Dust can be removed from air using any of several technologies. Most involve cycling the air through a filter that traps or attracts dust particles to withdraw them from the air. In actuality, most tabletop and room air cleaners send air through a gauntlet of filters: a foam pre-filter to screen out large particles, the primary filter or electrical-attraction device to catch smaller particles, a carbon filter to remove odors and some gases, and a post-filter for any remaining particles.
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, filters are highly effective. These filters were originally designed for removing radioactive particles from air. Today, HEPA filters are in certain portable and room-sized 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, cost is a disadvantage of HEPA filters—they generally must be replaced once every year to 18 months, depending upon the extent of 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, filters were 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.
“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% to 95% 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 $30 to $45.
Though ordinary 99-cent furnace filters catch about 75% of bulk dirt, they capture very little— often less than 3%—of minute pollutants. Slightly more effective for a forced-air system are treated filters. These disposable products, which typically sell for under $10, are coated with dirt-trapping glycol or mineral oil. Though they stop about the same percentage of bulk dirt, they block up to about 10% of the smaller, microscopic particles.
An electret, or electrostatic air 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. You just hose down reusable models to clean them, but they still have to be replaced periodically. Disposable models should be replaced quarterly. Depending on the make, an electrostatic filter blocks between 10% and 35% of microscopic particles. Some are sold as kits; the frame is permanent and the filter pad is replaced every couple of years.
A negative-ion generator, or ionizer purifier, uses tiny charged wires or needles to create ions (gas molecules with a negative charge) that latch onto airborne particles. Some types have a fan in the unit that speeds and improves the cleaning and distribution of air. These are noisier but more effective than other types that don’t have a fan. All produce trace amounts of ozone and oxidants. With this type of air cleaner, airborne particles tend to stick to walls, floors, tables, and other surfaces near the unit.
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% of bulk dirt and 85% of microscopic particles. Whole-house electronic air cleaners, sold through heating contractors, cost $300–$900 installed, depending on the complexity of the installation.
Ozone generators produce significant amounts of ozone—the main ingredient in air pollution. They have come under heavy scrutiny for the serious health risks they present. In 2007, the state of California banned their sale. Beware of buying or using a room air purifier that produces ozone.