When sunlight streams in through a window, do you see a galaxy of dust particles in the air? Believe it or not, even on the sunniest day, most dust isn’t visible. In fact, every cubic foot of air in the average home is loaded with about 3 million dust particles.

"Dust" is a catchall term that describes the minute residue sloughed off by a house and its occupants. It comes from fireplace, cigarette and cooking smoke; skin and pet dander; molds, mildew and fungi; pollen and plant spores; and a variety of other sources. Of course, dust is most visible where it settles and collects...on furniture or—worse—the hidden crannies and top shelves visited by a dust cloth only once each spring.

But dust isn’t just an embarrassment waiting to be discovered by your mother-in-law. It can be a serious pollutant, particularly for allergy sufferers, asthmatics, people with bronchial problems and those who are hyper-sensitive to airborne particulates. For everyone, breathing air laden with bacteria and other contaminants is less than ideal. Dust is, after all, pollution, and the Environmental Protection Agency estimates that indoor air is two to five times as polluted as outdoor air.

Though noxious odors and gasses are often best eliminated by ventilation, dust, dander, spores, smoke and other particulates are most effectively removed with an air cleaner. Air cleaners are made as self-contained, tabletop or room-size appliances that serve small areas or single rooms. Whole-house filters attach onto a forced-air furnace and clean the air that’s cycled through the system by the furnace’s blower.

Types of Air Cleaners

Broadly, air cleaners work by mechanical filtration, electrical attraction or a combination of the two techniques. Mechanical or "media" filters strain dust from the air as a fan propels the air through the filter membrane. Electrical-attraction cleaners use a principle similar to "static cling"—they charge particles in the air or a collection device in the filter (or both) to create a magnetic attraction between particles and filter.

Most tabletop and room air cleaners actually 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 gasses and a post-filter for any remaining particles.

How Effective Are They?

An air cleaner’s effectiveness is measured by how thoroughly it captures particles and the amount of air it can process in a given period of time. It’s only as good as its ability to catch particles of all sizes and process enough air to make a difference.

To this end, the finer the sieve-like action of a mechanical filter, the smaller the particles it can block. However, the finer the sieve, the more quickly the filter will clog up with dust. Though most mechanical filters actually catch more dust as they become more saturated, airflow grows increasingly stymied, reducing overall effectiveness. The best mechanical filters have a very fine sieve and a large surface area to maximize both dust collection and airflow.

Filters that use electrical attraction to grab particles are different. Most don’t slow air as they "fill up," but their charge is diminished by the accumulation of particles. They’re most effective when they’re clean and actually when the air passes through them at a relatively slow rate.

Dust particles are measured by the micron (one micron equals 1/25,400 of an inch). The human eye can see lint, pollen and dust particles larger than 10 microns (a sharp pencil dot is about 200 microns in diameter). Most air cleaners eliminate a large percentage of visible dust that passes through them, but to be really effective, an air cleaner should eliminate most microscopic particles, too.

The standard of effectiveness for eliminating tiny particles was long ago set by the HEPA filter (see below), which is tested at its ability to eliminate particles that average .3 micron in size. This is the particle size most likely to be inhaled and deposited in the lungs and the most difficult for mechanical filters to catch. More recently, the ULPA filter (see below) was developed and is capable of removing particles as small as .1 micron.

The effectiveness of a filter is tested and given ASHRAE (American Society of Heating, Refrigeration and Air-conditioning Engineers) ratings. The first rating, Initial Staining Dirt Efficiency, is a critical measurement of how well the filter removes microscopic staining particles such as tobacco tar, allergens and grease. The second rating, Average Arrestance Test, refers to the percentage of ordinary dust, hair, lint and other large-particulate matter removed. When comparing ratings, it’s important to know which of these two is being listed.

The Association of Home Appliance Manufacturers (AHAM) certifies some portable air cleaners with a Clean Air Delivery Rate (CADR). This measures how many cubic feet per minute (CFM) of air are cleared of a certain size particle. Three particle sizes are rated: smoke (minute), dust (medium) and pollen (large). The higher the CADR rating, the less time the unit needs to remove a given quantity of certain particles from a specified room size. AHAM-certified models are marked with CADR data.

Buying Considerations

Whether to choose a whole-house or self-contained model depends on the type of heating system in your home. If you have a forced-air system, you should buy a unit that attaches to it since the effectiveness of a portable model will quickly be defeated by the system’s circulation of unfiltered air throughout the house.

When you shop, be sure you’re comparing apples to apples. With a tabletop or room air cleaner, pay attention to how many CFM (cubic feet per minute) of air move through the unit. For each 250 square feet of space, figure you’ll need about 100 CFM. Room models can handle up to about 400 CFM and tabletop models top out at about 175 CFM.

Also be sure the model you choose isn’t too noisy or drafty in its output and consider one that has intake and output on opposite sides of the unit for best efficiency. Check replacement filter prices, life and availability and be sure filters are easy to access.

If the unit contains a HEPA filter, find out how big it is. Obviously, a 12-by-16-by-6-inch filter will be much more effective (and costly) than an 8-by-8-by-6-inch filter.

Prices for room models run from about $180 to $1,000, depending on the quality of the mechanics and filtration; tabletop units cost from $60 to $180.

Whole-house units work silently, efficiently and clean a great deal of air—about 1000 CFM (cubic feet per minute). On the downside, they’re relatively expensive, require installation by a contractor, and unless you leave the fan on, they only move air when the furnace or air conditioner kicks on.

Whichever type you purchase, if you choose a quality product and maintain it properly, you’re sure to see a marked improvement in the quality of your home’s air.

How Air Cleaners Work

Get Top Performing Real Estate Agents