The Ultrasonic Humidifier: worth a second thought for indoor air quality

The Ultrasonic Humidifier: worth a second thought for indoor air quality

Aaron Whittemore, Water INTERface Science Communicator

We are all aware of outdoor air pollution and how it can be detrimental to our health. Industry, transportation, and even wildfires can deteriorate air quality and create health risks. However, indoor air pollution can be just as detrimental, and many people may even be unknowingly compromising the air within their own homes.

Particulate matter, a mixture of solid particles and liquid droplets that exist in air, is a harmful agent within air pollution. Particulate matter particles are extremely small and when inhaled can cause a range of health impacts including irritating and compromising the respiratory system. Particulate matter comes from a variety of sources, but within homes an unexpected, but common source is our drinking water. Particles within water can transfer back in forth with the air across a figurative boundary scientists call the “air-water interface.” A fairly common way for drinking water particles to cross this boundary is through the use of ultrasonic humidifiers.

Ultrasonic humidifiers have an internal device that vibrates and creates extremely fine liquid droplets that are ejected into the air as a cool mist. These fine droplets help increase humidity, but Wenchuo Yao, a VT Water INTERface researcher and PhD student in the Civil and Environmental Engineering department, conducted research that found ultrasonic humidifiers can substantially increase the levels of particulate matter within the indoor environment. Yao ran several different experiments that used different types of drinking water to fill ultrasonic humidifiers and then measured indoor air quality. The results were striking. When high quality tap water was used as ultrasonic humidifier fill water, the particulate matter in the air exceeded ambient air standards set by the EPA. Air quality became even worse when using tap water with high levels of minerals. Yao used a model to estimate how this degraded air quality might impact human health, finding that many small and potentially dangerous particles would be inhaled and settle within human’s lungs. Further, inhalation risk was greater for young children, which could potentially impair lung development.

Due to these risks, it is not advisable to use tap water to fill ultrasonic humidifiers. It is much safer to use distilled water that does not contain high amounts of minerals and thus will not transfer as many small particles to the air. Another alternative is to use steam humidifiers, which do not release small particles in large numbers, though the steam they emit can be hot and pose a risk to young children.

Andrea Dietrich, a Water INTERface Interdisciplinary Graduate Education Program (IGEP) professor from the department of Civil and Environmental Engineering, was pleased with the outcomes of this interdisciplinary research, which has the potential to improve human health. “The world has all kinds of issues, but luckily we have all kinds of scientists who, when they put their heads together, can tackle those important to improving health and wellbeing,” said Dietrich.

Also see:

Yao, W., Gallagher, D.L., Dietrich, A.M.  An overlooked route of inhalation exposure to tap water constituents for children and adults: aerosolized aqueous minerals from ultrasonic humidifiers. Water Research X, 2020. 9, 1 December 2020,

Yao, W., Dal Porto, R., Gallagher, D.L., Dietrich, A.M.  Human exposure to particles at the air-water interface: influence of water quality on indoor air quality from use of ultrasonic humidifiers. Environment International, 143, 105902;

Yao, W., Marr, L., Gallagher, D.  Dietrich, A.M.  Emission of potential inhalable insoluble mineral particles from ultrasonic humidifiers. Water Research, 64 (14899):1-9, 2019.


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