Professor Caleb Finch, “There is no safe level of particulates”.
Controlling humidity in a home is a continuous and challenging battle with many factors determining the outcome: climate and weather outside the home, occupancy, activities, and conditioning equipment types inside; with infiltration and ventilation bridging the indoors to the outdoors. A balanced climate, like here in Illinois, will force occupants to deal with both spectrums, from hot and humid to cold and dry and everything in between. It is a tricky task for occupants and mechanical equipment alike.
We have written extensively about moisture removal and its challenges in our Handling Humidity Series, but adding humidity to a home has its own concerns. Primary among them is not creating issues while trying to solve a “dry” home problem. Mold and condensation, which impact occupant health and deteriorate buildings, are the most common concerns when trying to increase humidity levels in a home during cold months. These issues are associated with improper control of humidification, but another relatively unknown issue can occur even with proper humidity management; particulates.
In this article, we discuss humidifiers and how some humidifiers can create a “snowstorm” of particulates in your home. In 2021, Virginia Tech published an article discussing humidifiers and their association with high particulate levels, and our testing confirms these findings and aligns with our recommendations to use steam humidifiers or distilled water.
Our 2017 article (The Perfect Dust Storm) describes how indoor and outdoor particulates impact us. Sometime between 1970 and 1980, homebuilders sealed houses to a level in which indoor particulates became more prominent than outdoor particulates. The quadrupling rise of asthma in the US, as noted in our article, strongly correlates with the progression of house-sealing. And now, sadly, we live with increasing frequency of wildfires and airborne infections. This makes the reduction of any source of indoor particulates so important.
We conducted experiments in a home using a “cool mist” (atomizing) humidifier and a steam humidifier to show the differences between these two common methods for humidifying a home in the winter. The data in this article comes from a conventional “newer” home built in 1996 with central HVAC gas furnace and a split system AC, located in the basement, providing comfort conditioning. MERV11 filters are used in the central HVAC system. A CERV2 unit supplies fresh, MERV13 filtered air to the central HVAC unit’s return duct with the CERV2 return pulling air from the basement. There are two floor levels above the basement with the second floor having the bedroom areas where the humidifiers were located during the tests. Figure 1 shows the humidifiers on table tops within the two bedrooms.
Cool mist and steam humidifiers operate differently. Cool mist dehumidifiers produce droplets by mechanically agitating and breaking liquid, including minerals and microbes, into small droplets. The liquid droplets are vaporized by heat from ambient air in the room. Heat taken from ambient air cools a room. Energy from room air vaporizing water droplets comes from the home’s heating system. Minerals and microbes from the water reservoir are left suspended in the air as the droplets evaporate.
Steam humidifiers have electrodes in the water reservoir to directly heat and boil water, releasing steam into room air. A room with a steam humidifier will feel warmer because no room air cooling occurs. Minerals and microbes are left behind in the steam humidifier water reservoir.
We collected three 4-day sequences of particulate data for the house using CERV-ICE (CERV-Intelligently Controlled Environment) dashboard. Particulate sensors were installed in the return and fresh air streams to measure the indoor as well as the outdoor particulates. Both particulate mass and particulate count were recorded. If you are a CERV owner using CERV-ICE, this data dashboard will look familiar to you, with an exception (Hint: what you see is related to a big announcement coming next month).
Cool Mist Humidifier – Tap Water
Figures 2 and 3 below show the first four-day test period using a cool mist humidifier with regular tap water. During nighttime, particulates rapidly filled the entire house. The indoor particulate sensor is located in the CERV2’s return air, just above the return air filter. The particulate mass and particulate count readings are both high. Particulate mass, “PM”, exceeds Build Equinox’s healthy indoor air quality standard (PM10 < 10µg/m3) each night when the cool mist humidifier is used. Particulate count, less familiar than particulate mass but equally important, is less than Build Equinox’s healthy indoor air quality standard (PC0.3 < 40,000#/liter).
This is where things get interesting…if you’re somewhat nerdy like us. Particulate mass exceeding our recommended particulate limit (PM10 < 10µg/m3) with particulate count less than our recommended limit (PC0.3 < 40,000#/liter) indicates particulates are relatively “big” (greater than 1µm). Road dust, cooking (eg, pan frying) particulates, and the particulate “snowstorm” created by this cool mist humidifier. Our April 2023 newsletter discussion of particulate mass and particulate count explains more about small and large particulates.
Smoke, respiratory generated aerosols, and microbes tend to be smaller, more numerous particulates. For example, 37,000 particulates with 0.3µm diameter have the same mass as a single 10µm particulate. At 10µg/m3, a liter would contain 19 of the 10µm particulates or 710,000 of the 0.3µm particulates. The bigger particulates lodge in your upper respiratory system where coughing, sneezing and mucous pumping cilia move larger particulates to your mouth where you swallow it, cough, or spit. Smaller particulates move into your lower respiratory system where microbes may launch an attack, and other fine particulates move across the alveoli blood membrane barrier to be distributed to every organ in your body.
Cool Mist Humidifier – Distilled Water
So, what if we remove the components from the water that create the unwanted particulates? Using distilled water in the cool mist humidifier eliminates nearly all minerals and other contaminants that are left behind when the water is vaporized. The set of data from January 20-23, 2025 has the cool mist humidifier operating at night using distilled water. Figures 4 and 5 below show particulate mass and particulate count data for the home with distilled water cool mist humidification. No particulates are generated by this humidification as salts and minerals were removed from the water in distillation processes. This is a real improvement over the tap water cool mist humidifier and one that can result in big health implications. The lone uptick in high particulates was the result of a brief cooking event.
Steam Humidifier – Tap Water
The set of data from January 13-19, 2025 has the steam humidifier operating at night. Figures 6 and 7 below show particulate mass and particulate count data for the home with steam humidification. No particulates are generated by steam humidification, like the cool mist distilled humidifier, as steam generation leaves salts and minerals in the water as in distillation processes.
The particulate snowstorm created by the cool mist humidifier using tap water is a real problem one should avoid. If you recall from figure 2 house air is seen to have 20,000 particulates per liter in the air during nighttime. Adults breathing 8 liters per minute will inhale more than 75,000,000 particulates during 8 hours of nighttime humidifier operation. These particulates, even if composed of relatively benign substances, can become “fomite aerosols” or carriers of microbes as some of them are inhaled and then exhaled with infectious microbes from our lungs. Comfort and air quality in homes is complex and trying to remedy one variable can often be unintentionally detrimental to another, so it is exceptionally important to continue to learn, educate, and adapt as new information becomes available.
