Should You Use a Humidifier to Prevent COVID-19 or Flu?

Airborne virus particles travel well in dry air. Here’s how using a humidifier can help.

Catherine Roberts, Consumer Reports Senior Health & Food Reporter.

By Catherine Roberts
Senior Health & Food Reporter

Published January 10, 2020·Updated March 2, 2022

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After two years of living with COVID-19—a disease caused by the airborne SARS-CoV-2 virus—many of us are paying much closer attention to the cleanliness of our air. The circulation of plenty of fresh air inside along with high-quality air filtration can help reduce the spread of not only COVID-19 but also influenza and other respiratory viruses.

And there’s another strategy that can help make our air safer to breathe: humidifying. You might not immediately associate moist air with clean air, but a large body of evidence suggests that more humid air—in the range of about 40 to 60 percent humidity—can help cut down on airborne virus transmission.

Much of that evidence comes from studying the flu, says Linsey Marr, PhD, a professor of civil and environmental engineering at Virginia Tech in Blacksburg who studies airborne viruses. “We kind of had this general understanding of how viruses survive as a function of humidity before the pandemic,” she says. Now, some newer data shows that the SARS-CoV-2 virus follows the same pattern. “So no surprises,” Marr says.

Here’s what we know about how humidity makes it harder for viruses to spread, and what to know about using a humidifier in the fight against respiratory illnesses.

The Physics of Airborne Particles and Humidity

SARS-CoV-2 and influenza are viruses that can be transmitted by both visible droplets (of saliva, for example) and by tiny respiratory particles that float in the air and can be inhaled. While droplets generally fall to the ground quickly, airborne particles can remain suspended for minutes or even hours. The greater the concentration of viral particles in the air, the greater the risk that someone breathing that air could become infected.

One main way humidity is thought to reduce that risk is by straightforward physics. Respiratory aerosols consist of fluid that’s a mixture of virus and other substances. When they are released through breathing or talking, for example, they may partly evaporate. If the air is dry, they’ll evaporate a lot and lose a significant amount of their size. But if the air is humid, they won’t evaporate as much, remaining larger and—critically—less able to float around.

In humid conditions, the heavier viral particles will fall out of the air more quickly, meaning less virus will be left to potentially inhale. Once these particles land, the virus could potentially be transmitted by touch, although—particularly with SARS-CoV-2—that type of transmission is less of a concern.

A study published in January in the journal Clinical Infectious Diseases provides experimental evidence of this phenomenon. Researchers recruited 11 people from the University of Oregon who’d tested positive for COVID-19. They participated in a series of experiments conducted over the course of three days.

Each participant would enter a small, environmentally controlled room (alone) and complete a series of activities, including talking, coughing, walking, and other tasks, while the researchers varied different conditions in the room, including humidity. Researchers compared how much virus was in the air and how much virus fell to the ground under conditions of low humidity (about 29 percent on average) and high humidity (about 54 percent on average). They found that for every 12 percentage points of humidity added to the air (within a range of approximately 20 to 70 percent humidity), the amount of virus present in the air fell by about half.

“That’s a really big deal,” says study author Kevin Van Den Wymelenberg, PhD, professor of architecture and director of the Institute for Health in the Built Environment at the University of Oregon. “It’s kind of like masking the room instead of the person.”

The study involved only 11 participants, so its findings will need to be confirmed by additional research. But it’s the first of its kind to gather in-person data on the effect of humidity on the physics of airborne SARS-CoV-2 particles.

Other Effects of Humidity

Humid air may have other effects on the transmission of respiratory viruses. Some research suggests that the flu virus has a harder time surviving in humidity between about 40 and 60 percent, and that it thrives more in drier air. Results of these studies vary, however, Marr says, based on the flu strain studied and the designs of the lab conditions, so the overall extent of this effect isn’t clear.

Other research has evaluated (PDF) how humidity may affect the body’s ability to fight infection. The mucus in our nose and respiratory tract helps trap pathogens and pollutants we breathe in before they can cause infection. But in drier air, this mucus can become dehydrated, impairing its ability to protect against infections.

What About Seasonality?

Humidity likely plays a role in the seasonality of flu and other respiratory viruses, at least in the temperate areas of the globe.

During winter, colder weather forces more people to congregate indoors. And the heating, ventilating, and air conditioning systems widely used to heat buildings and homes also significantly dry out the air, leading to low indoor humidity, says Seema Lakdawala, PhD, an associate professor at the University of Pittsburgh School of Medicine. Airborne transmission of viruses happens mainly indoors, and the dry air makes this happen more efficiently.

The COVID-19 pandemic has complicated this picture somewhat, however, Lakdawala says. “Transmission is still happening indoors,” she says. “But we’re getting transmission efficiently in the summertime, and in the wintertime, and in multiple regions.”

The question of why COVID-19 does not follow a seasonal pattern is one scientists are trying to answer right now. It may be that while humidity affects the SARS-CoV-2 virus to some extent, other factors are more significant in driving its transmission at the moment, such as many people’s relative lack of immunity (compared with, for example, the immunity people build up over time to the flu) and its high level of contagiousness. It’s possible, as more and more people are vaccinated against or infected with SARS-CoV-2, that the spikes in cases could fall into a more seasonal pattern.

“It will be interesting to see what happens in the summer of 2022 now, when we have so much more immunity in the population from both vaccination and natural infection,” Lakdawala says.

Tips for Humidifying

“I do think humidification . . . can impact transmission,” Lakdawala says, and could be an important layer of protection to add, particularly during the months when many respiratory viruses are circulating all at the same time.

Of course, it’s not something you should rely on exclusively, says Marr at Virginia Tech. Good ventilation and the filtration of air will have a more protective effect, she says, that’s backed up by more research. And masking up indoors is an important layer of personal protection as well. Still, humidifying could help, too, and it won’t hurt.

Van Den Wymelenberg says that if someone in your household gets sick with COVID-19, using a humidifier, along with a HEPA-equipped air purifier, could be helpful in the room where they’re recovering, particularly during the winter, when it might be too cold to open a window for ventilation.

Be sure to keep the humidity level between 40 and 60 percent. If it’s higher than that, you risk encouraging the growth of mold, bacteria, and dust mites. If you live in a particularly cold climate. Marr recommends trying to limit humidity a bit further, to about 50 percent, to avoid the possibility of causing condensation on your walls or windows, which could also lead to mold growth.

CR’s experts also recommend opting for a cool-mist humidifier if you have children, because the tanks on warm-mist devices contain water that’s hot enough to cause burns. Also, if you have a cold, warm mist can cause nasal passages to swell and make breathing more difficult, according to the Food and Drug Administration.

Here are a few of our top-rated humidifiers.

Catherine Roberts

Catherine Roberts is a health and science journalist at Consumer Reports. She has been at CR since 2016, covering infectious diseases, bugs and bug sprays, consumer medical devices like hearing aids and blood pressure monitors, health privacy, and more. As a civilian, her passions include bike rides, horror films and fiction, and research rabbit holes. Follow her on X: @catharob.