When sunlight streams in through a window, do you see a galaxy of dust particles in the air? "Dust" is a catchall term that describes the minute residue sloughed off by a house and its occupants. Of course, dust is, after all, pollution, and the Environmental Protection Agency estimates that indoor air is two to five times as polluted as outdoor air.

This type of pollution 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 is not 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.

Visible dust is about 10 microns in diameter. Respirable dust–the type that can lodge in your lungs–is more commonly about .3 micron. (For comparison, a sharp pencil dot is about 200 microns in diameter.)

Noxious odors and gasses are often best eliminated by ventilation, but dust, dander, spores, smoke, and other particulates are most effectively removed with an air cleaner. 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.

The best types of filters are called ULPA and HEPA filters. If the unit contains one of these filters, 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. 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.