By Robert McIlvaine and Karen Vacura, The McIlvaine Company
Over the years, focus in the air filtration industry has shifted from just particulate capture to also include adsorption of gaseous contaminants. In fact, the label “molecular contamination” was created to identify this additional danger to products made in cleanrooms.
Now a new category of contaminants, nanoparticles, has emerged. There is controversy over the ability of traditional HEPA filters to capture these particles. One reason stems from a debate over the ability to measure these particles. If nanoparticles can’t be accurately measured, then how can we know whether they’re captured?
As new studies show that inhalation of small particles is a more serious health issue than previously believed, air filtration becomes more important than ever. It not only protects the product, but the cleanroom worker as well.
Markets and revenues
The market for filters in cleanrooms not only includes the high-efficiency final filters but also the prefilters used to protect the final filters. Typically, HEPA filters are replaced every few years, whereas prefilters are replaced every few months. Since the price of the prefilter is only a fraction of the price of the high-efficiency filter, the total cost of prefiltration is much less than that of final filtration.
Revenue in the cleanroom air filter market is closely associated with the growth of the individual industry. When the semiconductor industry is strong, for example, larger orders for HEPA filters are seen.
Major international air filter companies include Camfil, SPX, and American Air Filter (AAF). 3M is a large supplier of prefilters, but not final filters.
The larger air filter companies have plants and sales offices located internationally. AAF, for example, has 18 locations worldwide, and media companies such as Hollingsworth & Vose are now producing in China.
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Due to the growth in disk drive, semiconductor, and flat panel display industries, Asia has become the largest geographic market for cleanroom filters (see Table 1).
Classification
Air filters can be classified by type. Electronic filters, for example, use an electrical field to trap charged particles. Voltage is applied to a field through which the air stream passes, and then charged particles are collected on oppositely charged capture media or plates.
Gas phase filters utilize activated carbon adsorption. For cleanroom applications, the carbon is impregnated on the filter, rather than granulated, increasing material stability and keeping the carbon from breaking off.
Filters can also be classified by efficiency. The European standard EN779:1993 is based on an average of two different test methods. First a filter is tested with untreated outdoor air and then again with the addition of synthetic dust. The outdoor-air test establishes dust spot efficiency (the filter’s ability to remove large particles) and the second test determines the filter’s arrestance value (the filter’s ability to capture a mass fraction of coarse test dust).
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The U.S. counterpart to this standard is the MERV rating, which is based on ASHRAE Standards 52.1-1992 and 52.2-1999. These standards measure arrestance, dust spot efficiency, dust holding capacity (the total amount of dust a filter is able to hold) and particle size efficiency (different particle size ranges for a clean and incrementally loaded filter). Table 2 compares the standards.
HEPA filters, and the even more efficient ULPA filters, are typically made with microfiberglass media. However, synthetic membranes have been developed and are providing competition for the traditional media.
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The cleanroom industry accounts for only nine percent of the total yearly air filter revenues, but it accounts for 23 percent of the high-efficiency filter revenues (see Table 3).
Health risks of nanoparticles
Nanotechnology manufacturing processes have been studied and shown to pose a risk of worker exposure to nanoscale particles (NSPs). The National Institute for Occupational Safety and Health (NIOSH) and related federal agencies are looking at worker health and safety in the midst of innovation and technical progress.
Further study is needed on the best way to capture and dispose of NSPs in order to reduce health risks. Also, according to a recent CleanRooms article (January 2006, p. 10), the information available needs to be better organized. The International Council on Nanotechnology and the Center for Biological and Environmental Nanotechnology at Rice University have teamed up to establish an environmental, health and safety database as a first attempt to organize the massive amounts of information on the impacts of nanoparticles.
One theory on nanoparticle health risks maintains that current HEPA filters may be completely adequate to protect workers. HEPA filters trap 300 nm particles with a capturing efficiency better than 99.97 percent. But some measurements demonstrate that they also trap NSPs down to 3 nm with even greater efficiency. Tests reveal that airborne NSPs behave enough like gases that their random (Brownian) motion makes it possible for them to hit and stick to the filter. Generally, particles larger than 300 nm are collected by impaction due to particle inertia, and particles smaller than 300 nm, behaving more like a gas, tend to be collected by diffusion. The 300 nm “valley” between these two different particle collection mechanisms is often quoted as the most penetrating particle size for filter media.
On the other hand, since these particles act like gases, will they be captured by HEPA filters? Rather, capture by gas adsorber filters may be the best solution.
Fine-particulate mortality confirmed
Michael Jerrett of the University of California analyzed a subgroup of 22,905 southern Californians drawn from a much larger study of 1.2 million subjects enlisted twenty years ago by the American Cancer Society (ACS). The subjects were enrolled in the study in 1982, and between then and 2000, there were 5,856 deaths. Jerrett compared each death with estimated long-term PM2.5 concentrations (referring to particulate matter that is 2.5 μm or smaller) at the location of each person in the cohort, or study group. Jerrett established a link between higher levels of particles and increased rates of death from lung cancer and-especially in diabetics-heart attack, as well as a number of other causes. He also showed particles to be two to three times more toxic than previous analyses.
Whatever their effects, particles first enter the body through our lungs, or more accurately, the respiratory tract, which serves two purposes: to transfer oxygen to red blood cells, and to remove from them waste gases, principally carbon dioxide. The respiratory tract is shaped like an upside down tree, stout at the trunk (the trachea) and dividing into progressively smaller branches (the bronchus and bronchioles). Finally, there are the leaves (alveoli), which are so small that between 274 and 700 million of them are in a human lung. This is where the actual gas transfer occurs.
Only one cell thick and in very close contact with each other, the lining of the alveoli and the surrounding capillaries average about one micron in size. Oxygen passes through this air-blood barrier quickly and into the blood in the capillaries. So, too, do particles, or at least the smallest of them, traveling with the oxygen-rich blood to the left side of the heart, where it is quickly pumped throughout the body.
Exactly what happens next is unclear. It is certain, however, that noncrustal particles (from combustion and other man-made sources) possess toxic properties. Exposure to noncrustal PM2.5 is followed by increased heart rate and blood viscosity, as well as a wide range of other responses. Some of these are immediate, while others occur over time. While not benign, crustal particles (from natural, environmental sources) do not approach the toxicity of those from diesel engines, coal-fired power plants and other combustion sources.
Ten years ago, scientists were at a loss to demonstrate how things so small might possess such a lethal effect. Today there are several plausible biological explanations for fine-particulate toxicity and it’s reflected in the vast difference between the current standard-setting process and that of the mid-1990s.
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Of the plausible explanations for how fine particles adversely affect humans, two are most important: restructuring of the respiratory and circulatory systems; and inflammation-induced changes in heart function and blood composition. Inflammation is a complex, major response of the immune system to tissue damage and infection, and according to National Center for Health Statistics1, fine particles are the 9th leading cause of death in the United States (see Table 4).
Conclusion
The air filter industry is facing new challenges, one of which is to determine the performance of filters on nanoparticles. Recent data showing greater health problems as a result of small-particle inhalation will push the industry to improve filtration efficiency. The rising cost of energy will encourage the industry to achieve higher performance with lower pressure drop. And bioterrorism and new airborne diseases are yet additional challenges for the filter industry.
Cleanrooms will continue to be a major market for air filter manufacturers. The concept of cleanroom technology, however, has expanded beyond its traditional applications and is now applied more broadly. For example, filters will protect diplomats in embassies, which for all intents and purposes are positive pressure cleanrooms. Even the interior of an automobile, with its HEPA filter system, has become a miniature cleanroom.
References
1. http://www.cdc.gov/nchs/data/hestat/finaldeath03_tables.pdf#2
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Robert McIlvaine is president and founder of The McIlvaine Company in Northfield, IL. The company first published Cleanrooms: World Markets in 1984 and has since continued to publish market and technical information for the cleanroom industry. He can be reached at [email protected].
Karen Vacura is the air filtration market editor for The McIlvaine Company. She can be reached at [email protected].