Why Some Experts Say Humidifiers Could Help Against Covid-19

Your warm, dry home can be a hotbed for Covid-19 infections, but is a humidifier helpful and safe?

Photo: MICROGEN IMAGES/SCIENCE PHOTO LIBRARY/Getty Images

When a coronavirus particle takes flight from an infected person’s mouth or nose, encased in a tiny airship of spit or snot, it must quickly invade another human host and unlock a respiratory-tract cell before all its spiky “keys” melt away like a Wicked Witch splashed by water.

How long the virus remains viable and infectious — stable, as scientists say — depends on the environment. “The virus is more stable as the temperature and the humidity decrease, and it is less stable as the temperature and humidity increase,” says Lloyd Hough, PhD, head of the Hazard Awareness and Characterization Technology Center at the Department of Homeland Security.

Humidity not only affects the virus itself, but also the front lines of our immune system. From the nose on down, the human respiratory tract filters out particles that would do us harm. That filtering system and other aspects of our immune system don’t work as well in dry air, somewhat like how a dry sponge doesn’t clean as well as a wet one.

The specifics are below. But here’s the upshot, from Akiko Iwasaki, PhD, a professor of immunobiology at Yale University: “When cold outdoor air with little moisture is heated indoors, the air’s relative humidity drops to about 20%,” Iwasaki says. “This dry air provides a clear pathway for airborne viruses, such as [the virus that causes] Covid-19. In addition to this, our immune system’s ability to respond to pathogens is suppressed by dry air.”

“The virus is more stable as the temperature and the humidity decrease, and it is less stable as the temperature and humidity increase.”

Iwasaki is one of several scientists calling on the World Health Organization to offer guidelines for minimum humidity levels in public buildings. Further, she tweeted recently, “Let’s humidify our homes.”

However, given the evolving science on a virus still only months old, Hough and other scientists are not convinced a humidifier in the home will offer significant protection against the coronavirus compared to proven ways to scrub or dilute the air in your home, including upgraded air filters, portable air purifiers and, when possible, improving ventilation by letting in fresh outdoor air.

Humidity is relative

The amount of water vapor in the air is measured as relative humidity. Too much feels stuffy. Too little can cause dry skin, cracked lips, dry nasal passages, and even a bloody nose, and really dry air can aggravate allergies and asthma plus encourage viral infections. The ideal range for comfort and good health is 30% to 60%, according to the Environmental Protection Agency. Relative humidity of 60% or more encourages mold growth.

Hough’s center in Homeland Security’s Science and Technology Directorate, which studies chemical, biological, and explosive hazards, has been supporting DHS-sponsored research at the National Biodefense Analysis and Countermeasures Center on how the coronavirus reacts to sunlight, heat, and humidity. Sunlight is the most effective of the three at disabling the virus, he says. Humidity’s role is tricky.

Coronavirus particles from an infected person, expelled by talking or just breathing, are encased in tiny droplets of water and mucus. Larger droplets fall quickly, often within six feet or so. Smaller ones, called aerosols, are mostly invisible and can stay airborne. Regardless of a droplet’s size, its moisture begins to evaporate immediately upon expulsion, and the effect is more dramatic on the smaller aerosols.

The filtering system and other aspects of our immune system don’t work as well in dry air, somewhat like how a dry sponge doesn’t work as well as a wet one.

Drier air speeds evaporation, which can occur completely within a few seconds with aerosols, Hough tells Elemental. The faster the better, as far as the coronavirus’ stability is concerned. Here’s why: Remember those spikes you’ve seen on that ubiquitous coronavirus illustration? They’re like little keys that fit tiny locks in human cells, letting the coronavirus sneak in and co-opt our cellular machinery to reproduce itself. But when virus particles leave one human host, inside a droplet, the spikey keys and other external structures begin to break down.

“Those keys are kind of melting in the environment,” Hough explains. “As a result, the keys no longer fit and the virus can’t infect.”

Here’s the tricky part: If the evaporation of the moisture around a coronavirus particle is really quick, in low humidity, the keys are more likely to survive than when evaporation takes longer, loosely similar to how freeze-drying food means it won’t rot even if stored at room temperature. Once dry, the virus will still decay, but much more slowly, and it can remain infectious for hours.

Fast evaporation has two other important effects, Hough explains: The smaller particles can stay airborne longer, and they can more easily penetrate deeper into the lungs of another person. Larger droplets tend to lodge in the nose, for example. Smaller virus-packing aerosols may pass into the trachea and upper lungs. The smallest — including dried-out, bare coronavirus particles — can plunge deep into the tiniest airways of the lungs, increasing the risk of severe infection.

A review of 10 international studies on humidity’s effect on airborne transmission of influenza, SARS-CoV2, and other coronaviruses, published in the journal Aerosol and Air Quality Research in August, reached similar conclusions: “If the relative humidity of indoor air is below 40%, the particles emitted by infected people absorb less water, remain lighter, fly further through the room, and are more likely to be inhaled by healthy people,” says study leader Ajit Ahlawat, a staff scientist at the Leibniz Institute for Tropospheric Research in Germany.

Why viruses love winter

Winter is a wonderland for viruses. Scientists don’t know the exact equation for transmission risk, but they know the major factors. For starters, lower outdoor temperatures, lower humidity, and less sunlight all favor virus survival. Those environmental conditions also drive people indoors, where closeness and longer exposure times make human-hopping easier for the virus.

“Another important difference is that in wintertime, we heat our buildings, which usually means that we try to close them up to keep the heat in, and this reduces the ventilation rate of outdoor air, allowing aerosols to build up more,” says Linsey Marr, PhD, a civil and environmental engineering professor at Virginia Tech.

The indoor heating, by lowering humidity further, lowers our biological defenses.

Iwasaki and her colleagues, in a study using the influenza virus on mice, found that low humidity dries out air passages, from the nose to the lungs, damaging little hair-like structures on cells that are responsible for sweeping away viruses and other particles. These cilia, as they’re called, normally wave about in a thick layer of mucus, like kelp on the seafloor, trapping incoming particles. When the mucus layer dries, it shrinks, and the cilia are immobilized and flattened away from the flow of air like a slicked-back haircut, letting virus particles fly right past.

Low humidity also hampers the ability of airway cells to repair damage caused by a virus, Iwasaki‘s team found. And in a 1–2–3 punch, dry air also prevents certain proteins in infected cells from sending threat alerts to nearby cells, hindering a key component of the immune system.

Iwasaki’s team expects all these negative effects to occur in humans, too.

Humidifiers have their own risks

Despite the emerging view that humidifiers can reduce the risk of potential respiratory infections, including Covid-19, conclusive studies on humans are lacking, and the appliances are not without risks.

“This is an unproven approach and has potential for very bad side effects,” says Donald Milton, MD, a professor of environmental health at the University of Maryland who has studied the specific risks of humidifiers as well as airborne virus infections more broadly. “I don’t recommend it.”

Even with proper monitoring of humidity levels in a home, condensation can occur in cold spots like closets or within exterior walls — anywhere the warm heated air meets up with cold air — promoting mold growth, Milton says. Mold can cause allergies and trigger asthma.

Another caution: Humidifiers aren’t simple set-and-forget appliances. Whether portable or installed in central heating systems, humidifier filters must be changed regularly and the units must be cleaned thoroughly. “Otherwise, they tend to become contaminated with mold and bacterial growth that may be blown through the house,” according to the Brigham and Women’s Hospital website.

The bottom line

Marr, whose Virginia Tech lab studies the airborne transmission of infectious diseases, sums up the evolving state of all this research with a lot of qualifiers and a lukewarm endorsement of humidifiers.

The coronavirus indeed survives better in the cold, dry, and dark conditions of winter, Marr says. And so it’s “possible,” she tells Elemental, that a humidifier in the home could lower transmission risk. “Raising humidity from 20% to somewhere between 40% and 60% could have two beneficial effects,” she confirms. “The virus doesn’t survive as well in the air and on surfaces at the mid-range humidities, and our immune response might be stronger at the mid-range humidities. There isn’t any harm in humidifying to this level.”

But what about the risk for mold? “Yes, this is an important caveat,” Marr says.

Editor’s Note: The Mayo Clinic provides an overview of different types of humidifiers, how they work, and how to measure humidity levels in your home and use the appliances safely.

Explainer of things, independent health and science journalist, author, former editor-in-chief of LiveScience and Space dot com.

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