Bacterial Removal To Be Expected (Per cent) Ultrahigh-efficiency filters High-efficiency filters 90 to 99 Medium-efficiency filters 60 to 90 Roughing filters: fibrous, metallic, oiled, and screen types 10 to 60 Electrostatic precipitators 60 to 90 Air washers and scrubbers (low-pressure-drop type) 20 to 90 to evaluate the relative importance attached to delivering bacteria-free air to various hospital rooms. For example, the contamination of concern in hospitals is produced mainly in the room itself, rather than brought into the room from the outside, except for problems associated with recirculation of cooled or heated air. In most cases, outside air contains few viable organisms, and except in unusual situations only a small percentage of these are pathogens. Building ventilation systems remove aerosols in most United States hospitals through dilution of the room air by a turbulent flow of incoming clean air. The contamination builds up in the turbulent atmosphere in proportion to the rate of generation, the location and rate of exhaust, and the degree of contamination of the incoming air. Even if all air entering the room is sterile, this sterile air mixing with the room air in turbulent flow merely dilutes the aerosol being generated and does not assure a sterile environment. If ventilation and generation of the aerosol within the room remain constant, the concentration of the organisms in the air will, in time, reach an equilibrium or steady concentration. The exact value of this concentration will depend upon the rate at which the aerosol is being produced, the methods and velocities by which the diluting air is supplied to the room, and the exhaust locations and velocities by which the mixture is removed. This raises the question as to whether it is necessary that every organism be removed from the recirculated air in the turbulent hospital ventilation systems. One may need only to remove a sizable fraction of the organisms. Mathematical studies show that there is a significant difference between the number of microorganisms removed by the 60 per cent efficiency filter and the number removed by the 90 per cent efficiency filter, but comparatively little difference between the 90 and 100 per cent efficiency filters. Thus there is considerable benefit in using a relatively efficient (90 per cent) filter as opposed to a relatively inefficient (60 per cent) filter, but little additional benefit is gained by using a filter that is essentially perfect. Before deciding upon the type of air cleansing device to be used for a building area, a realistic assessment should be made concerning the degree of atmospheric cleanliness that is desirable. The installation of an efficient biological air-cleaning system to serve certain areas of hospitals, research installations, industrial plants, and civil defense shelters does not in itself ensure freedom from biological contamination. To maintain the atmosphere of one or more rooms at a low level of bacterial contamination, it is necessary to establish a system of differential air pressurization. Use of a pressurized air system will minimize the flow of air from corridors, work rooms, laboratories, and other parts of the hospital, where the concentration of bacteria will be relatively high, to areas where a minimum bacterial count is desired, such as nurseries and operating rooms. The pressure differential between the surgical room and the hall VOL. 53. NO. 12, A.J.P.H.

6 AIR FILTRATION OF MICROBIAL PARTICLES way adjoining it should be from 0.1 to 0.3 inch (water column). The surgical room should have the higher pressure. To facilitate such a balance the use of a cubicle or air lock is recommended. If one is entering or leaving the pressurized area through an air lock, the first door should be closed before the second door is opened. The use of mechanical or electrical interlocks to prevent doors being opened at the same time should be considered for doors that frequently are left open. Such an arrangement prevents a sudden drop of pressure and is the best safeguard against flow of contaminated air into clean areas. As an added precaution, it is good policy to provide a downward wash of clean air within the air lock from the ceiling through the floor to remove easily dislodged contaminated particles from the clothing of persons passing through the air lock. Now let us assume that one is working in a bacteriological laboratory handling pathogenic microorganisms. In such a situation the air pressures are reversed. The laboratory should be under reduced air pressure so that the laboratory air, which may contain pathogens, is discharged through a filter or incinerator rather than to adjacent rooms. Differential pressure minimizes movement of laboratory air into the other rooms. If there are several rooms in which pathogenic bacteria are handled, then increasing degrees of reduced pressures should be provided so that the more hazardous rooms have the greater reduced pressure. In this manner the direction of air flow will always be toward the more hazardous room. Summarizing what has been said, if the incoming air must be kept entirely bacterial free, then a roughing filter followed by an ultrahigh-efficiency filter is T)PES OF FILTERS IJtra-Hgh Efficiency Hig VEfficie y......, Medium-Efficiyeny ~~ ~~~~.:...:. Figure 2 DECEMBER

7 the system of choice. Normally, however, cleaning the air to this extent is not necessary and a filter of lower efficiency is adequate. High-efficiency, medium-efficiency, or even good roughing filters may be used where lower standards are satisfactory (Figure 2). Ultraviolet light and air incineration normally are not applicable for the problems under consideration. The subject of air filtration of microbial particles has been covered very briefly. For additional information on this subject, one may refer to the monograph,* "Air Filtration of Microbial Particles," written by personnel at the Biological Laboratories and the Public Health Service. * Decker, Herbert M., et al. Air Filtration of Microbial Particles. PHS Publ. No Washington, D. C.: Gov. Ptg. Office (June), Mr. Decker and Mr. Buchanan are associated with the U. S. Army Biological Laboratories, Fort Detrick, Frederick, Md., and Mr. Hall and Mr. Goddard are associated with the Technical Development Laboratories, Public Health Service, Savannah, Ga. This paper was presented before the Laboratory Section of the American Public Health Association at the Ninetieth Annual Meeting in Miami Beach, Fla., October 15, WHEN YOU CHANGE YOUR ADDRESS You won't want to miss the Journal when you change your address so please tell us well in advance what your new address will be. It is not sufficient merely to notify the Post Office, for they will not forward your copies unless you pay extra postage. As a result, your Journals are returned to this office and the Association pays the postage to r them to you. When you write giving a change of address be sure to give your old address as well. Enclose the IBM label from a recent Journal. Please give us six weeks advance notice and you will not miss a single issue. Write to: The American Journal of Public Health, 1790 Broadway, New York 19, N. Y VOL. 53, NO. 12, A.J.P.H.