Preventing Humidifier White Dust And The Importance Of Distilled Water
A deeper look into the emissions of ultrasonic humidifiers with 3 different water hardness.
White dust is a widely discussed subject in humidification. People have concerns about inhaling whatever their humidifier is emitting into the air, especially when it comes to children. A common solution to mitigate this problem is to use distilled water in your humidifier. After testing three different water types (distilled, tap, and hardened) to operate the humidifier, we found that the type of water you use clearly impacts the production of white dust.
The Harder The Water You Use, The Dustier Your Environment Will Get
Only two types of humidifiers usually produce white dust: ultrasonic and impeller. Impeller humidifiers are now practically obsolete, so we focused on ultrasonic humidifiers.
Since ultrasonic humidifiers emit small droplets of liquid water, whatever is in the water will be carried out and added into the air. This means that the quality of your air will be directly impacted by the content of the water used in your ultrasonic humidifier.
All tap water has some dissolved minerals in it, mainly calcium and magnesium. Calcium carbonate is measured to determine the water's "hardness." You would typically see water hardness of less than 200 ppm (parts per million) in your tap water.
| Hardness (ppm) | <60 | >60 and <120 | >120 and <180 | >180 |
| Soft | Moderately hard | Hard | Very hard |
We measured our tap water (in Montreal) to be around 120 ppm, which is considered moderately hard. However, this might vary significantly based on where you live. The United States has several natural water sources with water hardness reported to exceed 1000 ppm. That's almost 10x the hardness of our tap water. Intrigued, we decided to see what would happen if we used water with that degree of hardness in an ultrasonic humidifier. We dissolved calcium chloride into our tap water until we measured a hardness of around 1000 ppm. Note that we used calcium chloride instead of calcium carbonate because the latter is very difficult to dissolve in water, whereas calcium chloride is much more water-soluble.
We ran the same ultrasonic humidifier (Levoit OasisMist 450S) in the same conditions, only varying the water used in the unit. Then, we monitored the white dust production at PM1.0, PM2.5, and PM10 (Particulate Matter) levels during each run. We used the Sensirion SEN55 PM sensor to monitor the white dust concentrations at around four feet from the humidifier.
When we used distilled water, the PM readings went up slightly, peaking at around 4 µg/m3 after 35 minutes. This isn't a significant increase and wouldn't be noticeable in everyday life.
However, when we used our tap water, the PM readings exceeded 50 µg/m3 after 35 minutes. This is a noticeable and significant increase in PM numbers. 50 µg/m3 is almost twice as much as measured in a Canadian home with smokers. However, these numbers might not be considered hazardous compared to our last test with the hardest water in the world.
The graph shows that after 35 minutes, our PM sensor was saturated and couldn't report any higher readings. The white dust production was so high that after only one minute, the PM readings exceeded 50 µg/m3, which is close to the number we got after 35 minutes of runtime using our tap water. This amount of dust in the air is simply hazardous and should be avoided at all costs.
Evaporative And Steam Humidifiers Are Inherently Safer
Water contains much more than just dissolved minerals. It contains bacteria, viruses, and fungi (or mold). These bacteria, viruses, and mold are usually harmless when ingested, which is why we don't get sick when we drink potable water. However, they could become pathogenic if inhaled.
If you leave the water in your humidifier long enough without cleaning, mold and bacteria will start forming inside the tank. The pink slime you might see is usually colonies of bacteria multiplying in your damp and warm humidifier. This is why it is very important to clean your humidifier regularly in accordance with the manufacturer's recommendations.
We tried to measure the amount of bacteria that could be emitted into the air when using ultrasonic humidifiers. However, this requires very specialized equipment to incubate the bacteria in Petri dishes and then analyze whether it's actually the same bacteria that was in the tank. As such, we decided not to perform this test.
Steam humidifiers usually don't suffer from that effect thanks to their design, which boils water and, in turn, sterilizes it.
Evaporative humidifiers also have the advantage of emitting almost nothing but water vapor. When we ran several of the evaporative humidifiers we tested using tap water, we observed no mentionable increase in PM numbers whatsoever. This means that even if bacteria were to multiply significantly in your humidifier, they would most likely not be emitted into the air. Nonetheless, you should still clean it and change the wicks regularly.
Some manufacturers seem to be actively trying to find a way to keep bacterial and fungal growth under control in their non-steam humidifiers. The Dyson AM10, the Sejoy ASN-13, and the Vicks V3900 use UVC rays to sterilize or neutralize bacteria, viruses, and fungi. The Objecto H4, on the other hand, uses a small heating element to heat the water up to 60°C (140°F) to sterilize it, making it more power-hungry and less efficient than other ultrasonic humidifiers.
Conclusion And Recommendations
Regularly cleaning your humidifier is very important to control bacterial and mold growth and avoid any health risks, especially when using ultrasonic humidifiers. If a constant supply of distilled water isn't feasible, an evaporative humidifier can be a viable option. However, evaporative humidifiers are usually noisier than their ultrasonic counterparts. If low operating noise is your top priority, use distilled water in your ultrasonic humidifier to avoid white dust in your air or on your furniture.