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Healthy Home Basics - Air Filters

By HHI Staff

From The Healthy House Answer Book: Answers to the 133 most commonly asked questions. Questions 81-87.


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81. Won’t a good air filter remove all the pollution in my house?


In most cases, filtration isn’t the single answer, but it can be part of the answer. To have good indoor air quality, you first need to apply the three Healthy-House Design Principles of eliminate, separate, and ventilate. Once that’s been done, the indoor air should be pretty good, and you can use a filter to remove any minor pollutants that are left. A good filter is not a substitute for ventilation because filters can’t remove moisture from the air and they can’t supply oxygen.


If you want to try and use filtration to clean up the air in a problem house, you’ll need a very powerful system that will filter the air several times an hour. This will be expensive, breezy, and noisy, and it won’t be as effective as applying the three healthy-house design principles first.


If you decide to use a filter, there are three ways to do so. First, you can use a portable room-sized filter unit. There are a number of companies that offer these free-standing units.


Portable filters work best in a single room, with the door closed, and the filter left running continuously. Second, for whole-house filtration, you can let the fan on your forced-air furnace or central air conditioner run continuously so its filter will remove pollutants passing through the system. Third, you can use a filter with a general ventilation system to filter the incoming air—air that isn’t always as clean as we’d like it to be. In some cases, it can make sense to combine a forced-air heating/cooling system with a ventilation system. That way, one filter (and one set of ducts) can serve both systems.


82. My furnace already has a filter. Isn’t that good enough?


This is a common misconception. Furnace filters are furnace filters—not people filters. They exist primarily to remove very large particles that can damage the bearings in a fan motor, or clog the coil in a central air conditioner. People need much more efficient filtration.


83. I’ve seen ads offering filters that are 90% efficient. Are they any good?


While there are hundreds of different pollutants, from a filtration standpoint, they can be placed in two categories—particulates (small particles) and gases. You need a particulate filter to remove particulates and a gas filter to remove gases. There isn’t a standardized way to evaluate gas filters, but there’s more than one way to measure particulate-filter efficiency—and this leads to some misleading advertising and a great deal of confusion.


Particulate-filter efficiency is most-often measured with either an Arrestance test or an Atmospheric Spot-Dust test. Both are described in detail in a technical standard called ASHRAE 52-76. The Arrestance test really isn’t very useful because it only measures how well a filter captures large particulates—something all filters do fairly well. For example, a standard 1"-thick furnace filter is about 80% efficient when measured on an Arrestance test. For most residential applications, it’s better to use the Spot-Dust test—on which a standard furnace filter is only 3-5% efficient.


So, to answer the question, a filter that’s 90% efficient on an Arrestance test isn’t nearly as good as a filter that’s 90% efficient on a Spot-Dust test. We routinely see ads for 90%-efficient filters, and many use the Arrestance test (although they rarely say so on the ad). They’re certainly better than a standard furnace filter, but not as efficient as you might think.


84. What are the different types of air filters, and how efficient are they?


Electrostatic air filters are made of plastic that’s manufactured to hold a charge of static electricity. Because of this charge, particulate pollutants cling to the filter. It’s the same principle that causes some clothing to cling. These filters are often advertised as being 90% efficient. When they’re evaluated on a Spot-Dust test (#83), they’re 10-15% efficient. While that may not sound like much, it’s 3-4 times as good as a standard furnace filter. These filters actually do a reasonable job with most larger mold and pollen particles. They’re not too expensive, and they can be easily installed in an existing forced-air heating/cooling system. Some can be cleaned and reused, but others must be replaced when they get dirty. Filtrete is one popular brand made by 3M, but there are several others.


Electrostatic precipitators also operate with static electricity, but they require an electric current to function. They’re commonly called electronic air cleaners, and are widely available through heating/cooling contractors. They have electrical wires inside them that cause particulates to become charged, and collector plates that capture the charged particulates. When these filters are clean, they’re typically an impressive 90% efficient on a Spot-Dust test. But, as the collector plates fill with dust, they lose more and more efficiency. It isn’t unusual for the efficiency to drop to 20% in just a few days if the air in a house is very dusty. They’re easy to clean—you just slide the filter unit out of its housing, and place it in a dishwasher. Manufacturers usually recommend washing them at least once a month—but some people find that cleaning is necessary every week. There’s nothing to replace with these filters, but they do produce a small amount of ozone during normal operation.


Extended-surface air filters are 2-6" thick, and they’re made in a pleated, accordion-like shape. This gives them a great deal of surface area—something that’s important to minimize the resistance to airflow. Space-Gard is a popular brand with which heating/cooling contractors are usually familiar, but there are several others that are similar. Because of their thickness, they must be installed in a special housing in the furnace/air-conditioning ductwork. They typically last for a year or two depending on how dirty the air is, and preform at 25-45% efficiency on a Spot-Dust test. This is good enough to capture most common mold spores and pollen particles, so these filters work very well for many allergy sufferers. However, the filter itself is made of fiberglass or polyester fibers held together with a resin, and some sensitive people are bothered by the minor odor of the resin.


HEPA (high efficiency particulate accumulator) filters are extremely efficient. They typically register 98% or higher on a Spot-Dust test, but they’re usually evaluated with an even-more-accurate DOP-Smoke-Penetration test. A HEPA filter is actually a specialized kind of extended-surface filter. In order to minimize the resistance to airflow through them (which can be considerable), they’re usually several inches thick. For residential heating/cooling systems, you must use a more-powerful fan motor just to get very much air to move through these filters. HEPAs are the most efficient particulate filters you can buy. They were originally developed for use in atomic-energy labs to filter out tiny plutonium particles. While it’s a good idea to remove as many particulates as possible from the air, in most residential applications, HEPAs are really often overkill. They can also have a slight odor that bothers some sensitive people.


All the filters mentioned so far are designed to capture particulates. To remove gases from the air, you must use an adsorption (spelled with a D) material, of which activated carbon is the most widely used. When a gas such as formaldehyde passes through an adsorption filter, it clings (adheres) to it. After a while, the adsorption material can’t hold any more gas molecules, and the filter must be replaced. Activated alumina (brand names include Purafil and Carusorb) is another adsorption material, and it actually does a better job of capturing formaldehyde than activated carbon. For these filters to be effective, and to last more than a few days, they generally need a considerable amount of adsorption material. The best filters weigh at least several pounds, but there are some on the market that only contain a few ounces of activated-carbon dust. The heavier filters can have a great deal of resistance to airflow, and they can’t be easily adapted to residential applications—unless the fan motor is replaced with a more-powerful model.


If you’re very sensitive and want a very efficient filtration system, you should know that all filters add something to the air that wasn’t there before. They typically remove far more than they add, but sensitive people can react negatively to the filter itself. For example, the slight odor of an electrostatic air filter, an extended-surface filter, or a HEPA filter really isn’t an issue for most people. However, it can bother some sensitive persons. So can the ozone from an electrostatic precipitator, and some people are bothered by activated carbon manufactured from coal but not carbon made from coconut shells.


While it’s fairly easy to get most of the whole-house filters we’ve discussed through heating/cooling contractors, it’s more difficult to locate a residential supplier of adsorption filters containing very much material, or HEPA filters. If you’re interested in these approaches, Allermed Corp. (31 Steel Rd., Wylie, TX 75098, 214-442-4898) and Pure Air Systems, Inc. (P.O. Box 418, Plainfield, IN 46168, 800-869-8025) make units that can be used with a residential forced-air heating/cooling system.


85. Several people have told me to get an ozone generator. What do you think?


In most cases, we don’t think they’re a good idea. Ozone is a very-well-studied air pollutant that can cause eye, nose, and lung inflammation. When there’s a smog alert in a major city and people are told to stay indoors, it’s because ozone levels are high. So why, you might ask, are ozone generators being sold as a solution to indoor air pollution problems?


Well, it’s because manufacturers have taken a grain of truth and stretched it.


There's evidence ozone can chemically react with some VOCs and the end product of the reaction is water vapor and carbon dioxide—two innocuous compounds. However, this reaction takes place slowly. In the meantime, the ozone can be reacting negatively with your lungs. Ozone can also kill bacteria and mold. But, if it’s strong enough to that, it’s not doing you any good either. In some cases, ozone can react with pollutants in the air and form new bothersome compounds that weren’t there before.


Minnesota, North Carolina, Florida, and California have all come down hard on ozone-generator manufacturers in recent years. These states claim there’s no evidence the devices help improve indoor air quality, and they cite a number of reports showing ozone to be harmful to people.


A situation where ozone generators can be helpful is with smoke damage. Ozone has long been known to react with smoky odors and neutralize them. However, ozone should only be used in a smoke-damaged house when it is unoccupied, and the house should be thoroughly aired out before the occupants return.


86. Is a negative-ion generator a filtering device?


Yes and no. A negative-ion generator uses electric current to produce negative ions (free electrons). The ions attach themselves to particulates in the air causing them cling to oppositely charged walls and furnishings. This can result in slightly cleaner air, but dirtier walls. Some negative-ion generators have a built-in filter that captures the charged particulates.


Sometimes these devices are not sold for their filtering ability at all. There are studies showing if there is an overabundance of negative ions in the air, some people can have a feeling of well-being—they just feel better. On the other hand, these devices create a certain amount of ozone that other people find irritating.


87. Didn’t NASA find that house plants could clean the air naturally?


Again, yes and no. In the 1980s, NASA performed a fairly simple experiment. They placed a house plant inside a sealed chamber with a certain amount of formaldehyde. Then, after waiting a while, they measured the concentration of the formaldehyde. Like magic, the formaldehyde was gone. Further research showed that it wasn’t the house plants that consumed the formaldehyde—it was specific kinds of bacteria living in the soil in which the plants were growing. Anyway, NASA’s simple experiment was widely reported in the press as a revolutionary new idea—house plants as a cure for air pollution. Unfortunately, the real world inside houses is more complicated than the environment inside a NASA test chamber.


In NASA’s experiment, a certain quantity of formaldehyde was used. In most houses, there are manufactured wood products which contain a significant reservoir of formaldehyde—enough to release the gas for several years. When plants are in an environment where there’s a continuously outgassing source of formaldehyde (either in a house or in a test chamber), several things happen. First, the soil bacteria starts consuming the formaldehyde as food. Then, the formaldehyde source (particle board, or whatever) starts outgassing faster to try to raise the concentration back up to what it was. At the same time, the plant is releasing a certain amount of moisture into the air, and formaldehyde outgases faster as the relative humidity goes up. The net result is a slightly lower concentration of formaldehyde—but not a significant reduction.


This is one of those simple solutions to a complex problem that just doesn’t work. Plus, if you have very many plants indoors, the relative humidity can become high enough to result in a mold problem.


(Note: This article is part of the original HHI Archives, and was believed to be accurate at the time of writing. The views expressed in this article are those of the author, and do not necessarily represent those of The Healthy House Institute, LLC.)


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Healthy Home Basics - Air Filters:  Created on February 3rd, 2008.  Last Modified on February 28th, 2011


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