A conventional HEPA filter contains a single, continuous sheet of material made largely of glass fibers. This paper-like sheet of material is pleated into a corrugated fold, with aluminum separators between each pleat. The pleated filter element is then treated with a polyurethane compound that bonds it into a hard frame with a depth of somewhere between 150 and 300 millimeters. The air in the room is then pushed through the filter and pumped back out. As with any other filter, the amount of air that can be drawn through the filter decreases over time as the filter becomes clogged with more and more contaminant particles. A clogged filter can even burn out the motor on the fan that drives air through the filter. Most filter manufacturers devise warranties that take this limitation into account, so the warranty is voided if a customer fails to replace the filter unit on a regular schedule, usually every six months or so.
The effectiveness of HEPA filters is limited, not only by how clogged those filters happen to be, but also by how much air can be drawn into and away from the screen. One homemade HEPA unit kit recommends a five-inch computer fan, especially the 120 volt variety, for most standard filter sizes. Commercial HEPA filters allow for a choice of fan speeds to improve efficiency and limit noise.
HEPA filters alone are effective on almost all contaminants, but not on viruses. A virus is usually something like a hundredth of a micron across, which is only a third of the distance across a trap pocket in HEPA filtering material. However, viruses attached to dust particles and other larger objects will be caught by the material. The problem is, that only covers about a tenth of all viruses floating in human living spaces.
Air quality in a filtered volume can be affected by what’s called the “black wall” effect. HEPA filters rely on a negatively charged field of ionized oxygen atoms around the unit. If the unit is wall-mounted, then dust will accrue in a dark layer on surfaces close to the ionizer, usually for a meter or so away from the unit. That’s because walls are flat, spacious surfaces that carry their own positive charge. This charged area attracts negative ions, which are now stuck to dust particles being drawn into the filter. Make sure to clean the walls around an ionizing HEPA filter regularly.
The Department of Energy has warned that proper installation and maintenance of any filter unit is essential to the success of that unit. There also seems to be a need for HEPA filtration devices that are better able to monitor their own progress, warning consumers when filters are overly clogged or air flow becomes insufficient.
By now it shouldn’t be too hard to imagine the advantages and disadvantages of HEPA filtration.
Advantages Of HEPA Air Purifiers
We all like to breathe. It just seems to be one of those things we try to do in any given day. And as long as we’re breathing, it only makes sense to breathe the cleanest air possible. This is true for any large animal life form, but especially for people with asthma, allergies, or other respiratory ailments. For such people, air quality can literally mean the difference between life and death, because airborne pollutants can trigger asphyxiation attacks. Even for the rest of us, air quality is tied directly to life expectancy. In 1995, the American Journal of Respiratory and Clinical Care Medicine reported that “long-term exposure to high levels of fine particulates is associated with higher mortality risks and shorter life span.” Regular cleaning and vacuuming can remove dirt and other visible contaminants, but as one HEPA filter manufacturer warns, “it’s what you can’t see that can hurt you.” That’s because studies have shown that over 99% of all particles in the air are smaller than a micron across. That’s below the level of human vision, so good luck trying to find such particles with a Dust Buster. And because they’re so light, they can float in the air for several days. Vacuuming can make the situation even worse by tossing settled particles back into the air. Only HEPA filters are able to capture indoor air pollutants that small.
Among those pollutants are mold spores. Over a quarter of all children, and a tenth of all adults, are allergic to certain varieties of mold, including Aspergillus, Penicillium, and Stachybotrys. The more we learn about mold, the more it seems clear it has a dramatic effect on the number of allergy cases reported in the United States. For example, in 1999, doctors at the Mayo Clinic did a study that found all but four percent of the sinusitis cases they treated derived in part from infections to airborne mold spores or fungi.
HEPA filters are tremendously useful in eliminating spores and other contaminants from the air we breathe. According to Blueprint for Health, a 1997 publication of the American Lung Association, “HEPA filtration is recommended for those individuals who suffer from allergies, asthma or other respiratory problems.” It doesn’t get much clearer than that. Standard electronic air filters are not as effective as HEPA filters, capturing less than a fifth of all particles smaller than .3 microns.
Disadvantages Of HEPA Filtration
In with the good air, out with cold cash. Replacement filters are expensive, running as high as forty dollars. In less than a year, some customers will spend more on HEPA filter replacements than they spent on the entire unit. Luckily, there are good HEPA filters out there for a reasonable cost, and remember, it’s really only necessary to replace the filter unit about twice a year. Just make sure to take the twice-a-year rule seriously, because otherwise, organisms captured by the filter can reproduce on the surface of and permeate the filter. HEPA filtration does not impede the reproduction of organisms, and some experts consider this problem serious enough to constitute a biological hazard.
There are other disadvantages to HEPA filtration. Because they use a tightly constructed material to trap airborne particles, they affect airflow in a room. They can even cause a substantial drop in air pressure. And Americans concerned about homeland security should pay special attention to a report in Transactions on Plasma Science, which warned, “Conventional HEPA filtration may be ineffective against emerging biological threats.” As those threats become clearer in the post-9/11 world, the report (released in 2000) may be less worrisome. For example, HEPA filtration is virtually a hundred percent effective in removing particles in the one to two micron range—a category that includes anthrax spores. Just remember that according to the U.S. Centers for Disease Control, OSHA, and the Nuclear Regulatory Commission, HEPA filtration alone is not enough to prevent pathogen contamination; it must be accompanied by positive pressure ventilation. That way, air from the outside world cannot enter protected airspace and carry with it biological agents that might infect people inside before HEPA filters have a fair chance to capture those agents.
Now that we’ve weighed the advantages and disadvantages of HEPA filtration, you might be sold on buying a HEPA filter unit—but which, and how much will it cost?
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