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Air filters can definitely improve the air quality in your house, but the big questions are “How much?” and “Is it enough?” If the air is relatively clean to begin with, a small portable air purifier may be all you need to remove the few pollutants. With very contaminated air and sensitive occupants, a huge, complicated, expensive system may be required. Different types of filters or media are designed to remove different types of pollutants, so it is important that you know what is in the air before selecting a filter. Most indoor air quality experts agree that it is best to clean the air by other means first (for example, eliminating or separating pollutants from the living space, and ventilating), then use filtration if necessary.
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Filtration can be an attractive option for a couple of reasons. First, the outdoor air that is being brought indoors may not be very pure and clean. It could be laden with exhaust gases, pesticides, pollen, or mold spores. Second, if the indoor air is filtered, less ventilation air is needed to flush out the pollutants that are generated indoors. It should be kept in mind that filtration is not a substitute for ventilation because filtration can only clean contaminants out of the air; it cannot provide the oxygen that is depleted as we breathe.
The full range of air pollutants can be placed into two broad categories: particles (more correctly called particulates) and gases. Particulates include mold spores, pollen, asbestos, house dust, dead insect parts, etc. Gases include combustion gases, formaldehyde, and the hundreds of other VOCs floating in the air. Tobacco and wood smoke are composed of both gases and particulates. The type of filter that works well with particulates does nothing to remove gases, and vice versa. To remove both categories of pollutants, you need two different types of filters.
Particulate Filters
The standard 1"-thick furnace filter falls into this category. It is designed to remove particulates from the air. Unfortunately, it doesn’t do a very good job of it. Actually, its primary purpose is not to clean the air for occupants, but rather to help protect the fan motor in the furnace. It is a furnace filter, not a people filter. A motor doesn’t need a very efficient filter. People require more, and fortunately there are filters available that do a much better job: extended-surface filters.
Extended-Surface Filters
With their very fine pores, extended-surface filters can have more resistance to the flow of air than a standard furnace filter. In order to compensate, they are often manufactured in a pleated accordion shape. This configuration results in much more surface area and less overall air resistance. When such a filter starts to get dirty, it actually becomes more efficient because the pores get smaller as they become clogged. But clogged pores also mean increased resistance to air movement. If a filter has too much resistance, the fan won't be able to blow air through it as easily, so it won’t operate as effectively.
An extended-surface filter is usually made of fiberglass or polyester fibers held together with some type of resin. While it is doubtful if any fibers could be released into the air stream, some sensitive individuals are bothered by the slight odor of the resin. This odor can often be eliminated by removing the filter from the cabinet and heating it in a 200 degree oven for a couple hours. When doing this, be sure to have the range hood turned on at high speed and have a kitchen window open. If you are very chemically sensitive, you should not be in the house when this is done, and the oven and kitchen should be aired out thoroughly when the process is complete.
HEPA Filters
HEPA filters are a special type of extended-surface filter. HEPA stands for high-efficiency particulate arrestance. They are sometimes called “absolute filters” because they are 99.97% efficient at removing particles .3 micron and larger. Developed originally for use in nuclear and biological laboratories, they are now available in both portable and central residential equipment.
HEPA filters are more costly than other particulate filters, but they represent a desirable benchmark as far as clean air is concerned. Since their pores are so small, they have considerably more resistance to air flow than other extended-surface filters. As a result, they often require a more powerful fan motor to drive the air through them. However, they will do nothing to remove gases from the air.
Gaseous Filtration
Particulate filters in some ways work much like a coffee strainer—they simply trap the pollutants that are larger than their pore size. Gaseous filtration requires a totally different approach: adsorption. Adsorption is a process by which gas molecules adhere to a solid surface. A crude analogy would be to compare an adsorption filter to flypaper. Once the adsorption filter has captured all the gases it can hold, it must be replaced.
Activated carbon (sometimes called activated charcoal) is one of the most common adsorption materials on the market. It can be made from coal, coconut husks, or a variety of other materials. Activated carbon does an excellent job of removing most gaseous pollutants from the air. Unfortunately, it is less effective at capturing gases with low molecular weights. Formaldehyde, a very common indoor pollutant, is one of the gases that activated carbon doesn’t adsorb particularly well.
To remove formaldehyde from the air, specially treated carbons are readily available. Activated alumina impregnated with potassium permanganate is another type of adsorption material that works well against formaldehyde.
Like extended-surface filters, adsorption filters can have a considerable amount of resistance to air flow. For very polluted air, a lot of adsorption material is necessary; therefore, a great deal of surface area is required for a fan to function properly. In commercial applications, trays filled with the adsorption material are arranged in an accordion shape to provide plenty of surface area. This approach is generally too costly for residential applications, although it can be done.
A more cost-effective approach for using an adsorption filter in a residence is to use less carbon (less air resistance) and replace it more often. As a rule, adsorption filters are quite expensive because of their size and special fan requirements. However, those with less carbon, while not as efficient, are much lower in cost, and within the budgets of many homeowners. Of course, there are some activated carbon filters on the market that have so little carbon in them that they probably are not worth the effort.
In a house with very contaminated air, an adsorption filter of small to moderate size may not be big enough to provide any noticeable improvement in air quality. In a relatively unpolluted house, such a filter can be used to “polish” the air.
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