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Adding humidity to the air during the winter months can improve the air quality in your home, and it can have significant positive effects on your family’s health. Portable, single-room humidifiers are by far the most popular option for home humidification, accounting for more than eight million installations annually in the U.S., as of 2011, according to the EPA.

Portable humidifiers come in two types, evaporative and ultrasonic. While both types add moisture to the air in your home, they do so in very different ways. As a result, each type of humidifier has its own drawbacks and benefits. This article will take a close look at how ultrasonic humidifiers function, compare them to their evaporative cousins, and determine which type of humidifier is the right choice for you.

What is an ultrasonic humidifier?

An ultrasonic humidifier is a very simple device. It includes a reservoir of water and a diaphragm or other type of vibrating element. The diaphragm vibrates at an extremely high frequency, so high that it is above the range of human hearing (which is why they’re called ultrasonic humidifiers). The vibrations propel microscopic water droplets into the air. Once in the air, the droplets evaporate, adding humidity to the air in the room. There is no heating of the water at any point, so these humidifiers are sometimes known as “cool mist” humidifiers. However, note that many evaporative humidifiers also accomplish evaporation without heating and are also referred to as “cool mist” humidifiers in marketing materials. The important terms to differentiate humidifiers are ultrasonic and evaporative.

Ultrasonic vs. evaporative humidifiers

How is an evaporative humidifier different from an ultrasonic one? To understand, first we need to explain the difference between water vapor in the air, and water droplets in the air. When water evaporates, it changes from its liquid form to its gas form. As a gas, water molecules are dispersed in the air along with the molecules of the other gases that make up air. Water droplets, on the other hand, are tiny blobs of liquid water that are light enough to be suspended in the air, at least temporarily. The difference is important for humidifiers because water droplets can carry substances that were present in the water with them, while water vapor can not.

As mentioned before, ultrasonic humidifiers use vibrations to send water droplets into the air. Evaporative humidifiers, however, evaporate water inside the humidifier and send water vapor into the air. Some accomplish this by heating the water into steam. These are known as steam humidifiers, or as “warm mist” humidifiers if the vapor is cooled before being emitted. Other evaporative humidifiers wick the water through a filter of some kind, then blow cool air across the filter with a fan. This evaporates the water and sends water vapor into the room.

Whichever method an evaporative humidifier uses, the important thing is that the water is turned to water vapor inside the humidifier, leaving any substances that were in the water behind. We will discuss why this can be both good and bad in more detail.

The benefits of ultrasonic humidifiers

Ultrasonic humidifiers offer a few advantages over evaporative humidifiers.

  • They are very good at humidifying the air. In fact, they can add so much moisture to the air that this advantage becomes a drawback, resulting in over-humidified air that causes dampness and mold. This is why it is necessary for an ultrasonic humidifier to have a humidistat that can detect humidity levels and shut off the humidifier when the air has enough moisture in it.
  • Ultrasonic humidifiers can be made extremely small and compact because the humidifying mechanism is so simple. The limiting factor for how small an ultrasonic dehumidifier can get is basically the water reservoir. So they are the right choice if you need to fit your humidifier in a very tight space.
  • The vibration of an ultrasonic humidifier is much quieter than the fan of an evaporative humidifier. However, ultrasonic humidifiers do emit an audible hum, especially noticeable in a quiet bedroom.
  • Ultrasonic humidifiers do not generate any heat, making them safer than steam humidifiers that boil water to produce water vapor. This could be especially important if the humidifier is used in a child’s room. However, many evaporative humidifiers also function without any heat and without boiling water.
  • You never have to replace a wick or filter on an ultrasonic humidifier. The wick/filter in an evaporative humidifier eventually collects a build-up of minerals left behind by the water, especially if hard water (water with a lot of minerals dissolved in it) is used. These filters must be replaced regularly, adding effort and operational cost.
  • Ultrasonic humidifiers consume very little electricity.

Drawbacks of ultrasonic humidifiers

All the drawbacks of ultrasonic humidifiers result from the fact that the humidifier releases water droplets into the air, not water vapor. This causes several problems.

First, while most of the water droplets will eventually evaporate into water vapor, some of them will settle out of the air and onto nearby surfaces before they have a chance to evaporate. This will be more of a problem if the air in the room is cold or if the air already has a high moisture content. The result is a damp area or even a puddle of water near the humidifier. This is why ultrasonic humidifiers need to be placed on a shelf or dresser instead of on the floor, so the water droplets have more time to evaporate before settling to the floor.

A much bigger problem is the fact that any substances in the water will be propelled into the air along with the water droplets. These substances will be either minerals or biological contaminants.

Minerals in the air from ultrasonic humidifiers

Minerals are present in all tap water, and if you know you have hard water in your area, your water has more minerals than most. These trace amounts of minerals are carried into the air along with the water droplets when an ultrasonic humidifier is in operation (in an evaporative humidifier, the minerals are left behind). At the very least, these minerals will settle out of the air and leave a fine white dust on surfaces near the humidifier. At worst, anyone in the room can inhale these minerals.

Because of the concern that people can inhale the aerosolized particles in the white dust, studies have examined the effects on the lung tissue of mice using tap water in ultrasonic humidifiers (Umezawa et al., 2013). Though the aerosol particles did not cause severe inflammation of the lungs in mice, the study authors concluded that demineralized water should be used instead of tap water to reduce any risk.

The EPA also recommends using distilled water instead of tap water in ultrasonic humidifiers. “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.”

Risk of microbial exposure

One of the dangers of ultrasonic humidifiers is an increased risk of microbial exposures. If the water in an ultrasonic humidifier, or the humidifier itself, is contaminated with bacteria or mold, it will emit those substances, exposing anyone in the room to them. This can result in allergic reactions or the spreading of disease, sometimes known as “humidifier fever.” This problem is not unique to ultrasonic humidifiers. All humidifiers have the potential to become breeding grounds for bacteria or mold. However, the EPA has found that ultrasonic humidifiers tend to disperse more biological agents than evaporative humidifiers.

There is one potential problem with ultrasonic humidifiers not related to water. While the vibrations they generate may be too high frequency for you to hear, they might be within the hearing range of your dog. There have not been any studies on the effects of ultrasonic humidifiers on dogs, but if you plan to use one in a room with dogs, the sound may be painful or uncomfortable for them. Watch your dogs’ behavior when the humidifier is running, and consider using an evaporative humidifier instead if it seems to bother them.

Ultrasonic humidifier concerns for kids and babies

Many people choose to use a humidifier in a baby’s room or kid’s room, especially when they have respiratory infections. However, there is no solid evidence that humidifiers can relieve symptoms for young children, though it can make them feel better. There was one case of accidental inhalation of mineral dust from an ultrasonic humidifier in a young infant, though the family used distilled water in the device (Daftary & Deterding, 2011). The infant recovered, and doctors who treated him though it may have been the chemicals found in the white dust. The study investigator documented this case because he wanted to draw attention to the fact that the lungs of babies and small children may be more sensitive to the chemicals produced by an ultrasonic humidifier.

All in all, millions of parents do purchase humidifiers for their sick children and the risk of this type of illness is probably very low. The EPA has not found any adverse effects for using humidifiers in general, though the agency recommends regular cleaning and using distilled water for ultrasonic humidifiers.

How to clean an ultrasonic humidifier

The solution to reducing risks for the types of problems mentioned above is regular and thorough cleaning and disinfecting of the humidifier. The EPA offers a simple guideline: “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.” Manufacturer recommendations should be followed when using cleaning agents, but a 3% solution of hydrogen peroxide is the EPA recommended disinfectant.

Choosing the right ultrasonic humidifier

If you decide to use an ultrasonic humidifier, which one you choose will depend on the room you need to humidify and any additional features you want.

Very few portable, single-room humidifiers have output ratings, mainly because these can vary tremendously based on air pressure, temperature, initial humidity levels and other factors. A more important factor when selecting a humidifier is the size of the reservoir and how often it will need to be refilled. If you are humidifying a large room that gets very dry, you will want a humidifier with a larger reservoir, especially if you will use it in a bedroom. You do not want to have to get up in the middle of the night to refill your humidifier. If your room is very small, you can use a humidifier with a small reservoir.

One very useful feature is a humidistat, which will automatically shut off the humidifier if the humidity levels get too high. This is very important for preventing excessively humid air that can lead to dampness or mold growth. Automatic shut-off when the reservoir runs low is another useful feature. Some ultrasonic humidifiers can be used with aromatherapy oils, so look for that feature if it is something you will want.

Maintain acceptable humidity levels for good air quality

Maintaining proper humidity levels in a dry climate or during winter is an important element of good indoor air quality. Excessively low humidity can lead to dry, scratchy throats, coughing and dry skin. Wood floors and furniture can be damaged by low humidity. And studies have found that low humidity actually increases the spread of bacteria (Arundel et al., 1986).

When using a humidifier, it is important to reduce the risk of introducing air pollutants like mold and bacteria in the air. It is also very important that relative humidity levels stay within the ideal range of 30-50% as recommended by the EPA, or at the very least, below 60%. This is to prevent excess mold growth and dust mites in your home. An additional way to maintain air quality is to use an air purifier. The Molekule air purifier is different from traditional air purifiers that simply trap pollutants like mold onto a filter surface; the PECO technology can destroy airborne mold and prevent any potential re-release of microbes into the air.

Portable humidifiers are a good option for humidifying specific rooms in your house, and ultrasonic humidifiers do offer some advantages over evaporative humidifiers. Regular cleaning is important no matter what type of humidifier you decide to use, especially to maintain good air quality in your home.

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