If indoor air is dry, you can add moisture using a portable humidifier. Choose from two types of portable cool-mist humidifiers: ultrasonic and evaporative. Each has significant differences in how they produce moisture. Read on to understand their unique qualities and to help you decide on the right one for you.

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Ultrasonic vs. Evaporative Humidifiers

Both ultrasonic and evaporative humidifiers serve the same function—to increase room humidity levels by adding water vapor to the air. Ultrasonic humidifiers do this by means of vibrations create by two ceramic plates, while evaporative humidifiers use a simple fan to speed up the natural process of water evaporating from a wick. Choosing between a design will depend on what features are more important to you.

Key Features

Ultrasonic Humidifier

An ultrasonic humidifier uses high-frequency sound vibrations to produce an extra-fine water mist that is then expelled to add moisture to the room. It's the quieter of the two types of humidifiers. The ultrasonic generally has no filter factored into its design, which saves operation costs. This type of humidifier is considered safer since there is no hot water present in the unit and therefore no risk of scalding. However, as with any humidifier that does not boil the water to produce a mist, there is a risk of bacteria being present in it and being discharged to the room. Therefore, disinfecting the humidifier becomes even more important than with a warm mist humidifier that does boil the water. 

Evaporative Humidifier

Evaporative humidifiers have been in use for decades and are considered the most common humidifying appliance. The most basic way to humidify a room is to place a pot or jar of water in the room and allow the moisture to naturally evaporate into the air to relieve dryness. An evaporative humidifier works on basically the same principle, but it uses a fan to speed up the evaporation of water and increase humidity in a room. The built-in fan draws in air that blows through a moist wick filter at the bottom of the humidifier. As the water evaporates into vapor, it's pushed out as water mist or spray into the air to increase humidity.

Appearance

Ultrasonic Humidifier

The most familiar portable ultrasonic humidifier is in the shape of a teardrop with the mist coming out from the top. There are numerous portable ultrasonic humidifier designs, including some models that have fillable tanks and others that require that you insert a water bottle. There are even mini ultrasonic humidifiers that you can place directly into a glass of water as its moisture source. Many ultrasonic humidifiers offer various additional features, such as aromatherapy, USB power, color-changing LED night lights, LED display pads, and built-in clocks.

If you're interested in a fun and decorative addition to your room, opt for an ultrasonic humidifier. Keep in mind, however, that its LED lights may keep you awake if the appliance is placed in a bedroom.

Evaporative Humidifier

Portable evaporative humidifiers are a bit bulkier in appearance because they need room for filters, fans, and fillable tanks. A wick filter can be quite large and takes up most of the space inside of a humidifier's casing. The type of filter used varies considerably depending on the appliance's design. With most, the filter will require routine cleaning or replacement. Some evaporative models are also designed to provide aromatherapy during the humidification process. Very few models in this category are non-filter units that simply churn out a water mist or vapor.

Humidifier Size

Both types of portable humidifiers come as compact, tabletop, or freestanding tower floor models that are on wheels for easy mobility. What matters is choosing the correct size of humidifier for your room. Humidifiers have ratings that show what square footage they'll cover within a 24-hour period. For example, a personal compact humidifier can humidify 25 square feet (best for a desk area), but a model rated for 100 square feet would be best for a small bedroom.

If the humidifier is too large and powerful for the space, condensation will begin to form on the windows, which can result in the growth of mold, mildew, and bacteria. The bottom line: Know the square footage of the room in order to choose a humidifier that is the right size.

Whichever type you choose to buy, always monitor your humidity levels. Ideally, humidity levels in a room should be between 30 percent and 50 percent. Some humidifiers have a built-in gauge that measures the room's humidity level. Otherwise, you can test it with an inexpensive device called a hygrometer.

Care and Cleaning

Ultrasonic Humidifier

Ultrasonic humidifiers tend to produce fine grayish dust that can settle around the room, since they have no filter to absorb minerals in the water. Some models, however, come with ceramic-type cartridges that can be refreshed, and these are effective to some degree.

Ordinary tap water often contains a notable mineral content, and with no onboard filters, ultrasonic humidifiers may turn this mineral content into a fine gray or white dust that goes into the air along with the water vapor. This mineral dust lands on furniture and electronics, and it is almost impossible to avoid. The amount of dust will depend on the source of your drinking water. Hard water is notably higher in dissolved minerals that will create dust when processed through an ultrasonic humidifier. Using distilled or filtered reverse osmosis water will reduce the amount of dust you need to clean, though this can be a pricey option. If this is a major annoyance for you, consider buying a unit with a decalcification filter.

Evaporative Humidifier

The evaporative humidifier is a popular design because of the filter's function which traps mineral scale and reduces the risk of fine white/gray scale dust filtering to the room. Mineral scale dust is usually barely noticeable.

Noise

Ultrasonic Humidifier

The most common places for a portable humidifier are the bedroom or the main living space, so it's no wonder that most consumers want a humidifier with quiet operation. If humidifier noise is your main concern, an ultrasonic humidifier is your best choice. An ultrasonic humidifier does not have a fan to add to the noise of the appliance. You should recognize, though, that no humidifier is entirely silent.

Evaporative Humidifier

If noise concerns you less than expense, choose a more affordable evaporative humidifier. Sound levels vary according to design, but all evaporative humidifiers will produce a notable amount of noise due to the internal fan that runs more or less constantly.

Operation

Set up for both an ultrasonic and evaporative humidifier is relatively simple. Add water to the reservoir, position the appliance 1 to 3 feet away from walls to avoid moisture damage, plug it in, and adjust your settings. The only difference you'll have with an evaporative humidifier is the simple installation of a new filter. In terms of operation, there is no advantage of one type of humidifier over the other.

Cost

Ultrasonic Humidifier

When the technology was still new, ultrasonic humidifiers typically were considerably more expensive than evaporative humidifiers. However, with increased popularity has come increased supply, and today's ultrasonic humidifiers are only slightly more expensive than the older evaporative designs. Highly rated units designed for a moderately large room (500 square feet) will start at around $75 and get more expensive from there.

Evaporative Humidifier

Once costing considerably less than ultrasonic humidifiers, evaporative humidifiers today are only a little less costly than ultrasonics. A good unit designed for a moderately large room (500 square feet) will start at about $50. However, when you consider the cost of replacement filters, the cost advantage of an evaporative humidifier becomes less relevant.

Lifespan

Both ultrasonic and evaporative portable humidifiers will last two to five years if they are properly maintained. Even the best-maintained humidifiers are prone to the build-up of mineral deposits over time that can be hard to remove, and that will indicate the need for a new appliance.

The Verdict

If you loathe changing or cleaning a humidifier filter or you simply want a quieter humidifier, chose an ultrasonic filter-free model. With the increased popularity of this design, you now have many, many options when it comes to ultrasonic humidifiers. But you should expect to combat dust in the room more frequently.

For slightly lower pricing, choose an evaporative humidifier, but understand that filters need periodic changing or cleaning to keep them running efficiently. Locate filter replacements before buying a humidifier and always have a spare filter on hand.

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Use and Care of Home Humidifiers

A PDF version of this publication is available:  Use and Care of Home Humidifiers (pdf)

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On this page:

• Introduction
• Types of Humidifiers and Associated Pollutants
• Recommendations for Use and Care
• Can I Use Tap Water in My Ultrasonic or Impeller Humidifier?
• Additional Sources of Information

On other pages:

• Biological Pollutants’ Impact on Indoor Air Quality
• Indoor Particulate Matter
• The Indoor Microbiome

Introduction

Humidifiers are commonly used in homes to relieve the physical discomforts of dry nose, throat, lips, and skin. The moisture they add to dry air also helps alleviate common nuisances brought on by winter heating, such as static electricity, peeling wallpaper, and cracks in paint and furniture. However, excess moisture can encourage the growth of biological organisms in the home. These organisms include dust mites, which are microscopic animals that produce materials causing allergic reactions to household dust, and molds.

Several studies have shown that ultrasonic and impeller (or "cool mist") humidifiers can disperse materials, such as microorganisms and minerals, from their water tanks into indoor air. Proper care and cleaning of ultrasonic and impeller humidifiers are important for reducing potential exposures to microorganisms, such as bacteria and molds. Microorganisms often grow in humidifiers which are equipped with tanks containing standing water. Breathing mist containing these pollutants has been implicated as causing a certain type of inflammation of the lungs.

Using water with lower mineral content will reduce exposures to these materials (see Can I Use Tap Water in My Ultrasonic or Impeller Humidifier?).

The young, the elderly, and those people with lung diseases or respiratory allergies may be particularly susceptible to certain types of airborne pollutants. However, if you follow the recommendations for the use and care of home humidifiers provided in this fact sheet, the potential for dispersal of microorganisms and minerals from your humidifier should be reduced.

Types of Humidifiers and Associated Pollutants

Console humidifiers are encased in cabinets which are designed for floor use.

Portable humidifiers are smaller and more readily moved.

Central humidifiers are built into heating and air conditioning systems, and humidify the whole house.

The two types of humidifiers which generally appear to produce the greatest dispersions of both microorganisms and minerals are:

• Ultrasonic, which create a cool mist by means of ultrasonic sound vibrations.
• Impeller, or "cool mist," which produces a cool mist by means of a high-speed rotating disk.

Two additional types of humidifiers can allow for growth of microorganisms if they are equipped with a tank that holds standing water, but generally disperse less, if any, of these pollutants into the air. These are:

• Evaporative, which transmit moisture into the air invisibly by using a fan to blow air through a moistened absorbent material, such as a belt, wick, or filter.
• Steam vaporizer, which create steam by heating water with an electrical heating element or electrodes. "Warm mist" humidifiers are a type of steam vaporizer humidifier in which the steam is cooled before exiting the machine.

Note: Steam vaporizer and evaporative humidifiers are not expected to disperse substantial amounts of minerals.

Recommendations for Use and Care

It is important to use a humidifier only when conditions require it, to use the correct moisture setting for existing conditions, and to clean it thoroughly.

The possible health effects resulting from the dispersal of microorganisms and minerals by home humidifiers are not fully understood. Meanwhile, it may be prudent to reduce the potential for personal exposures to these materials by taking the following precautions, particularly when using ultrasonic and impeller humidifiers.

• Empty the tank, wipe all surfaces dry, and refill the water in portable humidifiers daily to reduce any growth of microorganisms; follow the manufacturer's instructions for changing water in console humidifiers. Be sure you unplug the unit from the electrical socket first.
• Use water with low mineral content to prevent the build-up of scale and the dispersal of minerals into the air. See Can I Use Tap Water in My Ultrasonic or Impeller Humidifier? for information on using water with low mineral content.
• Clean portable humidifiers every third day. Empty the tank and use a brush or other scrubber to clean it. Remove any scale, deposits, or film that has formed on the sides of the tank or on interior surfaces, and wipe all surfaces dry. Again, be sure you unplug the unit.

Follow the manufacturer's suggestions on the use of cleaning products or disinfectants. In the absence of specific recommendations, clean all surfaces coming in contact with water with a 3% solution of hydrogen peroxide. If you use any cleaning or disinfecting agent, rinse the tank thoroughly with several changes of tap water to prevent dispersal of chemicals into the air during use.

• Follow the manufacturer's directions on cleaning and maintaining console and central (furnace mounted) humidifiers. In particular, if the humidifier contains a tank, do not allow water to stand in the tank for extended periods of time, and keep the water clean.
• Keep steam vaporizer humidifiers out of the reach of children. Steam and boiling water may cause burns.
• Do not humidify to indoor relative humidity levels exceeding 50 percent. Higher humidity levels may encourage the growth of biological organisms in the home. Hygrometers, available at local hardware stores, may be used to measure humidity levels. Some humidifiers contain a built-in humidistat which may be adjusted to the proper moisture level. If water condenses on windows, walls, or pictures, relocate the humidifier, lower its humidistat setting, or reduce its use.
• Do not permit the area around the humidifier to become damp or wet. If dampness occurs, turn the output volume of the humidifier down. If the humidifier output volume cannot be turned down, use the humidifier intermittently. Do not allow absorbent materials, such as carpeting, drapes, or tablecloths, to become damp.
• Follow the manufacturer's instructions regarding the use, maintenance, and replacement of any materials supplied with the humidifier. Use appropriate materials as recommended by the product manufacturer.
• Clean the humidifier, as directed, at the end of the humidifying season or when the product will not be in frequent use. Before storage, make sure all the parts are dry. Dispose of all used demineralization cartridges, cassettes, or filters. Store the unit in a dry location. After storage, clean the unit again and remove any dust on the outside.
• Stop using your humidifier and contact your physician if you have respiratory symptoms which you believe are associated with periods of use of your home humidifier, even if you are following maintenance directions.

Can I Use Tap Water in My Ultrasonic or Impeller Humidifier?

Researchers have documented that ultrasonic or impeller humidifiers are very efficient at dispersing minerals in tap water into the air. In addition, some consumers are bothered by a "white dust" that may appear on surfaces during use of these devices. Most importantly, minerals in tap water may increase the development of crusty deposits, or scale, in humidifiers. Scale can be a breeding ground for microorganisms. Retarding the growth of scale is the most compelling reason to find alternatives to tap water. For this reason, or if white dust is a problem or you wish to minimize your exposure to minerals in the tap water as a matter of prudence, you should either:

1. Use bottled water labeled "distilled." While distilled water still contains some mineral content, it will likely contain lower mineral content than most tap water. Distillation is the most effective method for removing minerals from water.

Two additional demineralization processes, deionization and reverse osmosis, remove most of the minerals from water, but are generally less effective than distillation. Water demineralized by these two processes would, on the average, be expected to contain a higher mineral content than distilled waters. "Purified" water may be produced by any of these three or other similar processes.

Be aware, however, that not all bottled water is produced using demineralization processes. Bottled waters labeled "spring", "artesian" or "mineral" have not been treated to remove mineral content.

2. Consider using demineralization cartridges, cassettes, or filters if supplied or recommended for use with your humidifier.

Be aware, however, that the ability of these devices to remove minerals may vary widely. Further research is needed to determine how well, and how long, these devices work. Watch for the appearance of "white dust," which would indicate that minerals are not being removed.

Also, in areas of the country where the mineral content in the tap water is high, using distilled water may be less expensive than cartridges, cassettes, or filters.

Additional Sources of Information

For additional information on home humidifiers and other consumer products, contact the U.S. Consumer Product Safety Commission (CPSC) or see the CPSC Safety Alert Dirty Humidifiers May Cause Health Problems.