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Chapter 1

History of the Development of HEPA filters

This chapter discusses the history of the development of high efficiency particulate filters (HEPA) for military use and the evolution of high efficiency particulate filters for respiratory protection, nuclear weapons research, nuclear energy research, the space race and cleanroom industries. This chapter includes the first of many actual "field examples" that show the application of a particular concept in the text.

A. German Gas Masks

The development of the HEPA filter dates back to the 1940s and WWII. They were originally developed by Germany using asbestos fibers as the filtration media in gas masks. In the early days of World War II, the British sent filter paper extracted from captured German gas mask canisters to the U.S. Army Chemical Warfare Service Laboratories (CWS) in Edgewood, Maryland. 89

The German filter paper was made of finely ground up asbestos dispersed in esparto grass. They had unusually high particle retention characteristics, acceptable resistance to airflow, good dust storage, and resistance to plugging from oil-type screening smokes (a deficiency of the resin-wool filters then used by the British forces). The U.S. Army Chemical Warfare Service and the U.S. Naval Research Laboratory developed a similar "collective protector" filter unit using a cellulose-asbestos media. The first successful paper produced for the U.S. Navy contained Bolivian crocidolite and was called H-60. The paper produced for the U.S. Army also contained Bolivian crocidolite and was first designated H-64, but later renamed CWS Type 6. It was formulated from northern spruce sulfite and sulfate pulp (approximately 76 percent), cotton waste (approximately 15 percent), and crocidolite asbestos (Bolivian or African crocidolite and African esparto grass) (approximately 14 percent). Penetration was 0.025-0.04 percent based on a methylene blue stain-intensity test procedure. 115

The Hollingsworth and Vose Company in Massachusetts manufactured these in large quantities for the military on conventional papermaking machinery.

The U.S. Army Chemical Corps "collective protector" filters were also known as absolute, super-interception, and super-efficiency filters. However, in 1961, the generic acronym HEPA filter was coined by Humphrey Gilbert, a former Manhattan Project safety engineer. It came from the title of a 1961 Atomic Energy Commission (AEC) report called High-Efficiency Particulate Air Filter Units, Inspection, Handling, Installation. 87 Since that time, the term "absolute filter" has fallen into disuse because it is not technically accurate."

B. US Nuclear Weapons Manufacturing

Large size HEPA filters were first used to remove radioactive particles from the air during the United States Manhattan Project while researching and constructing the first atomic bombs. They were developed by Arthur D. Little Co.

In developing these HEPA filters, researchers focused on the ability to capture solid particles in the air. These particles were created through the condensation of gases and liquid aerosols into solid matter during the production of weapons-grade materials. Researchers considered the condensation nuclei of radioactive iodine vapors to be the most harmful exposure risk to research personnel because iodine is taken up by the thyroid and can produce thyroid cancer.

C. U.S. Nuclear Energy Reactors and the Space Race

During the 1950s, the nuclear weapons production and the emerging nuclear power generation industry were the driving force for further development of HEPA filtering technology. The U.S. Atomic Energy Commission (AEC) developed HEPA filters using a fiberglass-asbestos media that was more durable than the previous cellulose-asbestos filter material. As the space race began in 1958, the U.S. developed the first all-fiberglass HEPA filters. However, fiberglass-asbestos filters continued to be used in some applications through the 1970s.

By this time, the Army and the AEC were well aware of in-use performance problems of HEPA filters. Although these filters were designated as HEPA, during actual use, they were found to be less effective. Consequently, in order to insure the safety of workers in nuclear weapons production, they needed to test the performance of HEPA filters to insure that they actually performed to the theoretical specification. In 1962, The AEC set up its first Filter Test Facility at the Hanford graphite mediated weapons grade uranium and plutonium production reactor facility in Hanford, Washington. The Hanford site stayed in production until September 2009 when it was shut down for safety reasons. The Hanford reactor was the same type of reactor that caught fire during "safety testing" in Chernobyl in 1986. 109.

Field Example # 1: In a lesson in not learning from history or not sharing "secret" weapons production information:
In 1957, fires occurred at two of the US graphite-moderated, air-cooled reactor similar to the Chernobyl reactor. This was at AEC's Rocky Flats Plant and in the Windscale facilities. These fires showed the need for developing noncombustible exhaust air HEPA filters and lead to development of all fiberglass HEPA filters.

Had these facts been known to the Chernobyl operators that two fires in graphite mediated reactors had occurred in the US, our Russian counter parts may have been more careful in testing their graphite mediated reactor.

The U.S. Army made the design specifications for HEPA filters available to the public in the 1960s. Two of the first HEPA filter design standards for the nuclear industry were Mil-F-50168 "Filter, Particulate, High-Efficiency, Fire-Resistant," and Mil-F-51079 "Filter Medium, Fire-Resistant, High-Efficiency." These standards date to 1965. The AEC included these in Health and Safety Notices 212 and 306.

HEPA filter technology spread to the development of cleanrooms for technologies in other industries including aerospace, nuclear power, pharmaceutical production and later in transistor and integrated circuit production.

In 1974, responding to the change in filter media technology to fiberglass, the AEC issued RDT M-16-3T "HEPA Filter Medium, Glass Fiber (MIL-F-51079 with Modifications and Additional Requirements)" and RDT E-9-1T "HEPA Filters" (AACC CS-1 With Additional Requirements). In 2003, the requirement of Mil-F-51068 and Mil-F-51079 were incorporated into ASME AG-1, Section FC. Committee on Nuclear Air and Gas Treatment (CONAGT). Today, the U.S. nuclear industry's HEPA filter design and construction specifications in AG-1 are essentially the same as those in England. These newer specifications reflected that HEPA filters are now made of many different materials including fiberglass, Teflon, nylon, ceramic sintered metal, polypropylene, polyethylene terephthalate and other materials.

D. PHEAF Equipment

Most of the HEPA-filtered equipment described above are stationary systems that are installed in laboratories or manufacturing facilities. Over the past few decades, the development of the asbestos, lead, mold, illegal drug labs, other hazardous material clean-ups and fire/water damage remediation / restoration industries has resulted in a huge increase in the use of portable HEPA-filtered equipment. Portable HEPA-filtered equipment is a special subset of HEPA-filtered equipment that is subject to highly variable conditions and physical abuse. Consequently, special acronyms for this type of equipment were developed by OEHCS, Inc. PHEAF (pronounced "feaf") is an acronym for a "portable high efficiency air filtration." This acronym can be used for air filtration equipment and vacuums that meet the PHEAF classes in this standard. (Minimum of 90% effectiveness.)

The next chapter discusses the initial aerosol methods that were developed to test HEPA filtration efficiency of individual filters in both fixed systems and in portable equipment.



Chapter 2

THE HISTORY OF THE DEVELOPMENT OF THE DOP AEROSOL TEST METHOD

A. Tyndall Scattering

All current HEPA filter testing and evaluation methodologies are based on optical particle measurement technology - that is, the ability to detect light reflected off of particulate matter in the air. Interestingly, the technology for monitoring particles in the air has not changed all that much since its inception in 1860s.

John Tyndall conducted the first demonstration of the existence of small particles in the air in the 1860s. Tyndall devised a very bright light source and shined the light into a dark box through a thin slot. Air was passed through the box and the observation hole in the box allowed the viewer to see the individual particles moving with the air through the box. This effect was known as Tyndall Scattering.

Field Example # 2 : Early Air Purifier ?

In the lab, Tyndall came up with a simple way to obtain "optically pure" air. He coated the inside walls of a box with glycerin. After a few days, the air inside the sealed box was observed to be particulate-free under examination with light beams, because the various floating-matter particulates had ended up getting stuck to the walls or settling on the sticky bottom of the box. This was an early example of an "air purification" strategy. He observed that particles diffused and settled out of the air over time.

B. Counting Particles with Light Microscopy

During the early part of the 20th century, particle concentrations in the air were measured using liquid impingers. In this industrial hygiene sampling method, air was bubbled through triple distilled water to collect the particles in the water. The water was then placed on a glass slide that had a 1 milliliter well and covered with a glass cover. The particles and their size were then optically counted under a microscope. Results were reported as millions of particles per cubic foot (MPPCF) or particles per cubic meter.

However, this technique was not sufficiently accurate for evaluating the effectiveness of absolute filters, (the 1950s term for HEPA filters.) The purpose of these absolute filters was to remove very small particles from the air, especially condensation particles of gas warfare agents and biological warfare agents. These particles were too small to be assessed accurately using this light microscopy technique. Clearly, a better assessment method was necessary.

C. The Methylene Blue Stain-intensity Procedure

The first test method for checking the performance of the U.S. Army Chemical Corps ?collective protector? filters was a methylene blue stain-intensity test procedure. This test method was used in gas mask testing during the early part of WWII until 1941. It was an extension of a water filter test method.

A test aerosol was generated from methylene blue dye dispersed in a water solution. The aerosol was evaporated and the dried particles impacted on the filter media.115 This test method quantified the amount of blue color that would come through a filter onto a collection material. This test procedure showed a 99.96 - 99.975 % filtering efficiency (0.04- 0.025 % particulate penetration) for the typical gas mask.

This procedure is no longer used because once a filter has been stained, it can no longer be periodically retested with this method. However, it could still be used to find a leak in a defective HEPA filter in some applications.

Field Example # 3 : A Glowing Report ?

Another method of finding a leak in a defective HEPA filter is to dust the overall filter with an ounce or so of UV blacklight powder. Make sure to choose one with very small, micron-size particles. www.maxmax.com/aUVPowderUltraVis.htm). The UV powder will find its way to the leak area after a few minutes of operation. Then you can scan the filter with a UV light to find the leak.

D. Initial Theories on the Most Penetrating Particle Size

As seen in the previous section, the early science of air filtration testing was based on water filtration science. However, more advanced scientific research specifically targeting HEPA filters was needed. Consequently, the National Defense Research Council contracted with various researchers including Nobel Laureate Irving Langmuir to develop the theory and physical equations for particle retention using fibers or small granules as the filtration media.

Langmuir's research and theoretical analysis concluded that there were two principal mechanisms involved in the retention of small particles in the air passing through HEPA filter media. These included:

(1) inertia - (now described as inertial impaction), which affected particle sizes substantially greater than 1.0 micrometer (µm) in diameter when moving through a devious flow path in a bed of porous material; and
(2) diffusion - which affected particles with diameters substantially smaller than 1.0 µm. 120

Langmuir's analysis was later modified by Ramskill and Anderson 158 to include a third phenomena involved in small particle physics. This added :

(3) interception - which states that if a particle passes within ½ of its diameter to a filtering fiber, it will impact the surface of the fiber and stick to the fiber surface due to adhesion.

Ramskill and Anderson's theoretical analysis concluded that the combined collection effects of all three of these physical forces on a particle would be minimal when the particle was 0.3 µm in diameter. Therefore, particles that were 0.3 µm in size would be the most difficult to capture with the filter media. This theoretical 0.3 µm particle size was defined as "the most penetrating particle size" or MPPS. 158

Based on this theoretical analysis, Langmuir advised testing gas mask filters with smoke of this 0.3 µm particle size to determine a filter's minimum retention efficiency. Langmuir also stated that when particles with diameters greater or smaller than 0.3 µm were present, they would be removed at higher efficiencies than the 0.3 µm size test particles. Even though this MPPS was only estimated by theoretical calculations, this size became dogma and remains to date as the defining characteristic of most HEPA filters.

(For a more detailed discussion of current theories of the MPPS, see Section E below and Chapter 6, Section B.)

E. LaMer and Sinclair's Development of the DOP Aerosol Test Method

Langmuir's early theoretical calculation of a MPPS lead to the development of the Dioctyl Phthalate (DOP) test method by Victor LaMer and David Sinclair.

Sinclair and LaMer developed a DOP generator that uses an evaporation-condensation process to generate 0.3 microns size particles. In their equipment, the evaporated DOP condenses on nano-sized (10-9 meter) seed particles and air ions generated by electrical sparks. These condensed particles grow to a size range of 0.1 - 2.0 µm with a median 0.3 micron particle size. These are referred to as a heterogeneous DOP aerosol since the droplets are not all exactly the same size. In order to generate this size of particles the evaporated DOP concentration must be in the range of 100 - 200 mg/m3.

The last piece of equipment necessary for assessing the effectiveness of HEPA filters by this method was designed to measure the concentration of the DOP aerosol on both sides of the filter. This was done with the development of the Optical Mass Concentration Meter, later renamed as the Forward Angle Smoke Q107 Penetrometer. The Q107 Penetrometer was used to measure particle concentrations before and after the HEPA filter. This was basically a glass tube through which air containing the DOP particulates was sampled. A high intensity white light illuminated the smoke through a slit opening and a photomultiplier tube measured the amount of light reflected off the particles. Initially, the Q107 Penetrometer was manufactured for the military by Air Technologies. Sinclair and LaMer?s filter test method was used by the National Defense Research Council from 1942 through 1945. This filter test method became the U.S. for testing HEPA filters and it is still part of the current Mil Standard 282. 119

However, in 1949, seven years after Langmuir's initial theoretical work, some questions began to develop as to the accuracy of 0.3 µm particles being the most penetrating particle size. Victor LaMer 116 of Columbia University performed many experiments to test Langmuir's theory of a minimum filterable particle size. LaMer's research showed that 0.3 µm was a close estimate of minimum filterable particle size, but collection efficiency continued to declined as particle size decreased below 0.3 µm. 117

Other research showed that forces not taken into account by Langmuir (e.g. mid-size particle interception, flow rate, naturally occurring electrostatic charges on particles and filter media) also affect collection efficiency and the minimum filterable particle size. Today, it is generally recognized that the most penetrating particle size is actually close to 0.1 microns. 98

F. U.S. Mil Standard 282

The first declassified U.S. method for testing HEPA filters dates to 1952. It specifies the DOP method developed by LaMer. It was published by the U.S. military as MIL F-10462A(CmlC) 30 October 1952. It was superseded on 28 May 1956 with MIL-STD-282-Protective Clothing, Gas-mask Components and Related Products: Performance-Testing Method. www.wbdg.org/ccb/FEDMIL/std282.pdf.

In this method, DOP liquid is heated and vaporized, then condensed to become DOP particles with a mean diameter 0.3?m. This process is called thermal generation or the "hot DOP method" (as opposed to newer "cold DOP methods" where compressed air is used to aerosolize DOP at room temperature.) The total amount of air and smoke used in this test method was 12 cubic feet. Mil-Std-282 has been updated four times with the latest one being Mil-Std-282(4): 12 January 1995. 138

G. Test Methods for the Nuclear Weapons and Power Industries

After the issuance of Mil-Std-282, the majority in new HEPA filter research was done by the nuclear weapons and nuclear power industry. At the same time, the cleanroom industry (pharmaceutical, electronics, NASA, etc.) was developing and using this nuclear research as the basis for their airborne particulate clean room standards.

In 1970, the Atomic Energy Commission published the "Air Cleaning Handbook" for nuclear facilities. In this handbook was the AEC's HEPA filter construction specifications and testing method. This was ORNL NSIC-65 "Design, Construction and Testing of High-Efficiency Air Filtration Systems for Nuclear Applications". ORNL NSIC-65 was substantially updated in a second edition in 1976 as ERDA 76- 21 "Nuclear Air Cleaning Handbook." 55 This AEC test method remained as the industry standard until 1980.

In 1980, after reviewing the complexity and shortcomings of Mil-Std-282, the Los Alamos National Laboratory (LANL) proposed an alternative test method called the "High Flow Alternative Test System." 55 This method used more recent technology such as standardized Laskin nozzles and impactors instead of a DOP generator to produce aerosols. More significantly, it used a laser aerosol spectrometer instead of the aerosol measuring instrument on Q-107 Penetrometer. The LANL test results were equivalent to that of the MIL-STD-282 method, but efficiency was also measured for various particle sizes. However, this method required a longer test time to measure the effectiveness of each particle size range and increased data error due to the difficulty in generating specific particle sizes. Therefore, this method was never widely used. 55

Today, the current Department of Energy (DOE) Nuclear Energy documents for HEPA filter specification and testing are E-342, E-343, E-3-44, and E-345 standards. The most applicable standard for in-field HEPA filter testing is NE E-341T "In-Place Testing of HEPA Filter Systems by the Single Particle, Particle-Size Spectrometer Method."

H. Test Methods for the Cleanroom Industries

In 1968, the cleanroom industry's trade association, the American Association for Contamination Control, published its first HEPA filter standard AACC CS-IT. This standard did not rely on Mil-Standard-282 but used a different weapons-related document from the Edgewood Arsenal. This was known as "Instruction Manual for Installation, Operation and Maintenance of Penetrometer, Filter Testing, DOP, Q107" (Document Number 136-300-175A, ?Instruction Manual for the installation, operation and maintenance of Penetrometer, filter testing, DOP, Q107?, U. S. Army, Edgewood Arsenal, Aberdeen, Maryland). 60

During the 1980s, the American Association for Contamination Control went out of business and the predominant cleanroom industry trade association became the Institute of Environmental Sciences and Technology (IEST). Within the last few decades, high-efficiency particulate air (HEPA) filters have been joined by "ultra low penetration air" (ULPA) filters for situations where even greater filter efficiencies are necessary. (ULPA filters are rated at 99.99 - 99.999 % efficient.)

During 1992 and 1993, IEST issued a series of HEPA/ULPA filter test standards for different requirements and applications These included:

IEST-RP-CC007.1:1992, "Testing ULPA Filters",
IEST-RP-CC001.3:1993, "HEPA and ULPA Filters" and
IEST-RP-CC-006.2:1993, "Testing Cleanrooms".

In 2005, IEST issued another standard IEST-RP-CC001.4. "HEPA and ULPA Filters". This Recommended Practice describes eleven levels of filter performance and six grades of filter construction. In October 2009, IEST issued another recommended practice standard IEST-RPCC034.2 "HEPA and ULPA Filter Leak Tests." This Recommended Practice covers definitions, equipment, and procedures for leak-testing high-efficiency particulate air (HEPA) filters and ultra low penetration air (ULPA) filters in the factory as they are produced, at the job site before they are installed, and after they are installed in cleanrooms and in unidirectional-flow, clean-air devices.

1. The Aerospace Industry

The aerospace industry uses HEPA and ULPA filters in cleanrooms to control airborne particle levels during the assembly of rockets and satellites. This is done because particles can interfere with electrical connections, cause corrosion, cause seals to leak, result in undesirable coatings on mirrors used in telescopes, and contaminate extraterrestrial ecospheres.

More recently, it has been essential to use HEPA air filtration to control mold and bacteria spores that can also be in the air during the assembly of International Space Station components. This was based on the experience with mold and bacteria growth on the Russian space station Mir in the 1980s 116 which resulted in extensive damage to the vessel and its ultimate demise. (See video documentary titled "Mutant Mold in Outer Space," OEHCS Publications, 2012.)

Lastly, HEPA-filtered air is necessary to control particles during the assembly of infrared spy satellites for tracking the movement of personnel and equipment at night. Typically, the rocket and satellite construction facilities are cavernous and as many as 14 stories tall. They are the largest HEPA-filtered enclosed spaces in the world.

Field Example # 4 : A Ceiling Too High ?
In early 2009, OEHCS, along with a major restoration firm was asked to submit a proposal on how to clean and certify the largest rocket and satellite cleanroom assembly facility in the world. This facility was so tall that no powered personnel lifts existed that could reach the necessary height for cleaning the upper sections of the building. A custom personnel lift had to be built costing over $750,000. However, the real challenge in this project was how to test and certify over 1,000 HEPA filters built into walls that were up to 100 feet high so that the space could meet the cleanroom Class requirement for the discharge air and the room itself. This procedure required people on each side of the filter at the same time that had to work in exact coordination to test the filters, find leaks and fix them.

2. The Pharmaceutical Industry

The pharmaceutical industry uses HEPA-filtered air (Class 100 or less) in what are termed ?sterile fill? operations. Sterile fill operations are used for drugs that break down in elevated temperature and therefore cannot be heat sterilized after they are manufactured. In these drug manufacturing facilities, sterile bottles are filled with sterile drug solutions on high speed filling machines adapted from the food industry. The HEPA-filtered air distribution systems on these machines are designed to prevent particles, mold and bacteria from getting into the solutions during the fill operations.

3. The Electronics Industry

The electronics industry has the most stringent HEPA filtration requirements. This is because particles in the air can interfere with the production of microcircuits where the size of the "wires" used on circuit boards are now smaller than most particles in the air. Obviously if a particle from the air got onto one of these tiny circuit paths it would prevent the wire from being created and make the circuit board inoperable. Consequently, new cleanroom classes had to be developed for these extremely strict particle filtration requirements. The most stringent is a Class 1 room. This is 100 times cleaner than the Class 100 rooms used in sterile fill operations in the pharmaceutical industry. This is very, very clean air.

4. The Nanoparticle Industry

The nanoparticle production industry, along with research on nanoparticles is the latest area to use HEPA/ULPA filtration. HEPA/ULPA filters have become a critical control method to prevent personal exposure to these very small particles. Many nanoparticles are similar in size as biological cells and biochemicals (enzymes, hormones, etc.) Hence, some nanoparticles maybe "biologically active." Although significantly more research on the potential long-term health implications of nanoparticles is needed, there is a proactive effort in many nanoparticle industries to control employee nanoparticle exposures.

The next chapter will discuss the development of laser particle counter technology and subsequent methods that were developed to test HEPA filtration efficiency of individual filters and to test HEPA filtration effectiveness of portable air filtration devices.