HEPA Filters

History of HEPA Filters

• developed during WWII atomic bomb research for containment of radioactive aerosols
• called “superimpingement” or “ superinterception” filters; later referred to as “absolute” filters
• first prototype filters used esparto grass as the filter medium
• in 1950s glass fibers were introduced into the paper
• in 1960s specifications were standardized and called HEPA filters
• in 1970s asbestos was removed
• in 1960 the first laminar flow bench was invented at Sandia National Laboratory

Typical Characteristics

At a glance, HEPA filters have the following characteristics:

• Most submicron semiconductor fabrication lines use Type-D ULPA filters as an improvement over traditional HEPAs for Class-1 and Class-10 environments.
• Usual size is 3 ft. x 6 ft. x 5.875 in. frame.
• When new, maximum pressure drop is 1 in of water = 0.036 psi
• Each ft² of opening corresponds to about 50 ft² of paper area
• Designed for 90 lfm air velocity, or 45.7 cm/sec.
• Designed for entraining 500 - 1000 grams of dust per 1000 cfm
• Are sealed into the ceiling using gel-sealed T-bars
• Typical lifespan is several years if air is properly prefiltered

Industries and Applications

HEPA air filters have been traditionally used in hospital operating and isolation rooms, pharmaceutical and computer chip manufacturing, as well as in other applications requiring "Absolute" Filtration. Today HEPA air cleaners, vacuum cleaners and air filters are used in a wide variety of critical filtration applications in the nuclear, electronic, aerospace, pharmaceutical and medical fields. HEPA air cleaners, vacuum cleaners and air filters are required by law to be used in all equipment for asbestos, lead, toxic chemical and mold abatement. These HEPA filtered products must meet the strict Military Standard 282 HEPA filtration efficiency test.  Today, HEPA filters are used in a broad range of industries including: 

• Microelectronics (eg. semiconductor cleanrooms)
• Pharmaceutical
• Bio and gene technology
• Chemical industry
• Nuclear air ventilation
• Waste incinerators
• Hospital operating rooms
• Emergency burn centers
• Cosmetics
• Medical industry
• Food industry
• Optical industry
• Automotive industry
• Surface engineering
• Precision engineering
• Nanomaterials
• Space industry
• Military equipment
• Power and energy plants
• Controlled and ultraclean environments for critical technologies
• Movie theatre industry
• Portable residential air cleaners

HEPA and ULPA filters are best applied in situations where high collection efficiency of submicron Particulate Matter (PM) is required, where toxic and/or hazardous PM cannot be cleaned from the filter, or where the PM is difficult to clean from the filter. HEPA and ULPA filters are typically utilized for applications involving chemical, biological, and radioactive PM. HEPA and ULPA filters are installed as the final component in a PM collection system, downstream from other PM collection devices such as electrostatic precipitately or baghouses. (Heumann 1997)

HEPA and ULPA filters are specifically designed for the collection of submicron PM at high collection efficiencies. They are best utilized in applications with a low flow rate and low pollutant concentration. Filter outlet air is very clean and may be recirculated within the plant, in many cases (AWMAI 1992). They are not sensitive to minor fluctuations in gas stream conditions (Heumann. 1997). Corrosion and rusting of components are usually not problems. Operation is relatively simple. Unlike electrostatic precipitators, HEPA and ULPA filter systems do not require the use of high voltages therefore, flammable dust may be collected with proper care (AWMAI 1992). Filters are available for a range of dimensions and operating conditions. Commercial filter systems and housings are available in several types of configurations to suit a variety of installation and operation requirements. These systems have many built in features such as testing and monitoring equipment.

HEPA and ULPA filters are useful for collecting particles with resistivities either too low or too high for collection with electrostatic precipitated (AWMA. 1992). Unlike baghouses which require workers to enter the collector to replace bagel HEPA and ULPA filters systems are designed to replace filters outside the collector housing. This makes them ideal for applications involving hazardous air pollutants (HAPs) or toxic PM. The collected PM is tightly adhered to the filter media for subsequent disposal. Bag in/bag out procedures that may be required by OHSA are easily performed with the filters (Heumann. 1997).

Construction and Function

When designing HEPA filters, there are several items that must be considered: application, environment, efficiency required, physical geometry constraints, structural requirements, system volumetric flow requirements, system operational pressures, existing air handling equipment and its capabilities, as well as maintenance, ergonomics, cost, and manufacturability.

Most HEPA filters are constructed from a mat of randomly arranged special glass fibre sheet pleated in a “V” pattern like a folded paper fan with corrugated aluminium separators between the folds. This is attached to a sturdy base, forming the core of the filter.

HEPA and ULPA filters generally contain a paper media. Newer filter designs may contain nonwoven media which utilizes recently developed fine fiber technology (INDA, 2000). Generally, the filter media is fabricated of matted glass fiber such as borosilicate microtines (EPA, 1991). The small fiber diameter and high packing density of both the paper and nonwoven media allow for the efficient collection of submicrom PM (Gaddish, 1989). The waste gas stream is passed through the fibrous filter media causing PM in the gas stream to be collected on the media by sieving and other mechanisms, as mentioned below. The dust cake that forms on the filter media from the collected PM can increase collection efficiency (EPA, 1998a).

The filter media is pleated to provide a larger surface area to volume flow rate. For this reason, HEPA and ULPA filters are often referred to as extended media filters. Close pleating, however, can cause the PM to bridge the pleat bottom, reducing the surface area (EPA, 1998a). Corrugated aluminum separators are often employed to prevent the media from collapsing (Heumann, 1997). The pleat depth can vary from 2.5 centimeters (cc) (1 in.) up to 40 cm (16 in.). Pleat spacing is generally between 12 to 16 pleats per in., with certain conditions requiring fewer pleats, 4 to 8 pleats per in. (EPA, 1998a).

The most common designs are a box filter cell and a cylindrical filter cell. In a box cell the pleated media is placed in a rigid, square frame constructed of wood or metal. The air flows from the front to the back of the filter. Box packs are approximately 60 cm (24 in.) in height and width and 6 to 30 cm (3 to 12 in.) in length (EPA, 1991). The media in a cylindrical filter cell is supported by inner and outer wire frameworks. A metal cap seals the media at one end. Air flows from the outside to the inside of the filter. This allows a higher air flow rate than a box cell since more surface area is exposed (Vokes, 1999). Typical cylindrical packs are 50 centimeters (cm) (20 in.) in diameter and 35 to 60 cm (14 to 24 in,) in length (Vokes, 1999).

Both the box and cylindrical cells seal the media to the frame or cap using polyurethane, epoxy, or other commercially available adhesive. A metal grill protects the media face from damage. The filter cell is mounted to a holding frame using a gasket or fluid seal. The filter is generally mounted on the clean air plenum (EPA, 1991). The filter can be mounted directly in the duct or in a separate housing. HEPA and ULPA filter systems require pre-filtering for large diameter PM. HEPA and ULPA filter systems are generally the final component in a PM removal system (Heumann, 1997).

The HEPA and ULPA filter cells are generally utilized as a disposable-type filter. As discussed previously, when the filter cake buildup results in unacceptable air flow rates, the filters are replaced. In most designs, replacement of the filter cell takes place at the clean air plenum and outside of the housing unit. This reduces the risk of exposure to PM by the maintenance workers. This feature is especially important.HEPA filters differ in terms of 

• Filtration efficiency
• Configuration (size and shape)
• Materials of construction
• Fire resistance
  

Key metrics affecting function are fiber density and diameter, and filter thickness. The air space between HEPA filter fibers is much greater than 0.3 μm. The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. Just as for membrane filters, particles so large that they are as wide as the largest opening or distance between fibers can not pass in between them at all. But HEPA filters are designed to target much smaller pollutants and particles are mainly trapped (they stick to a fiber) by one of the following four mechanisms:

Filtration by interception Direct interception works on particles in the mid-range size that are not quite large enough to have inertia and not small enough to diffuse within the flow stream. These mid-sized particles follow the flow stream as it bends through the fiber spaces. Particles are intercepted or captured when they touch a fiber. With interception, particles following a line of flow in the airstream come within one radius of a fiber and adhere to it. Particles that are farther than one particle diameter will not be removed by this process. This is one reason for the high fiber volume density of the 200 cfm media. The more dense, the higher the probability of particlecapture. This effect is dominant from about 0.1μm up to about 1μm.

Filtration by inertial impaction Inertial Impaction works on large, heavy particles suspended in the flow stream. Inertial impaction occurs when large particles are unable to quickly adjust to changes in the flow stream around fibers. The particle, due to its inertia, impacts a fiber and is captured. This effect is dominant from around the 0.5 μm region up to around 5 μm. Larger particles are unable to avoid fibers by following the curving contours of the airstream and are forced to embed in one of them directly; this increases with diminishing fiber separation and higher air flow velocity.

Filtration by Brownian diffusion Diffusion (also known as Brownian Diffusion) works on the smallest particles. Brownian diffusion is perhaps the most mysterious of the filtering effects since it tends to defy common sense. Very fine particles in the air stream will collide with gas molecules and create a random path through the media. The smaller the particle the longer the particle will zigzag around. This random motion increases the probability of the particle contacting a fiber. This effect is dominant for all particlessmaller than 0.1μm.

Filtration by sieving Sieving, the most common mechanism in filtration. Sieving stops large particles that are just too big to fit through the open areas of the filter. This  includes all particles above 5 μm in size and larger. As you go smaller in particle size, say between 1μm to 5 μm, occasionally some of these particles get through, but the efficiency for removal is still well into the 99.9999+% range. This is still due primarily to sieve effectand the beginning of inertial impaction effect.

The particle capture effects mentioned are all subject to how the filter media is made. Fiber diameter, spacing, fiber cross section, and media thickness are big drivers in how effective a filter is. The smaller the fiber, the greater the small particle capture efficiency. The smaller the fiber spacing, the greater filter efficiency. The larger the cross section, the greater the capture capability.

Each of these has tradeoffs that must be considered however, when designing a filter. Glass and polymer fibers are the most common materials used in HEPA filters. Glass fibers can be drawn down to much smaller diameters than can polymers, 0.3um is very possible. This would suggest that an all glass fiber filter would be the best. If your not concerned about space, perhaps this would be correct. However, in order to put a large amount of media into a small space, media pleating is the best solution. Unfortunately glass is very brittle and the small fibers will break if folded too severely. This is one reason why polymer fibers are added to the media matrix. They add a significant amount of structural strength that allows pleating to be effective without significantly impacting filter performance.

Diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate above 0.4 μm. In between, near the 0.3 μm MPPS, diffusion and interception predominate.

The initial filter air flow resistance and final filter air flow resistance are typically measured as pressure drop across the filters.

The terminology of “True HEPA” is a loosely used marketing term. Various industries and institutions have different specifications as to how they define HEPA. For example the European Standard EN1822-1 defines a HEPA filter as ranging from 85% to 99.995% efficient against a 0.3 micron challenge. The standard adopted by commercial aircraft manufactures and many other industries is MIL-STD-282 Method 102.9.1 which requires the filter to capture 99.97% of 0.3 micron particles.

HEPA filters will have a label attach to them and normally contain the following parameters:

• Percent penetration at rated flow
• Resistance at rated flow
• Direction of flow
• Filter size
• Type of separator

Conditions That Will Damage HEPA Filters

• Moisture: 95-100% relative humidity
• Hot air: greater than 275 °F
• Fire: direct fire or high concentrations of particulate matter produced by fire
• High pressure: 8 in. of water, gauge (in. wg) internal or differential across filter media
• Corrosive mist: dilute moist or moderately dry concentrations of acids and caustics
• Any acid and some caustics will attack uncoated aluminum separators
• Hydrofluoric acid will attack the media
• Nitric acid will attack wooden boxes making highly flammable nitrocellulose
• Shock pressures

HEPA Filter Performance

HEPA filters provide a very high level of filtration efficiency for the smallest as well as the largest particulate contaminants. As defined by the Institute of Environmental Sciences and Technology, IEST-RP-CC001.3 and MIL-STD-282 Method 102.9.1, a HEPA filter must capture a minimum of 99.97% of contaminants at 0.3 microns in size. The 0.3 micron benchmark is used in efficiency ratings, because it approximates the most difficult particle size for a filter to capture. HEPA filters are even more efficient in removing particles that are smaller than 0.3 microns and larger than 0.3 microns. The fact that a HEPA filter’s removal efficiency increases as particle size decreases below 0.3 microns is counter intuitive. However, this is a proven and accepted fact in the filtration sciences.

Experimental penetration of particles through a HEPA filter have determined that approximately 0.1% in the 0.1 micron particle range will pass through the filter. If there are 100,000 particles 0.1 micron in diameter per cubic centimeter of air, then 120 per cubic centimeter of air will pass through a HEPA filter. In one day an average man will inhale 28 million particles in the 0.1 micron range through a HEPA filter.

HEPA filter performance is dependent primarily upon the four following characteristics:

Air Flow: HEPA and ULPA filters are currently limited to low capacity air flow applications. Standard filter packs are factory-built, off the shelf units. They may handle from less than 0.10 up to 1 .0 standard cubic meters per second (sm³/sec) ("lhundreds'' to 2,000 standard cubic feet per minute (scfm) (AAF, 2000; Vokes,1999). HEPA filter systems designed for nuclear applications require higher capacities. For these applications filter banks or modules are ducted together in parallel to increase air flow capacity (EPA, 1991). Commercially available modular systems can accommodate air flow rates in the range of 5 to 12 sm³/sec (5,000 to 40,000 scfm) (AAF, 2000; Vokes, 1999).

Air flow capacity is a function of the resistance, or pressure drop across the filter and particle loading. As the dust cake forms on the filter, the resistance increases, therefore, the air flow rate decreases. Since the filter is not cleaned the air flow rate continues to decrease as the system operates. After the pressure drop across the filter reaches a point that prevents adequate air flow, the filter must be replaced and disposed. For these reasons, HEPA and ULPA filters are used in applications that have low air flow rates or have low concentrations of PM (Heumann 1997).

Temperature: Temperatures are limited by the type of filter media and sealant used in the filter packs. Standard cartridges can accommodate gas temperatures up to about 93 °C (200 °F). With the appropriate filter media and sealant material, commercial HEPA filters can accept temperatures of up to 200 °C (400 °F). HEPA filters with ceramic or glass packing mechanical seals can accept temperatures up to 537 °C (1000 °F). (EPA 1991)

Spray coolers or dilution air can be used to lower the temperature of the pollutant stream. This prevents the temperature limits of the filter from being exceeded (EPA, 1998b). Lowering the temperature, however, increases the humidity of the pollutant stream. HEPA and ULPA filters can tolerate some humidity. Humidity higher than 95%, however, can cause the filter media to plug, resulting in failure (EPA, 1991). Therefore, the minimum temperature of the pollutant stream must remain above the dew point of any condensable in the stream. The filter and associated ductwork should be insulated and possibly heated if condensation may occur (EPA, 1998).

Pollutant Loading: Typical pollutant loading ranges from 1 to 30 grams per cubic meter (g/m³) (0.5 to 13 grains per cubic foot (gr/ft³)) (Novick, et at, 1992). Dust holding capacity compares the weight gain of the filter to the rise in pressure drop during a specific period of time (air flow volume). Typical inlet dust holding capacity range from 500-1000 g/1000 scfm (Gadish, 1989). As discussed above, the pressure drop across the filter is a function of pollutant loading. HEPA and ULPA filters are best used in applications that have low concentrations of PM, or prohibit cleaning of the filter (Heumann, 1997).

Other Considerations: Moisture and corrosives content are the major gas stream characteristics requiring design consideration. As discussed previously, humidity up to 95% is acceptable with the proper filter media, coatings, and filter construction. Filters are available which can accommodate corrosive gas streams with concentrations up to several percent. These filters are constructed of special materials and are generally more expensive. (EPA, 1991)

The dust-holding capacity of a filter is dependent on the shape, size, and density of the dust particles it is exposed to. In applications with high dust concentrations, the HEPA filter should be protected by replaceable prefilters upstream of the filter. For structural design purposes, 4 lb of dust load per 1000 cfm of rated capacity can be assumed.

HEPA and ULPA filters are typically operated under pressure of approximately 203 mm of water column (8 in. of water column). High operating pressures may rupture the filter. HEPA filters utilized in the nuclear industry have seismic requirements in addition to the performance characteristics discussed above. (EPA, 1991)

Individual HEPA and ULPA filter cells accommodate air floral capacities up to 1.0 sm³/sec (2000 scfm) (Vokes. 1999). Larger air flow capacities are required for some applications. such as the nuclear ndustry.
To increacre capacity, multiple fiIters are housed in bands or modules vvhich are ducted together. This allows a standard off-the-shell filter unit to be utilized for a variety of applications and air flow rates
(Osborn. 1998). In this type of design, dampers can be used to seal off a portion of the filters for maintenance (Evokes. 1999).

The number of filter cells utilized in a particular system is determined by the air-to-cIoth ratio, or the ratio of volumetric air flow to cloth area. The deletion of air-to-cIoth ratio is based on the particulate loading characteristics and the pressure drop across the filter media. Practical application of fibrous media filters requires the use of large media areas to minimize the pressure drop across the filter (EPA 1998a). The paper and nonwoven filter media used in HEPA and ULPA filters have a larger pressure drop across the filter than the woven fabrics used in bags. For this reasons HEPA and ULPA filters are utilized at lower airflow rates and lower particulate loadings than baghouse designs. As discussed previously, once the air flow rate through the filter system decreases to an unacceptable point, the filter must be replaced (Heumann. 1997).

Operating conditions are important determinants of the choice of materials used in HEPA and ULPA filter cells. Pollutant streams with high operating temperatures, high humidity, or corrosives require special filter media, sealant, materials, and coatings. These special materials increase the cost of the system. (EPA. 1991)

The paper and unproven media used in HEPA and ULPA filters have a significantly higher resistance than the woven fabrics that are used in bag filters. The high efficiencies of HEPA and ULPA filters require that the integrity of the filter seals be maintained. The filter media is subject to physical damage from mechanical stress (Heumann. 1997). Temperatures in excess of 95 °C (200 °F) or corrosive pollutant streams require the use of special materials in the filters which are more expensive (EPA. 1991). Concentrations of some dusts in the filter housing may represent an explosion hazard if a spark is accidentally admitted. Filter media can burn if readily oxidizable dust is being collected (AWMA. 1992). HEPA and ULPA filter systems require high maintenance and frequent filter replacement. Filter life may be shortened in the presence of high temperatures and acid or alkaline participates or gas constituents. High flow rates or dust loads will also decrease the operational life of the filter. HEPA and ULPA filters cannot be operated in moist environments. Hygroscopic materials, condensation of moisture, or tarry adhesive components may cause plugging of the filter media (EPA. 1991).

A specific disadvantage of HEPA and ULPA units is that they may generate a high volume waste product with a low density of pollutant. For HAP applications and chemicals biological, or radioactive toxic PM applications, the filters must be disposed of as hazardous waste. The waste is composed of the wood or metal frames, organic binders and gaskets, glass fiber media, and hazardous contaminants. (EPA. 1991).

What HEPA Filters Can and Cannot Do

Let us take a look at particles which may enter a cleanroom from outside air. If we assume air outside particle concentrations are about one million 0.5 micron and larger particles, it would not be an unreasonable assumption as levels are much higher in many areas of the country. In this case, the number of 0.1 micron particles will be about 35 times as many as for the 0.5 micron particles or about thirty-five million particles per cubic foot. Now let us assume we are using a 99.999% efficient HEPA filter (rated at 0.1 µpoint of least efficiency), we would filter out 34,999,650 particles but 350 one-tenth micron particles would remain. This is the maximum limit for a Class 100 cleanroom! Obviously, we need a number of additional methods to help address the required reductions of particle concentrations.

One way the methods is to use a small percentage of outside makeup air and mix it with recirculated air which gets progressively cleaner, a primary reason along with energy savings to use recirculated air. Another method is to use dual banks of HEPA filters, one set in the make-up handler and a final room set, which is a recommended general practice (using a dual bank also protects the final set from high levels of particle exposure, thus increasing their lifetime and reducing the buildup of pressure drops). At this point, we need to not only note, but also emphasize,that the use of any HEPA filters of less than 99.99% efficiency should never be considered. For even the crudest of cleanrooms, the initial cost savings, if any, will certainly be more than offset by certification costs and the poor results.

A 99.97% efficiency filter is tested with a gross leak measurement indicating that 0.03% of all upstream contamination may be passed through the filter. For a 99.99% filter, the test measures each small area so that no more than .01% of upstream contaminant may be passed in a individual leak. Since most of the filter does not have a leakage rate anywhere near the .01% limit, the result is that the gross leak is far less than 0.01%. Thus, a scanned 99.99% filter is far more than 3 times better than the gross leak tested 99.97% filter, generally on the order of from 1000 to 10,000 times better!.

It is just as important to understand what a HEPA filter cannot do as well as what it can do. No HEPA filter can reduce the amount of contamination introduced downstream of the filter. Repeat: No HEPA filter can reduce the amount of contamination introduced downstream of the filter. While this may seem inherently obvious, it is amazing how many times the excuse that the HEPA filters will take care of it is used!

If the only function of a HEPA filtration system were to provide clean air to the cleanroom, we could pump the room full of clean air and then turn off the filtration system! In fact, ninety percent or better of the function of a well designed cleanroom HVAC system is to remove internally generated contamination and prevent it from adversely affecting the critical product or process. Conversely, delivering clean air to the cleanroom is only ten percent or less of the function. With this in mind, we need to ask, "Where does this internal contamination come from?" ... 

• A person sitting or stopped generates about 100,000 particles per cubic ft.

• Sitting down or standing up generates about 2,500,000 particles per cubic ft.

• Walking generates about 10,000,000 particles per cubic ft.

• Horseplay generates about 30,000,000 particles per cubic ft.

• Grinding, sweeping, welding adds billions of particles per cubic ft.

• Two surfaces rubbing generate billions of particles per cubic ft.

• An open, non-airlocked door can add billions of particles per cubic ft.

• Process equipment adds particles

• Process materials add particles

• Maintenance activity adds particles

• Construction residue can generate massive particles throughout the life of the facility!

Thus we cannot depend upon the filtration characteristics of the HEPA to remove the internally generated contamination. We already know that it will only remove a given percentage of upstream contamination. Thus, we must utilize HEPA filters as the valuable tools they are in the cleanroom, but at the same time keep in mind the constraints of their use.

HEPA filters cannot remove contamination introduced downstream of the filter.

Microbially Tested HEPA Filters

Microbially tested simply means that a filter was tested against a particular bacterial, fungal, or viral particle challenge. Many industry and university studies have shown that a HEPA filter provides the same removal efficiency against a viable or a non-viable particulate challenge of the same size. The physical laws at work governing the removal efficiency of a filter media do not discern between a viable and a non-viable particle. The same capture mechanisms apply. Thus, the removal efficiencies for a viable and a non-viable particle are equivalent. The removal efficiency of the HEPA media against a 0.027 micron viral particle is dominated by the diffusion filtration mechanism. This mechanism provides a very effective means of removing very small particles, such as viruses. In fact, the smaller the particle, the higher the removal efficiency due to the diffusion filtration mechanism.

Residential Purposes

Every house with plants, pets or people is automatically polluted. According to the EPA, the air in most homes is at least two to four times more polluted than outside air.

Most of us spend up to 90 percent of our time indoors breathing polluted air and only 10 percent of the time breathing healthy, oxygen-rich outdoor air. The result is that many of us suffer from asthma, allergies and hypersensitivity.

According to the EPA publication The Inside Story: A Guide to Indoor Air Quality, the less fortunate can come down with respiratory diseases, heart disease and cancer after prolonged or repeated periods of exposure to some pollutants.

The American Lung Association reports that 24.7 million Americans have been diagnosed with asthma some time in their lives and that in 1999 alone, close to 2 million emergency room visits were attributed to asthma.

The two primary methods of preventing indoor air pollution are source control and cleaning the air.

Source control: If there are no pollutants, there is no pollution. Unfortunately we live in a very dirty world. On a practical level, source control is as simple as using pump bottles instead of aerosol spray cans, not letting anyone smoke inside the house and exhausting bathroom fans through the roof, not into the attic. It also means waterproofing and ventilating the basement so that it never gets damp and making sure the roof doesn't leak.

Cleaning the air: At the most basic level, the furnace filter takes hunks and chunks out of the air. The American Lung Association recommends upgrading furnace filters to at least the quality of the 3M Filtrete or other electrostatic filter. You also can upgrade to thick media filters, such as the Air Bear, or electronic air cleaners, such as the Trion Max 5 or the Honeywell Electronic air cleaner.

HEPA filtered air cleaners, air purifiers and vacuum cleaners are highly recommended for all allergy and asthma sufferers.

The American Lung Association recommends that proven source control strategies be employed in homes as a primary means of reducing exposure to pollutants, that is, getting at the real source of what causes pollutants and reducing it or removing it. However, physical studies which do not measure health effects do show that certain air cleaners are effective in removing certain indoor air pollutants. Thus, as an adjunct to effective source control and adequate ventilation, highly efficient air cleaners can be useful in further reducing levels of certain indoor air pollutants. More research on the health benefits of air cleaners is needed to provide complete evidence that would better address the circumstances of intended use.

Based on the limited available data, it can be concluded that if allergen sources are present in a residence, air cleaning alone has not been proven effective at reducing airborne allergen-containing particles to levels at which no adverse effects are anticipated. Cats, for example, generally shed allergen at a much greater rate than air cleaners can effect removal. Dust mites excrete allergens in fecal particles in sequestered environments (i.e., within the carpet or the bedding). For individuals sensitive to dust mite allergen, the use of impermeable mattress coverings appears to be as effective as the use of a laminar flow air cleaning unit above the bed. Source control should always be the first choice for allergen control in residences.

The reality in most residences is that total elimination of a pollutant source is not always possible or practical. Individuals with severe allergy and asthma symptoms, whose symptoms are not alleviated by other source control and ventilation strategies, may want to try an effective air cleaner in an attempt to aid in further exposure reduction. Although there is no proven health benefit from such a measure, some individuals report that they perceive air cleaners as useful in improving their health status.

Unfortunately, for residential use, HEPA filters can be noisy when used in air filter systems due to the fan and can be expensive due to electricity costs. To reduce noise, the air intake duct and intake fan is often located outside of a building, oftne on the roof of the building. Replacement filters can also be expensive as HEPA filters are not reusable. Despite this HEPA filters are easily the most effective filters available and their use can improve allergic symptoms dramatically.

HEPA filters (recommended by the Dept. of Homeland Security) are more effective than any other type of air filter at capturing dust, pollen, ragweed, dust mites, mold spores and other allergens.

HEPA Filter Classifications

HEPA and ULPA filters are classified by their minimum collection efficiency. Many international standards and classes currently exist for high efficiency filters (Osborn, 1989). In general, HEPA and ULPA filters are defined as having the following minimum efficiency rating (Heumann, 1997):

HEPA: 99.97% efficiency for the removal of 0.3 µm diameter or larger PM,

ULPA: 99.9995% efficiency for the removal of 0.12 µm diameter or larger PM.

Some extended media filters are capable of much higher efficiencies. Commercially available filters can control PM with 0.01 µm diameter at efficiencies of 99.99+% and PM with 0.1 µm diameter at efficiencies of 99.9999+% (Gaddish, 1989,, Osborn, 1989). Several factors determine HEPA and ULPA filter collection efficiency. These include gas filtration, velocity, particle characteristics, and filter media characteristics. In general, the collection efficiency increases with increasing filtration velocity and particle size. In addition, the collection efficiency increases as the dust cake thickness and density increases on the filter (EPA, 1998a).

Cleanrooms that require HEPA filtration will be categorized according to the level of contamination in the room after the air has been filtered out. Cleanrooms fall under one of the following classifications:

HEPA Filter Testing

Testing of the collection efficiency for HEPA and ULPA filters is performed under clean filter conditions. This is in contrast to continuously cleaned-type filters, such as baghouses, which are tested after reaching a steady-state pressure drop. Cleaned-type filters have nearly constant effluent panicle concentration whereas HEPA and ULPA filters have overall efficiencies which vary with particulate loading. (Heumann, 1997)

The efficiency of each filter is tested by the manufacturer before shipping. The user customarily leak tests the filter and the installation it is in on installation and annually thereafter. For nuclear applications, additional tests are required by the Department of Energy (DOE) and by the owner/operator after installation (Burchsted et at, 19791. There are two separate tests for HEPA and ULPA filter collection efficiencies. HEPA efficiency is rated using a thermal dioctyl phthalate (DOP) test. The test dust for HEPA filters is mono-sized, 0.3 µm diameter, DOP particles, generated by vaporization and condensation. Alternative aerosols can also be used as specified or required for given applications. A photometer measures the particle penetration of the HEPA filter by sensing the scattering of light. ULPA efficiency is tested using a particle counter upstream and downstream of the filter. An atomizer injects a solution of DOP, alcohol, and mineral oil in hexane to generate particles ranging from of 0.1 to 0.2 µm in diameter (Heumann, 1997).

Testing a HEPA filter involves the following requirements: 

• Access to the air intake to inject a special type of aerosol whose particle size should not penetrate the HEPA filter
• Mixing the air and aerosol in the upstream air (upstream being the contaminated air on unfiltered side of the HEPA filter)
• Uniform aerosol concentration upstream: making sure that the aerosol particles are uniformly dispersed in the upstream air
• Downstream mixing: Only required at the installation and not at the factory. At the factory, the aerosol exiting the filter is immediately sucked into a particle counter without being dispersed in a room. At the installation, the air needs to be dynamically moved into exhaust ducts to ensure leaked particles are captured.
• Uniform aerosol concentration downstream
  

Pre-Filters

HEPA and ULPA filters require pre-filtering to remove large PM or for dust concentrations greater 0.03 grams per centimeter squared (g/cm²) (0.06 pounds per feet squared (Ibs/ft²)). Pre-filtering may be performed in several stages. Mechanical collectors, such as cyclones or venturi scrubbers may be required to reduce large diameter PM. Standard baghouse or cartridge filters are required to filter out PM greater than 2.5 µm in diameter. (EPA, 1991) In high temperature applications, the cost of high temperature-resistant filter designs must be weighed against the cost of cooling the inlet temperature with spray coolers or dilution air (EPA, 1998b).

Maintaining HEPA Filters

HEPA filters require no cleaning or maintenance to maintain efficiency, and studies have proven that HEPA filters actually increase in efficiency with use over their 3 to 5 year life.

HEPA and ULPA filters are monitored for pressure drop across the filter media. Once the pressure drop becomes unacceptable, the filter must be replaced. The typical pressure drop for a clean filter is 25 millimeters (mm) of water column (1 inches (in.) of water column). An increase of the pressure drop in the range of 51 to 102 mm of water column (2 to 4 in. of water column) indicates the end of the service life of the filter (EPA, 1991, Burchsted et at, 1979). Newer filters are available which have clean filter pressure drops in the range of 6 to 13 mm of water column (0.25 to 0.5 in. of water column) (Burchsted et at, 1979).

The operation of the fiIter may require additonal equipment. Pressure sensors at the inlet and outlet may be required to measure the change in the pressure drop across the fiIter. This not only indicates when the filter should be replaced but also monitors the integrity of the filter system (EPA. 1991). For applications that require a DOP efficiency test to be administered in place, sampling and injection ports and a test apparatus may be required (EPA. 1991). A special fitting may be installed to facilitate bag in/bag out procedures (Vokes. 1999)

In addtition to an increased pressure drop (51 mm to 102 mm of water column), HEPA filters should be immediately replaced whenever it is exposed to:

• Water spray without protection by a demister or any time a filter is exposed to water spray from fire fighting
• Moisture: 95-100% relative humidity at temperatures higher than 130°F
• Hot air: higher than 275°F
• Fire (direct contact)
• Shock pressures greater than 1.7 psig.
  

Innovations in HEPA Filter Technology

Electrostatic filtration One technology currently being evaluated is the application of an electric field to the HEPA filter itself. This technology is currently being investigated under a Navy funded research effort by New World Associates in Fredericksburg Va. In typical filters when dust collects on a fiber, it tends to deposit on to the leading edge, or side, of the fiber facing the air stream. Through the application of an electric field, it has been noted that there could be a greater loading of dust on the down stream side of the fiber. Testing to date may support this since a significant increase in filter performance as well as dust load capacity has been achieved. However, as yet no smicrograph images of dendrite deposition have been obtained to determine this. Through the application of this technology drastic improvements to filter life, dust load capability, performance, and significantly reduced maintenance may be possible.

Electrostatic precipitation Following this same line of thinking, static precipitation technology is advancing and is under investigation. At Porton Down in the UK researchers are working with a low voltage system that they believe could drastically improve particulate filtration. The concept uses an electrostatic precipitator backed up by a HEPA filter. The filter Porton Down is using has a 99% efficiency for a single pass. If used in series with a HEPA filter, the life of the HEPA filter would increase drastically. Current HEPA filters are lasting up to four years in shipboard installations. If this technology were used, it is theoretically possible that the same filters that were installed when the ship was commissioned would be the same filters in use when the ship would be decommissioned. The reason for keeping the HEPA filter in the system is for backup in case the electrostatic filter fails. A similar approach is being developed by FILT AIR Ltd with its patented air ionizer. The product, known as the Sterionizer, creates a large supply of balanced positive and negative ions. While ionized air plays an important role in other applications such as removing static electricity, improving the quality of air for those who suffer from asthma, and preventing airborne contaminants such as viruses, ionized air also helps to reduce air contamination. Ions clinging to dust particles will weigh down the particles, slow their speed in the airflow, and cause them to stick to the filter media.

Finally, Nano Technology is being used to create new types of materials that either produce smaller fibers or a material with grid-like properties at the submicronic scale. These materials are nonwoven and are generated with processes unlike those used to create typical glass fiber media. The equipment required for Nano Technology manufacturing is rather expensive resulting in a more expensive filter media. Nevertheless, Nano Technology based filter media is catching on and will likely replace convential HEPA filter media in the not so distant future.