extended media filter, whole-house filter
Photo: Keller & Keller
Whole-house filters, like the extended media filter shown here, must be professionally installed in return-air ductwork.
Whole-House Filters

The most efficient way to filter household air is through your home's forced-air heating or central air-conditioning system. The filters are built into the return-air ductwork, trapping particles as air passes through. Such systems are passive; as long as the fan is running, they are constantly filtering all the air in your house. Whole-house filters come in four main types.

furnace filters
Photo: Darrin Haddad
Flat Filters: change forced-air furnace filters once a month
Flat filters
If you have a forced-air furnace, you've already got a rudimentary air-filtration system: That matted-fiberglass filter that should be changed once a month. "You can't change it often enough," says This Old House plumbing and heating expert Richard Trethewey. When it clogs with dust, it stops working and overworks the furnace. In fact, those filters are designed to protect your furnace from large particles of dust, and while they might keep surfaces in your house a bit cleaner, they won't block the microscopic particles that are most irritating to lung tissue. Pleated filters, which pack more material in the same amount of space, cost a few bucks more and do a slightly better job. By far the best pleated filters are electrostatically charged to attract allergens like pollen and pet dander. They cost around $15 and should be changed every two to three months.

Extended Media Filters
Photo: Darrin Haddad
Extended Media Filters: These 8-inch-thick filters require a pro to plumb them into the ductwork.
Extended media filters
Picture a stack of furnace filters about 8 inches thick and you get the idea of an extended media filter. These boxy units contain an accordionlike pile of filtration media, which makes them more effective than regular fiberglass filters. They require professional installation because the large filter holder must be plumbed into the ductwork. The price, including installation, ranges from $400 to $600; you'll need to replace the $40-to-$60 filter every year.

Electronic Filters
Photo: Darrin Haddad
Electronic Filters: These work paticularly well on smoke particles and never need replacing, though they must be scrubbed every few months.
Electronic filters
These high-tech units, sometimes called electrostatic precipitators, are also incorporated into the ductwork. As air passes through, a high-voltage current puts an electrical charge on particles. At the other end of the unit, oppositely charged collector plates grab the particles like a magnet. Electronic filters work especially well on smoke particles too small to be trapped in media filters. One independent test found such filters worked about 30 times as well as regular fiberglass filters. (There is no industry yardstick for measuring the effectiveness of whole-house units, because performance is affected by a home's blower and ductwork.) Unlike media filters, electronic filters never need replacing, but the aluminum collector plates must be cleaned in soapy water every few months. The process of charging particles, called ionization, may produce trace amounts of ozone, a lung irritant (see "Eye on Ions," page 66). Electronic filters cost $600 to $1,000 installed and require a 120-volt electrical outlet.

Ultraviolet filter
Photo: Darrin Haddad
Ultraviolet Filters zap airborne bacteria and viruses
Ultraviolet Filters
People worried primarily about germs can consider an ultraviolet filter. Typically, UV filters are built-in components, sold as add-ons to a whole-house electronic precipitator (as in, add on $400 to $800). The ultraviolet light zaps airborne bacteria and viruses into oblivion, which is why hospitals use UV air filters in tuberculosis wards. Of course, the bug has to reach the filter before it can be zapped; if someone sneezes in your face, UV technology won't help.

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