Kill your ultrasonic humidifier. I know this seems ridiculous, but the case is very simple:
- Breathing tiny airborne particles causes massive health problems.
- Ultrasonic humidifiers create lots of particles.
- While it’s uncertain if the particular particles from ultrasonic humidifiers are harmful, the risk is insane compared to the benefit.
That’s it. The current science says there’s a real chance that using an ultrasonic humidifier could cut years from your life. It’s not a sure thing, but it’s a real risk. Use an evaporative or steam humidifier instead.
Particles cause serious health problems
The evidence for this is strong. Outdoor particle levels vary throughout the world and statistics link these levels to health problems including heart disease, strokes, kidney disease, respiratory infections, and lung cancer. The mechanism appears to be that tiny particles make their way through the lungs into the bloodstream and thus other parts of the body. These foreign particles then activate the immune system causing all sorts of havoc.
A good heuristic is that that an increase of 33.3 PM2.5 μg/m³ costs around 1 disability adjusted life year. Correia et al. estimated something close to this from different counties in the US, and more recent data from many different countries confirms this. The most polluted cities in the world have levels around 100 PM2.5 μg/m³.
Ultrasonic humidifiers create tons of particles
Ultrasonic humidifiers create tiny particles of water, including any dissolved solids. The water quickly evaporates, leaving the solids in the air. Tap water contains lots of minerals, but even if you use distilled or deionized water, you still have the problem that humidifiers are warm moist environments. Bacteria grow quickly in them, again ending up as tiny particles.
Here’s a test I did, running a humidifier for one minute near a particle counter.
Ultrasonic humidifiers can easily push levels to several hundred μg/m³. If you used one for 8 hours a night and it created a level of 300 μg/m³, this would raise your average exposure by 100 μg/m³, meaning an expected cost of 3 disability adjusted life years.
You can expand this to read direct quotes from a bunch of papers.
Ultrasonic humidifiers […] create tiny droplets consisting of water and its impurities. Once the water evaporates, the impurities can remain suspended as airborne particles. One of the primary concerns for household humidifiers is their ability to aerosolize pathogenic microbes. […] This white dust was associated with respiratory distress in children if inhaled
Ultrasonic humidifiers were found to have a substantial impact on home indoor concentrations and personal exposures for SO42- as well as PM2.5 , likely as the result of aerosolization of sulfates and other dissolved minerals present in the water. For the two subjects with ultrasonic humidifiers, personal SO42- and PM exposures were approximately two and five times greater, respectively, than corresponding ambient concentrations.
When inhaled during an 8-hr exposure time, and depending on mineral water quality, humidifier aerosols can deposit up to 100 s of μg minerals in the human child respiratory tract and 3–4.5 times more μg of minerals in human adult respiratory tract. […] Distilled water should be used whenever possible to prevent respiratory irritation (USEPA, 1991), although there is little indication that consumers are aware of or follow this consideration.
The ultrasonic humidifier has converted all the non-volatile solutes in tap water into PM. […] notice an unexpectedly high concentration of SO42- in PM […] We have detected a compound with a mass-to-charge ratio equivalent to that of azelaic acid or structure isomers. This result demonstrates the possibility that organic compounds can be associated with humidifier PM. […] Operating a single ultrasonic humidifier with tap water resulted in PM2.5 concentrations up to hundreds of μg/m³.
The concentrations of metals/elements varies based on particle size and particle water content. Nonetheless, the same relative amounts of metals/elements are present in each particle. Thus, inhalation of small particles increased mass exposure to common tap water metals (e.g., calcium, sodium, magnesium), and also associated anions.
Average indoor air particle concentrations were 211 μg/m³-air, based on ICP-MS metals and elements without bicarbonate, which exceed USEPA PM2.5 and PM10 values for ambient air.
Still not convinced? You can also read Wang et al. (2020) or Fleisch et al. (2018) or Baxter et al. (2007) or Umezawa et al. (2013) or Highsmith et al. (1988) or Richard Saint Cyr or Stack Exchange or Awair. It isn’t controversial. This is a thing that happens.
The risk-reward ratio is insane
These calculations are uncertain. Our estimates for the harms particles cause are based on observational statistics, not randomized trials. Also, different types of particles surely cause different amounts of trouble, and we don’t know how bad the kind humidifiers make are. But this uncertainty goes in both directions. The harms could be less than we estimate, or they could be even worse.
You might calculate the odds differently. Say you’re 75% sure that ultrasonic humidifiers are harmless. Still, you still have to ask yourself if using an ultrasonic humidifier is worth the risk. Even a small probability of losing three years is something to take seriously.
Alternatively, you might really hate dry air. The good news is that steam or evaporative humidifiers cost $20-100, humidify the air just as well, and seem to create almost no particles. (See Park et al. (2020) or Paschold et al. (2003) or Highsmith et al. (1988) or the EPA or public health Ontario or the VA’s policy to prohibit ultrasonic humidifiers but allow steam.)
When there’s a possibility of a large harm, you have to be very confident that the danger is not real to justify ignoring the issue.
Using deionized water and keeping the humidifier clean
In principle, this could work. If there are absolutely no dissolved solids in the water, then no solid particles can be produced. This is basically what the EPA recommends: Use distilled water, and completely clean and dry the humidifier after every usage.
I don’t think this is practical. For one thing, note that filtered water isn’t good enough – it still creates lots of particles, although less than tap water. And you’re really going to clean and dry it every time?
More importantly, it’s very hard to avoid bacteria buildup. It happens within days and once bacteria have really taken hold, they’re almost impossible to fully remove (Grieble et al., 1970). The NIH recommends only using an ultrasonic humidifier for three days (with distilled water) and then throwing it away.
Why start with a fundamentally unsafe design, and then go to such lengths to try to make it safe?
The evolutionary argument
Some types of particles are surely more dangerous than others. Maybe we evolved some robustness to particles that are naturally occurring like those from dust or sea spray, since our ancestors have been breathing those long before humans existed.
Maybe, but are you sure? We never breathed particles at the concentrations or for the amount of time that a humidifier next to your bed represents. In any case, the bigger problem is still the insanely imbalanced risk-reward tradeoff. Unless you’re very confident, you can’t ignore the risk.