This article was originally published in the July/August 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online July/August 1996
Clearing the Air:
Upgrading the efficiency of the filter to remove more particles can help maintain the equipment as well as human lungs. However, all filters are not created equal. Some are much more effective than others at removing airborne contaminants, especially the smaller particles, which pose the biggest threat to health (see Figure 1).
ASHRAE Standard 52-76 describes three tests for determining a filter's efficiency at capturing particles. None of these tests measures a filter's ability to remove gaseous pollutants.
The results of this test are the ones most likely to be used in marketing claims that a given filter has 80% or greater efficiency. Removing 80% of the large particles is relatively easy but does little to protect human health or equipment life. To demonstrate how ineffective such a test is, try pouring table salt through a standard panel filter with a weight arrestance rating of 80% or greater. Be prepared to clean the floor, since the salt will pass freely through the filter.
Since ASHRAE Standard 52-76 sets guidelines for both the weight arrestance and the atmospheric-dust-spot tests, consumers can be easily confused by marketing claims. If a filter claims to be 80% efficient by ASHRAE Standard 52-76, be sure to look for the words atmospheric-dust-spot or arrestance to determine whether it is an efficient filter for small as well as large dust particles. Many products available at consumer retail outlets show weight arrestance and not dust-spot efficiency ratings.
ASHRAE Standard 52 ASHRAE is currently developing new guidelines for Standard 52 filter efficiency testing. The new test will measure minimum particle removal efficiency over a range of sizes, from 0.3 µ to 10 µ. The test will rate the performance of a filter over its entire life and account for gains or losses in efficiency. Standard panel filters will not meet the minimum performance guidelines for this new test.
|Table 1. Particle removal at various filter efficiencies|
|Atmospheric Dust-Spot Efficiency||Particles removed|
|10%||Good for capturing lint. Somewhat helpful for ragweed pollen. Not very good for smoke and staining particles.|
|20%||Fairly good at capturing ragweed pollen. Not very good for smoke and staining particles.|
|40%||Good at capturing pollen and airborne dust, some smudging and staining particles. Not very good for tobacco smoke particles.|
|60%||Very good for all pollens and most particles that cause staining and smudging. Partially helpful for tobacco smoke particles.|
|80%||Very good at removing smudging and staining particles, coal dust, oil smoke particles, and tobacco smoke particles.|
|90%||Excellent protection for all particles.|
Panel Filters The most common panel filters are the disposable spun glass or fiberglass type and the washable hog's hair products. They are usually 1 inch or less in thickness and fit in the standard filter slot for residential forced-air systems.
Panel filters are inexpensive to buy-they cost between 50 and $5-but they have dust-spot efficiencies of less than 5%. Their filtering capability actually increases as they get dirty, but this is accomplished at the expense of restricting air flow and increasing pressure drop. To avoid restricting air flow, they should be changed every one to three months. They do a poor job of protecting the forced-air system and offer human lungs almost no protection from particles.
Electrostatic filters are typically 1 to 2 inches thick and have low air flow resistance so that they can easily be substituted for a standard panel filter. However, they face load, meaning that dirt accumulates primarily on the surface facing the direction of air flow. Face loading can significantly increase the pressure drop and reduce efficiency. The time between cleanings or before replacement can vary from one month to an entire heating or cooling season.
Electrostatic panel filters use static electricity to attract charged particles in the air stream onto the filter surface. While not as effective as electrostatic precipitators, they do not require electricity, and are less expensive. However, they often face-load on the surface facing the direction of the airflow, which can increase pressure drop and reduce filter efficiency.
Electrostatic air filters typically have a dust-spot efficiency of 10%-15%. They are marginally effective at capturing small particles (1 µ or less) but more effective than a panel filter on larger particles (greater than 10 µ), such as mold spores and pollen.
Prices for electrostatic filters vary widely. Some products are sold for under $10; others are priced at over $125. These filters usually last longer than standard panel filters.
More efficient extended-surface filters have dust-spot efficiencies of 25%-45%. These filters are several inches thick and cannot fit in a standard 1-inch filter slot. They must be installed in a special housing in the return ductwork. The filter and installation costs can be several hundred dollars, with replacement filters costing around $30-$50.
A HEPA filter should be designed and installed by knowledgeable professionals and can cost several thousand dollars. Replacement filters cost approximately $150-$200 and can last a year or more.
To maintain their efficiency and avoid contamination, electronic air cleaners must be cleaned regularly. Be sure that there is easy access at the air handler to remove the metal plates for cleaning and to clean the housing.
Electronic air cleaners require electricity and must be fabricated to fit in the return ductwork. They can cost from $600 to $1,200, including installation. The power demands of these units vary from 20 W to 50 W, so the operating cost of some units will be more than double that of others. Depending on the unit, the price of electricity, and the amount of time the system operates, users can expect to pay between $3 and $30 per year in operating costs.
In producing the high voltage necessary to charge particles, electronic air cleaners also produce small amounts of ozone, a highly reactive gas composed of three oxygen atoms. Ozone can be an irritant to lungs, eyes, skin, and respiratory membranes. Most health experts feel that the small amount of ozone produced by a properly installed and maintained system poses little threat to healthy individuals. Nevertheless, extremely sensitive individuals may react to it. Some electronic air cleaners rely on a special adsorption filter (see below) to reduce levels of ozone entering the airstream; however, the long-term effectiveness of these adsorption filters is unknown.
An adsorption filter that removed a large percentage of gases (by weight) from the air would have to be several inches thick and would create too much resistance for a standard residential air handler. Most residential adsorption filters are about 1 inch thick and can remove some gaseous pollutants. However, a product's ability to remove specific gases, such as formaldehyde or other VOCs, varies. There is also the possibility of long-term desorption or release of gases, which can occur when the filter becomes saturated.
The pleated accordion shape of this filter is typical of many extended surface filters. These filters achieve higher efficiencies by increasing filter surface area, and can replace a standard panel filter without significantly restricting airflow.
According to an article published by the American Industrial Hygienist Association Journal, despite long-term and widespread use of these devices, there is a lack of evidence in the scientific literature that would support ozone as effective at low concentrations to remove organic contaminants from indoor air. Rather, scientific evidence exists that implies that low levels of ozone will not effectively remove most indoor air contaminants. Subjective claims of improved air quality may instead be explained by evidence indicating that ozone may act only to mask odors or to convert some odorous compounds to less odorous but potentially more toxic compounds.1
The Federal Trade Commission has signed agreements with manufacturers requiring them to cease unsubstantiated advertising claims or to provide competent and reliable scientific evidence to support marketing claims that ozone generators eliminate or clear specified chemicals, gases, mold, mildew, bacteria, or dust from the user's environment; the product does not create harmful by-products; and the product prevents or provides relief from allergies, asthma, and other specified conditions.2
Negative ion generators operate by releasing negatively charged ions into the airstream. These ions attach to dust particles, giving them a negative charge. The charged particles are then attracted to other surfaces in the home, such as walls, ceilings, and furniture, which have a more positive charge.
Although negative ion generators remove particles from the airstream, the particles are collected on room surfaces and can give them a dirty appearance. Some models include a filter to capture the charged particles before they can attach to room surfaces. Over time, the particles can lose their charge and be released back into the air. There is little scientific evidence to support the effectiveness of negative ion generators in removing air contaminants.
HEPA (high-efficiency particulate air) filters can achieve maximum efficiencies. Despite their tendency to restrict air flow, they are used when clean rooms are needed for either extremely sensitive individuals or in commercial settings. The whole-house filter shown here contains both a HEPA filter and a bank of adsorption filters, and can be used either with a forced-air heating/cooling system, or as a stand-alone unit.
Protect the Clean Air Duct-sealing crews regularly tell horror stories about expensive air cleaning systems that cause more problems than they correct, because dust and other pollutants are drawn into the home from poorly sealed installations. Since filters or air cleaners are installed in the return ductwork of a forced-air system, it is critical that all seams between their housings and the ductwork be airtight. Even tiny unsealed seams can draw in pollutants. To ensure an airtight installation, seal permanent connections with duct mastic. Use UL-181 metal tape to seal over loose-fitting filter openings and other areas that require access for maintenance. The tape can be cut to provide access, and it should be replaced if it dries out and no longer seals adequately.
Along with sealing the ductwork and building envelope, efforts should be made to reduce the sources of indoor pollutants. Paint, solvents, adhesives, and household cleaning solutions all contain potential indoor air pollutants. Reduction or removal of the sources is the sensible first step in cleaning household air.
Ironically, putting in a good quality air filter can actually reduce indoor air quality. Sloppy installation in the return can add a postfilter hole for air to enter the HVAC system, often from a pollutant-heavy area such as a basement or attic. Here, an inspector uses a smoke test to demonstrate the air leakage around an electronic air cleaner.
Notes 1. Boeniger, Mark F., Use of Ozone Generating Devices to Improve Indoor Air Quality, In American Industrial Hygienist Association Journal (June 1995): 596.
2. Decision and Order for complaint by United States of America Before Federal Trade Commission, in the matter of Alpine Industries, Inc., and Living Air Corp., corporations, and William J. Converse, individually and as an officer of Alpine Industries Inc. and Living Air Corp. Docket #C-3614, Federal Trade Commission, Sept 22, 1995.
Resources IAQ INFO (Indoor Air Quality Information Hotline), U.S. Environmental Protection Agency, P.O. Box 37133, Washington, DC 20013-7133. Tel:(800) 438-4318; (301)585-9020; Fax: (301) 588-3408.
Bower, John. Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems. Bloomington, Indiana: The Healthy House Institute, 1995.
Liddament, Martin W. A Guide to Energy Efficient Ventilation, Coventry, Great Britain, Air Infiltraton and Ventilation Centre, 1996.
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