Who needs a humidifier? Anyone who has uncomfortably dry or itchy eyes, throat, or skin, or whose asthma is a problem indoors during the heating season.

While indoor relative humidity should be 30 to 50 percent, it can drop significantly in winter, since cold air holds less moisture and dries as it's heated.

Humidifiers have improved over earlier models that spewed white dust in our tests. But that doesn't mean they all work equally well. Our tests show that manufacturer claims can be a poor guide to performance. Indeed, several small tabletop models fell well short of their claimed output and may not raise humidity as high as you’d like.

WHAT’S AVAILABLE

Holmes, Honeywell, Hunter, Kaz, and Reli-On (Wal-Mart) are the major brands. Others include Bemis, Bionaire, Emerson, La-lo, Sears, and Sunbeam. Humidifiers come three basic ways:

Tabletop. These cost the least and are fine for one room. Types include evaporative models, which use a fan to blow air over a wet wick, and warm-mist models, which use a heating unit to boil water before cooling the steam. Small tanks need to be refilled frequently, however. Evaporative models are noisy, and warm-mist models cost more to run. Price: $20 to $100.

Console. These models have large, powerful fans that rapidly blow air across water, generating large volumes of moist air. That is why they are better for multiple rooms. Console models are also efficient and can be placed unobtrusively. But all use evaporative technology and are relatively noisy. And the larger the tank, the harder it is to handle. Price: $80 to $140.

In-duct. These whole-house humidifiers are convenient, quiet, and efficient, making them least expensive to operate. Most are evaporative-bypass units, which tap into the air supply and return ducts. Some are warm-mist while others are nebulizers, which use a spray technology. Nebulizers can result in white dust, however. What’s more, in-duct humidifiers require forced-air heat, cost the most and often require professional installation. Price: $100 to $300, plus $100 to $200 to install.

IMPORTANT FEATURES

A good portable model should offer a humidistat and be relatively easy to carry, fill, and clean. The wick should be easy to replace. Also look for easy-to-use controls and tanks that fit beneath faucets. Some portable models can be programmed to turn on automatically. Digital displays are also a plus.

HOW TO CHOOSE

Be sure you’re willing to clean and disinfect a portable air cleaner regularly to prevent mold and mildew. Otherwise, consider an in-duct humidifier that’s plumbed into the water supply and drainpipes, need not be refilled, and has an easy-change filter serviced only once or twice a year. Then keep these shopping tips in mind:

Look for a humidistat. Whether it’s dial or digital, a humidistat controls humidity levels and shuts the unit off when it reaches the level you set. Models without one can let levels get high enough to allow humidity to condense on windows and other cold surfaces. Overhumidification can also cause mold and bacteria. Models that display room humidity levels and settings are best. Also be aware that most portable humidifiers won’t let you set humidity levels below 30 percent, which can cause window condensation when outside temperatures go below 20° F. You should lower humidity levels as the mercury drops outdoors.

Consider noise. Some warm-mist tabletop models make little or no noise beyond mild boiling and hissing sounds. Comparably sized evaporative models emitted 45 to 50 decibels on low settings and more than 50 decibels on high in our tests. For larger areas, consider buying a noisier console model and placing it away from sleeping areas. You’ll spend less than you would on several warm-mist tabletop models, and the water vapor travels quickly enough to benefit remote bedrooms if doors remain open.

Factor in running costs. While in-duct humidifiers are priciest, they cost only about $30 or so per year to run compared with as much as $350 or more for four tabletop models.

Consider your water. Some humidifiers have lower output with hard water and require more frequent maintenance. Nonetheless, you’ll find humidifiers of all types that work well with it.

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