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The spectrum of air filters and cleaners is differentiated in a number of ways. Here’s a closer look at the main types:
HEPA, which stands for High Efficiency Particulate Accumulation, is a highly effective filter originally designed for removing radioactive particles from air. Today HEPA filters are used in military, industrial and scientific applications and in certain portable and room-size residential home air filters. Made from finely woven glass particles formed into a pleated filter that has a very large surface area, a HEPA filter—by definition—removes 99.97% of all .3 micron particles from the air.
Unfortunately, high replacement cost is a real disadvantage to HEPA filters—they generally must be replaced once every year to 18 months, depending upon use, and they typically cost from $70 to $125. And because the filters are so dense, they require a strong blower to move air through, racking up higher energy costs. The larger the surface area of a HEPA, the better—so find out its square footage before buying one.
ULPA, which stands for Ultra Low Penetrating Air, is a filter developed for use in hospitals and manufacturing "clean rooms" such as those employed in the semiconductor industry. Now available in self-contained home air cleaners, the filter is an intercut web of micro-fibers that traps 99.99% of .1 micron particles from the air.
"HEPA-type" filters, made for both self-contained units and forced-air systems, have not met the rigorous standards of a genuine HEPA, but some, like the pleated types, are almost as effective. Most range from 25 percent to 95 percent effective at microscopic particle removal. Before buying one, be sure you know how good it is at blocking respirable-size particles. With continuous use, HEPA-type filters must be replaced every year at a cost of from $30 to $45.
Though ordinary 99-cent furnace filters catch about 75 percent of bulk dirt, they capture very little—often less than 3 percent—of the minute pollutants. Slightly more effective for a forced-air system are treated filters. These disposable products, which typically sell for under $10, have been coated with dirt-trapping glycol or mineral oil. Though they stop about the same percentage of bulk dirt, they block up to about 10 percent of the smaller, microscopic particles.
An electret or electrostatic filter uses synthetic fibers that create a static charge to attract particles. Costing from $15 to $100, electrostatic filters come plain or pleated, disposable or reusable. Though you just hose out reusable models to clean them, they still have to be replaced periodically. Disposable models should be replaced quarterly. Depending on the make, an electrostatic filter blocks between 10 percent and 35 percent of microscopic particles. Some are sold as kits; the frame is permanent, the filter pad is replaced every couple of years.
An electrostatic precipitating ("electronic") air cleaner draws particles in by fan and charges them with a series of high-voltage wires. A precipitating cell (a series of plates) that carries the opposite electrical charge attracts the particles as they pass by. You can buy portable versions or whole-house models that connect to the cold-air return plenum on the furnace. They are quite effective, removing about 95 percent of bulk dirt and 85% percent of microscopic particles. Whole-house electronic air cleaners, sold through heating contractors, cost $300 to $900 installed, depending on the complexity of the installation.
WHOLE-HOUSE AIR CLEANER
 
A negative-ion generator uses tiny, charged wires or needles to create ions (gas molecules with a negative charge) that latch onto airborne particles. A fan in the unit distributes the ions back into the room, but they tend to stick to walls, floors, tables and other surfaces, and may stain them.
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