According to manufacturers, 8 million or so humidifiers are sold in the United States every year. In Canada, it has been reported that nearly half of all households use a humidifier, perhaps owing to our harsher winters that bring about uncomfortable dry air. Asking ourselves if these portable humidifiers really do work and which type is best is important, but we should not neglect another crucial question: are they safe?
The Goldilocks zone for humidity
The purpose of using a portable humidifier is to increase the humidity in a room. Humidity is the concentration of water in the air. A certain amount of water in its gas (or vapour) phase is present in the air and this amount decreases as the temperature goes down, so winter air can be much less humid than summer air. Likewise, if the temperature of the air falls down to something called the dew point, there will be more water condensing than evaporating and we will start to see condensation on windows and dew drops on the ground.
Humidity can be measured in different ways. There is the actual amount of water vapour, known as absolute humidity. What we are more familiar with, thanks to weather reports, is relative humidity. To explain the difference between these two concepts, imagine 100 people inside a theatre that has 100 seats. In absolute numbers, there are 100 people. In relative numbers, the theatre has reached capacity: it has a relative occupancy of 100%. Now move those 100 people to a large sports stadium. There are still, in absolute numbers, 100 people, but they may only occupy 0.1% of the seats. Humidity works the same way. Imagine a room is cold and there is as much water vapour as there can be (relative humidity of 100%). As you increase the temperature, the amount of water that can exist in the vapour phase increases (like going from a small theatre to a giant stadium) and the relative humidity goes down in percentage points. Same total amount, different percentage because capacity has increased.
A frequent question we have received over the years at our office is why two humidity monitors give very different readings, and that often is because some monitors measure absolute humidity whereas others measure relative humidity. It’s a little bit like comparing two tape measures: one in metres and one in feet. Humidity sensors that measure relative humidity contain a film that attracts moisture from the air which creates a voltage difference or an electrical resistance change that can be measured; monitors that measure absolute humidity compare the resistance in a circuit fed by two probes, one in contact with ambient air and one inside a small chamber filled with nitrogen.
Humidity extremes can be beneficial to microorganisms and viruses. For example, low humidity is thought to help spread enveloped viruses (like the coronavirus and the influenza virus) by stripping respiratory droplets of their water content, leaving tiny aerosolized viral particles in the air. This is thought to play a role in why flu cases are more common in the winter, although this is a complicated and partially understood phenomenon. Meanwhile, mites and fungi thrive at high humidities. All this has led experts to declare a relative humidity of 40 to 60% as the Goldilocks zone: it’s comfortable for humans and not terribly good for the microorganisms we don’t like.
When it comes to health claims made in support of using a humidifier, especially in the winter months, we see many assertions but little evidence on the ground. Humidifiers may help with dry skin “but the literature is inconclusive.” They may help prevent the spread of flus and colds (especially in indoor spaces with lots of people, I presume, not so much in your apartment if you live alone), but no evidence they help once you’ve caught the common cold. As for asthma and allergies, Dr. Ceppie Merry looked at the studies done on this for the website Healthy But Smart and concluded humidifiers did not seem to help and might even increase the risk of allergy, asthma and eczema, although the quality of the evidence was again not great.
In researching this topic, I read that humidifiers don’t even increase the humidity of a room. This claim is based on a single study of 34 New York City apartments in which participants were asked if they owned a portable humidifier, not if they used it or how often they turned it on. Recent experiments clearly demonstrate that humidifiers do significantly increase humidity in a room, and my own personal experience with a humidifier and a hygrometer confirms this.
So a humidifier of the right size will increase the humidity in a room or house, which will typically increase comfort. But not all humidifiers are created equal and their impact on air quality is a rarely mentioned point of concern.
A breath of not-so-fresh air
There are a few types of portable humidifiers. They all work differently to turn liquid water into a vapour or a mist so that it can hang in the air and increase humidity.
Warm mist humidifiers simply boil liquid water in a reservoir to turn it into water vapour the way a tea kettle would. One concern has to do with young children: they could injure themselves by getting directly in front of a very warm jet of water vapour. However, you may think bacteria are not an issue since this type of humidifier boils water, but you would be wrong. Consumer Reports tested a number of warm mist humidifiers and when bacteria were present in the tank (either from normal growth after three days or because they were purposefully added in the tank), they found that almost all of these humidifiers emitted bacteria into the air. Labels such as “germ-free mist” or “antimicrobial material” offered no such protection. These bacteria may not always cause disease but could lead to an exacerbation of a pre-existing allergy or asthma.
There is a type of humidifier that did not emit bacteria in Consumer Reports’ testing: wick-based humidifiers. These machines have a fan that blows liquid water onto a thick paper wick that acts as a filter and that turns the water into a fine mist. They can be noisy and the wick filter has to be replaced every few months because it can become moldy. A similar type of cool mist humidifier uses an impeller (a rotor) to blow water out into tiny droplets that hang in the air.
The third type is perhaps the most worrisome. Ultrasonic humidifiers have become more popular in recent years. They generate a cool mist, so no risk of burns. They use little electricity and they tend to be quiet. They humidify the air by having a small vibrating bit in the water tank, the pulsation of which turns liquid water into vapour. It sounds like the perfect humidifier: quiet, safe, and efficient.
If you own one of those, you may have noticed a white mist on the surrounding furniture and wondered, “Am I breathing this?” Even though Consumer Reports does endorse ultrasonic humidifiers, some of their recommended models were recently tested by scientists and found to emit a large amount of very small particulate matter. The reason is that tap water contains common minerals and ions like calcium, magnesium, and sulphate, and less common substances like lead, arsenic, manganese, and copper. These minerals get aerosolized by ultrasonic humidifiers and we breathe them in. A warm mist humidifier, because it boils the water and leaves the minerals as a deposit inside the machine, does not emit anywhere near the amount of particulate matter as ultrasonic models, according to one recent study. As for impeller humidifiers, the one study I could find was published in 1988 by the Environmental Protection Agency and reveals the models they tested generated one-third the mass of particles that the ultrasonic humidifiers did. Less, but not nothing.
How much particulate matter is generated by an ultrasonic humidifier? A team in Virginia tested Consumer Reports’ “Best Buy” ultrasonic humidifier with different types of water and tested the air quality in a dormitory room over the course of eight hours. Water that was not ultrapure water from a laboratory but that contained some amount of minerals resulted in particulate matter concentrations comparable to what is measured during cooking.
Meanwhile, at the University of Alberta, researchers compared a warm mist humidifier to two identical ultrasonic humidifiers (“major brands” with “excellent customer reviews”) charged with tap water, filtered water, distilled water, or laboratory-grade ultrapure water. Scientists tend to use fairly neutral words in their papers, but the results of this experiment led these researchers to report “astonishingly high concentrations of indoor particulate matter” when using an ultrasonic humidifier with filtered or unfiltered tap water. What they measured is typically only seen outdoors “during extreme air pollution events in major metropolises.”
Before newspaper editors have a chance to craft headlines like “Why your humidifier is slowly killing you,” let me add a caveat. Those University of Alberta scientists were quick to point out that the particles they were detecting were minerals commonly considered to be harmless to humans... but the net impact of breathing in large concentrations of these particles over long periods of time remains unknown.
A clear way to avoid the generation of this white dust that may or may not pose a health risk is to use distilled water (water that has been boiled to leave most impurities behind), but a back-of-the-envelope calculation reveals it’s not a simple solution (unless your humidifier accepts demineralization cartridges). My own ultrasonic humidifier has a 3.5-litre tank. It uses up the whole tank in one night at close to the maximum setting, and that’s to humidify a bedroom with the door closed. This means I would need to buy 24.5 litres of distilled water every week. The cost of this measure is not negligible (roughly ten dollars a week) and we have to take into account the large number of plastic containers that have to be purchased, transported every week, and disposed of. You could distill your own water by evaporating tap water on the stove and getting it to condense, free of its minerals, on a bed of ice, but the process is very, very slow. The setup described in this CNet article would take 12 to 13 hours to yield enough distilled water to run your humidifier overnight.
It is also important to regularly clean a humidifier, but if harsh cleaners are used for this task, residues could also end up in the air we breathe. A particularly severe example of this was identified in South Korea in 2011 in what has become known as the “humidifier disinfectant disaster.” Harsh humidifier cleaners containing chemicals like polyhexamethylene guanidine were sold seasonally for home use to prevent the growth of bacteria. These chemicals were turned into aerosols by the humidifier, which led to many people developing a new type of lung disease. Some required lung transplantation and others died. These disinfectants were subsequently banned from being sold in Korea. Using vinegar to clean the base and a mild bleach solution (one tablespoon per gallon of water) to disinfect the tank, or just plain dish soap, should not cause problems.
Many government health agencies are reluctant to recommend the use of portable humidifiers. Public Health Ontario published an evidence brief on their use in a healthcare setting which points out issues with the dissemination of allergens and microorganisms, and the Government of Canada lists a number of potential health risks with humidifiers that don’t boil water. Portable humidifiers need to be diligently disinfected on a regular basis with mild solutions. Even then, ultrasonic humidifiers are known to emit large quantities of minerals in the air when tap water is used, which represents an unknown long-term health risk. Would pairing the humidifier with an air purifier solve the problem? Maybe, but it will need to be properly tested by researchers. One top-of-the-line humidifier has an integrated HEPA filter and an automated cleaning mode using ultraviolet light, but with $1,000 price tag, it’s out of the reach of most people even if it were demonstrated to solve the issue of particulate emissions.
Given how long portable humidifiers have been on the market, there is a remarkable paucity of studies on their risks. We keep a close eye on outdoor air quality. Maybe it’s time we turned that critical eye inwards.
Take-home message:
- Warm mist humidifiers boil water into a vapour and, while they don’t release minerals in the air we breathe, they can easily spread bacteria in the air if they are not regularly and diligently cleaned.
- Cool mist humidifiers that use a rotor to add a fine water mist to the air did not emit bacteria in a Consumer Reports test, but too few publicly available studies exist to know how many fine particles they may emit in the air.
- Ultrasonic humidifiers can emit large quantities of fine particles thought to be safe for humans, but no long-term studies have been done to test this (one solution is to use distilled water in the reservoir).
