Dust, hair and microscopic insect bodies are just a few of the particles that swirl through the air inside most homes. Removing the source of these problem particles may not be practical, especially if the source is pollution in the outdoor air. Many of the particles that cause problems are small enough to penetrate deeply into the lungs. Their small size allows them to slip past cilia and mucous intended to protect lungs from invasion.

If you think the standard furnace filter cleans the air, think again. Particles small enough to inhale pass easily through it. To have any effect, you'll have to look at better, and more expensive, filter options.

Air cleaners and filters can treat a whole house when installed in a forced air heating and cooling system. But your filter choice can affect air handling equipment and ductwork. It's important to choose the filter before making a final decision about the forced air system. Here's an introduction to filter selection.

Types of Particle Filters

Furnace filters have a loose pad of coarse glass fibers, lightly coated with an adhesive to help large particles stick. Furnace filters screen out only large particles, and have virtually no effect on airborne materials that cause allergies or sickness.

Passive electrostatic filters have polypropylene fibers that pick up a static charge as the air passes over them. The static charge helps to hold fine particles. These filters can be washed and reused. Passive filters work better than furnace filters and fit into the filter slot of most residential air handlers.

Pleated filters use a finer mesh, which catches smaller particles. Because the finer mesh also creates more resistance to air flow, the surface is "extended" to offer more area for air to pass through. Pleated filters are one to six inches thick. One or two inch filters will fit in the filter rack of many furnaces. Thicker filters are more effective, but require duct modifications.

HEPA is short for High-Efficiency Particulate Air filters. HEPA filters have hundreds of square feet of filter paper folded into a couple of square feet of space. They remove very small particles, including some viruses. Because they are several inches thick, they require duct modifications.

Electronic air cleaners appear in many modern forced air heating systems. Some units also use a panel-type prefilter and a carbon filter. Plates or wires in the unit have an electrical charge. Particles in the air passing over the charged surfaces pick up the charge. Oppositely charged plates further downstream collect the particles. These units generate a small amount of ozone, which may cause health concerns for some people. Carbon filters downstream remove some of the ozone. Electronic air cleaners draw between 20 and 40 watts of power. The unit holding the charged surfaces must be removed and cleaned often to maintain optimum performance.

Judging Filters and Air Cleaners

The results of standard tests can help you pick the right filter for your needs. Filter effectiveness is measured by two tests:

Arrestance tends to measure the filter's ability to remove the largest, heaviest particles. It's useful only for low-efficiency furnace filters. If you're concerned about small particles, especially breathable ones, arrestance doesn't give meaningful results.

Dust Spot Efficiency is a better way to measure removal of fine particles. Use it to compare pleated filters and electronic air cleaners.

Thermal DOP Efficiency applies to the very highest efficiency filters, such as HEPA filters. This is sometimes called "absolute" filtration.

Both arrestance and dust spot efficiency are defined in American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) Standard 52-89. Unfortunately, some manufacturers list filter "efficiency" or even "ASHRAE efficiency" in their literature, when they mean arrestance. Look for explicit mention of "dust spot efficiency." Thermal DOP efficiency is tested according to U.S. Military Standard 282. It's generally applied only to very high efficiency filters, such as the HEPA.

The filter industry divides filters into three groups by dust spot efficiency: Low-efficiency filters are less than 30 percent, with the typical furnace filter below 10 percent. Medium-efficiency are between 30 and 50 percent. High-efficiency are between 50 and 95 percent. "Absolute" filtration is a class by itself where dust spot efficiency is always above 95 percent.

The chart (above) illustrates how the effectiveness of different filters varies depending on particle size. Higher efficiency is usually better, but it can get complicated. Filters with 95 percent arrestance may catch almost all the lint, but few if any of the smaller particles. Many low- and medium-efficiency filters show their lowest efficiency between 0.1 and 0.5 microns. But the efficiency climbs slightly for the smallest particles (those below 0.1 microns). Two filters perform well across the entire range of particle sizes: HEPA filters and electronic air cleaners.

Resistance to Air Flow

Placing any filter in the air stream increases resistance and reduces flow. This is an important consideration for designing a forced air distribution system. Electronic air cleaners offer relatively low resistance. That's one reason for their popularity. Extremely sensitive people may want the higher efficiency or smaller particle removal of a HEPA filter. Unfortunately, the resistance of HEPA filters may require a more powerful blower. HEPA filters seldom make sense for homes, unless the occupants have extreme environmental sensitivity. Whatever filter you choose, be sure to account for the pressure drop of the filter.

The Filter Decision

Before selecting a filter, decide which particles to target. Then choose a filter that will be effective against that size particle. Not everyone needs to remove 100 percent of all airborne particles. The most effective filters tend to cost the most, so your decision should balance the cost against the effectiveness you need.

Most people will benefit by replacing a standard furnace filter with a pleated filter having a dust spot efficiency around 20 percent. These cost less than $10 at many home improvement retailers. The next step up is a medium-efficiency pleated filter that will remove most of the pollen, but still fits in the furnaces filter slot. It won't cost much more than the consumer grade, but you'll have to buy it from a heating contractor or air filter supplier. Higher efficiency pleated filters will probably require duct modifications.

Combustion gases (from a wood stove, gas range or cigarette) will require a filter that removes much smaller particles, such as an electronic air cleaner or HEPA filter. The electronic air cleaner ($600 to $800) can be a good choice. It's effective on a wide range of particle sizes and adds little pressure drop. With frequent cleaning it's around 90 percent efficient. HEPA filters are the most effective, but create significant pressure drop. Initial cost is around $500. Filters should last two to three years, but cost over $100 to replace.