During the winter months, many people combat dry indoor air with the help of a humidifier. Though there are many different kinds, ultrasonic humidifiers are one of the most popular devices that add moisture to the air by using tiny sound vibrations to convert water into a cool mist. In dry weather, ultrasonic humidifiers can help with dry skin, dry noses and coughs.
Juan was one new father who had been running an ultrasonic humidifier in his son’s bedroom for a couple years, using tap water. The device he used was advertised as safe with tap water, but when he decided to use a continuous air quality monitor in his son’s room as well, he was shocked to see that the level of fine particulate matter in the bedroom was off the charts. He reported very high indoor PM 2.5 concentrations (close to 200 µg/m³), similar to the particulate levels you would find when wildfire smoke hits the area. Juan also learned that the mist from the ultrasonic humidifier did not stay just in his son’s bedroom, but rather the ultrafine mist spread throughout the entire house.
Juan replaced the tap water in the ultrasonic humidifier with distilled water, and the particulate matter readings dropped drastically. But it brought up an important point: humidifiers may aid comfort, but they can be harmful if not used correctly.
How Humidifiers Work
When a furnace is running, humidity levels can drop, making the air in the room too dry. This can lead to a sore throat, dry nasal passages and dry skin. Using a humidifier can increase moisture in the air and make it easier to breathe and sleep at night. Ultrasonic humidifiers are a popular option for people to use in bedrooms because they are silent, energy-efficient and filter-free. But several studies have found that the mist that is propelled into the air can disperse materials, like microorganisms and minerals from their water tanks into the air, posing a potential health risk.
A Danger to Your Health
When an ultrasonic humidifier uses regular tap water, it can turn minerals and other tiny substances in the water into very small airborne particles that you can breathe deep into your lungs. These particles can include heavy metals and dissolved minerals that are so small that they can reach the parts of the lungs where oxygen passes into the blood (called alveoli). Over time, breathing in these particles can irritate the respiratory system, exacerbate asthma, and in some rare cases cause hypersensitivity pneumonitis (inflammation, fluid buildup, and possible scarring of lung tissue). In fact, studies have shown that the number of tiny particles released into the air when tap water is used is similar to outdoor air pollution levels.
Another health issue that occurs when using ultrasonic humidifiers comes from leaving water sitting in the reservoir. If not cleaned regularly, this can become a place where bacteria and mold grow. When that contaminated water is used, the organisms in it are released into the air as part of the mist. Breathing these contaminated particles can contribute to lung irritation or respiratory problems. When cleaning, you need to do so carefully as chemicals that might be used to clean the humidifier can also become a part of that mist.
Additionally, if the humidifier makes the air too humid (over about 50%), it can also encourage the growth of dust mites and mold in the room. These can make allergies and asthma worse and may trigger flare-ups. In general, children, older adults, and people with asthma or other lung problems may have stronger reactions to particles, microorganisms, or high humidity levels in the air.
How to Use Humidifiers Safely
Ultrasonic humidifiers are just one of many different types of humidifiers that are available for home use. Before purchasing anything, be sure to do some research and talk to your healthcare provider if you are unsure. No matter what you use, be sure to monitor humidity levels and follow a strict cleaning and maintenance schedule.
Ways to reduce health risks from ultrasonic humidifiers include:
- Use distilled or demineralized water instead of tap water. This lowers the number of particles released into the air.
- Clean the humidifier often. Unplug the tank so that you can empty and rinse the tank daily. Disinfect the humidifier regularly to remove any scale, deposits or film from the tank and other parts. Be careful to use natural products like vinegar instead of bleach or other chemicals that could be released into the air upon next usage.
- Clean and dry the unit before storing it for the season or whenever the humidifier is not in use.
- Monitor indoor humidity levels to keep humidity between 30% and 50%.
No matter what humidifier you use, it is important to never add essential oils, fragrances or use them in an enclosed space.
Learn more about how to keep your indoor air clean.
Blog last updated: March 4, 2026
