Clearing the Air

By: Max Alexander, This Old House magazine (Page 3 of 7)

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Breathe deep! Indoor air is full of microscopic nasties like these (magnified 90 times) that can trigger allergies and respiratory problems.

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.

Photo: Darrin Haddad

Flat Filters: change forced-air furnace filters once a month

Photo: Darrin Haddad

Extended Media Filters: These 8-inch-thick filters require a pro to plumb them into the ductwork.

Photo: Darrin Haddad

Electronic Filters: These work paticularly well on smoke particles and never need replacing, though they must be scrubbed every few months.

Photo: Darrin Haddad

Ultraviolet Filters zap airborne bacteria and viruses

Photo: Darrin Haddad

Portable Room Air Filters: These work well in houses with no central air or heating

Photo: Susan McWhinney

HEPA Filter

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Portable Room Air Filters

If your house has no central air-conditioning or heating, portable room air filters are the most practical choice. (Freestanding whole-house units exist, but they typically require attic installation and ducting to individual rooms.) Most portables employ highly effective HEPA filters, which are not generally used in whole-house systems because they need more-powerful fans than furnaces can provide . Some portable units, called ion air cleaners, use electrostatic precipitator technology. Ion units, which don't require fans, are typically quieter than HEPA models and cost less to operate because there are no filters to replace. But these units may produce trace amounts of the lung irritant ozone as a by-product of the ionization process

Portable units range in price from $150 to as much as $1,500, and there are ongoing costs. HEPA filters, which must be replaced annually, cost from $40 to more than $100. Portables can also be noisy, because it takes a lot of wind to push air through such a fine filter. Some manufacturers are addressing the noise issue with so-called smart filters that employ an optical sensor to judge when the air is relatively clean, then switch to a lower fan setting. Another strategy is to run a portable filter on high during the day, then turn it to low at night.

Portables powered by fans are rated by "clean-air delivery rate" (CADR), which measures both air movement and gunk-trapping effectiveness. It's important to buy a filter that's big enough. Manufacturers recommend that the CADR be at least two-thirds the room's area in square feet — so a 15-by-20-foot room (300 square feet) would need a filter with a CADR rating of 200. (The calculation assumes 8-foot ceilings.)