By now, you’ve probably got your coronavirus risk-minimizing routine down pat. Mask? Check. Social distance? Acquired. Spending time indoors with people outside your pod? Hard pass. You wash your hands. (But maybe not your groceries.) And that’s great. Keep it up. Because with the virus now surging to record levels in nearly every state, and hospitals starting to buckle under the strain, doing all these things is more important than ever. But as public health experts warn of a long and deadly winter ahead, there’s one more thing some scientists say we should be talking about: humidity.
Here's all the WIRED coverage in one place, from how to keep your children entertained to how this outbreak is affecting the economy.
By Eve Sneider
With winter comes plummeting temperatures, and the colder the air gets, the less water vapor it can hold. The way most buildings are heated only exacerbates the problem. Heating, ventilation, and air-conditioning (HVAC) systems suck in outside air and then heat it up, which zaps even more moisture out of it. These changes not only make it easier for respiratory viruses to hop from host to host, but dry air cripples the first few lines of defense your body has for preventing such viruses from establishing an infection. All of this could be a recipe for the coronavirus to wreak even more havoc in the coming months.
“A lot of indoor environments are bone-dry in the wintertime,” says Jeffrey Shaman, an infectious disease forecaster at Columbia’s Mailman School of Public Health. “That makes the virus more transmissible. And people are spending more time indoors. So a lot of factors are going to be working against us.”
A decade ago, a team of researchers led by Shaman looked at 31 years’ worth of data on influenza-like illness and weather patterns in the US. Over and over, they found that the biggest outbreaks happened in the winter when the weather was unusually dry. Lab studies with ferrets and guinea pigs showed similar patterns. The influenza virus spread most readily when the relative humidity inside the animals’ cages fell below 40 percent. (A typical indoor humidity range in warmer weather is between 40 and 60 percent.)
Aerosol scientists who study the flu, like Linsey Marr at Virginia Tech, have helped explain why that might be. In a 2012 study, her group showed that as relative humidity levels dip, the particles that people emit through talking or coughing get smaller and smaller. These particles are made up of mucus, salts, protein, and cell parts—but mostly water. The drier the air around them, the faster that water evaporates. And the smaller the particles get, the longer they can stay in the air, the farther they can travel, and the deeper into the lungs they can be inhaled. Any viruses lurking inside these particles go along for the ride.
If they land inside a susceptible person’s respiratory tract, that can spell trouble. Of course, the body is equipped with multiple layers of security to protect against would-be invaders. The first line of defense is a physical barrier maintained by the cells that line the nasal passages. Some of these cells secrete mucus—two layers of the slippery, stringy, substance, with two different viscosities. Other cells inside the nose and throat have tiny, anemone-like projections called cilia, which beat in synchrony in the more watery layer. That motion moves the thicker, top layer of mucus like a conveyor belt away from the lungs. This mucosal current catches any viruses or bacteria (or other irritants like pollen and ash) that land on it and sweeps them away to be swallowed or coughed out. But if the air is too dry, these mucus layers dessicate, squishing the cilia and immobilizing them.
In a 2017 study, researchers at Yale University School of Medicine found that mice housed at 10 percent relative humidity had a much harder time clearing the influenza virus from their respiratory tracts than mice housed in 50 percent relative humidity. Their mucosal flow slowed way down, as you can see in a stunning set of videos recently posted to Twitter by the study’s lead author, immunologist Akiko Iwasaki. Without a functioning mucociliary response, the virus successfully spread to the lungs of these mice at higher rates, and the animals got sicker than their counterparts that breathed more moist air.
Cold, dry air can also impair the second and third tiers of the body’s immune response. If a virus gets past the mucus river, its next aim is to find the cells that line the airway, known as epithelial cells, then get inside them and hijack their molecular machinery to make more copies of itself. From the moment that happens, it’s a race between how fast the virus can replicate and how fast the body can rally its defenses. When infected cells sense they’ve been compromised, they flip hundreds of genes on. Some of these encode the recipes for making weapons of molecular war—scissor-like enzymes for shredding the virus’s genetic code and net-like proteins that tether the virus to the cell’s membrane, preventing it from releasing clones of itself to infect other cells nearby. Some of the genes produce chemical distress signals known as interferons. These molecules recruit immune cells to come and join the fight, and their response is what creates inflammation and flu symptoms—coughing, fever, sore throat.
“Whether or not you get symptoms really depends on how far the virus gets on its quest,” says Ellen Foxman, an immunobiologist at Yale who did her postdoc in Iwasaki’s lab. She has studied how temperature affects this race between the immune system and viruses that cause the common cold. “Cold temperatures affect the speed at which the immune defenses get turned on. It slows them way down and gives the virus a huge advantage,” she says. Similar effects have been observed in mice housed at low humidity—in very dry conditions, their airway epithelial cells lost the ability to turn on those SOS signaling genes. But exactly how dry air affects the interferon response is still an area of active research.
In the world’s temperate zones, respiratory bugs ebb and surge with the seasons. Each year, influenza, rhinoviruses, and the endemic common cold-causing coronaviruses retreat as temperatures and humidity rise, only to come roaring back as summer turns to fall. These lab experiments help explain how low humidity is likely driving these seasonal swings. But very few scientists have been able to directly test how big an effect humidity can have on preventing infections among people in the real world. In fact, it’s only been done once.
In 2016, a group of researchers from the Mayo Clinic led by molecular biologist Chris Pierret worked with a local nursery school to install humidifiers in two of its four identically laid-out classrooms. They let them run from January through March, periodically swabbing samples in each classroom, and then tried to grow any viruses they found. During that period, the scientists didn’t find as much infectious virus in the humidified classrooms, and the students who spent time there called in sick with flu-like symptoms less often than the kids in the non-humidified classrooms.
Pierret got the idea from some of his work with InSciEd Out, a Mayo-affiliated nonprofit aimed at reforming science education to encourage doing research—not just memorizing facts and figures. One way the organization does that is to partner with schools and help their students and teachers conduct experiments with zebrafish, a model organism commonly studied by developmental biologists. A couple of years ago, one Minnesota school went all-in on the idea. Administrators installed fish tanks in classrooms and the school’s high-traffic computer lab. That winter turned out to be especially dry, which got to be a problem for maintaining all that new fish habitat. The dry air was sucking up moisture from wherever it could—including the newly acquired collection of aquaria. One of the teachers called Pierret to tell him how exhausted he was. “I’ve been doing nothing but filling fish tanks every day!” Pierret recalls him saying. It also turned out to be a bad year for respiratory infections. The majority of schools in the area reported higher-than-average student absenteeism due to flu symptoms. Only one school didn’t: The one with the fish tanks. “It really stood out,” says Pierret. “That was the hypothesis-driving event for us.” He and his colleagues went on to test that hypothesis in a local nursery school, publishing the results in the journal PLOS One.
Compelling as the Mayo nursery school study was, says Shaman, it’s difficult to make the leap from that one limited example to saying that humidification could be a game-changer against Covid-19. Scientists still aren’t sure if this new coronavirus will exhibit the same seasonality as other respiratory viruses, like strains of influenza and the viruses that cause the common cold. That’s almost impossible to tell in the first year of a pandemic, when the entire world is susceptible to a new pathogen. It might take a year or two for some degree of immunity to get established, before more subtle factors like climate emerge as playing a bigger role in transmission.
Read all of our coronavirus coverage here.
But people like Stephanie Taylor don’t want to wait that long. A physician and Incite Health Fellow at Harvard Medical School, Taylor is also a distinguished lecturer and member of the Epidemic Task Group at ASHRAE, the American Society for Heating, Refrigerating, and Air-Conditioning Engineers. For years, she’s studied the relationship between indoor air and human health. Taylor is among a group of scientists who think that fine-tuning the humidity inside buildings could save thousands of lives every year. In April, she started an online petition urging the World Health Organization to add relative humidity to its indoor-air standard recommendations. The WHO sets guidance for some indoor air quality issues, such as pollution and mold. But currently, it sets no limits on minimum humidity levels in public buildings. So far, more than 4,500 people have signed the petition.
This summer, she teamed up with researchers at the Massachusetts Institute of Technology to test her hunch about a connection between Covid-19 and humidity. Together, they pulled in data from 125 countries. In one bucket, they collected information about how different nations had prepared for and responded to the pandemic—annual health care spending, school closures, mask mandates, and other policies aimed at curbing the virus’s spread. In another bucket they gathered data about the toll of Covid-19, including confirmed cases. Into the third bucket went environmental data—temperature, humidity, air pressure, precipitation, sunlight, as well as spot measurements taken indoors to corroborate estimates of indoor relative humidity. Then they piped all this data into a machine-learning model and tasked it with finding the strongest connections.
Taylor says her MIT collaborators were sure the data analysis would turn up some other confounding variables that would disprove her hypothesis about the importance of indoor climate. But after three months of data crunching, they found that the most powerful correlation between national numbers of daily new coronavirus cases and daily Covid-19 deaths was indoor relative humidity. Even controlling for dozens of other factors, the data showed that as indoor relative humidity went up during the summer months in the northern hemisphere, deaths plummeted. In the southern hemisphere, the opposite was true—as humidity fell during those nations’ winter months, deaths began to climb. “It’s so powerful, it’s crazy,” says Taylor.
That work has not yet been published. But Taylor believes it’s the strongest evidence yet that humidity needs to be as much a part of the conversation about containing Covid-19 as is discussion of ventilation, masks, and hand hygiene. “It’s hard to prioritize one intervention over another; we need all of them,” says Taylor. “Humidifiers aren’t a replacement for masks or social distancing or ventilation. But when you have more humidification, it enhances all these other things we’re already doing.” At higher humidities, respiratory particles grow faster and fall to the ground earlier, so there’s a better chance that staying 6 feet apart from infectious people really will dilute how many bits of their aerosolized virus you might happen to inhale. In a recent modeling study, Japanese researchers found that air with 30 percent relative humidity can carry more than twice the number of infectious aerosols, compared to air with relative humidity levels of 60 percent or higher. That also means masks are more likely to block more of the particles coming out of people’s noses and mouths, because they tend to be better at trapping bigger particles than smaller ones. And it means that air purifiers (even cheap, DIY ones) will filter out a larger proportion of potentially infectious particles.
We know what you’re thinking: that now, on top of everything else, you’ve got to be measuring and monitoring the amount of moisture in your home! As if the Covid-19 risk mitigation calculus wasn’t complicated enough already. But Pierret says there’s a way to simplify things. “If you could only humidify one space, I’d make it the one where you’re sleeping,” he says, noting that in giving such advice he is speaking for himself and not for the Mayo Clinic.
In addition to keeping your mucus flowing and your cilia beating, research has shown that people sleep better in rooms with between 40 and 60 percent relative humidity. And during sleep, your immune system produces antibodies and other important signaling molecules. So the better you sleep, the more ready your body will be to fend off any future infections. Still, Pierret says that it’s important not to think of humidification as any sort of magical fix. You still have to wear a mask and wash your hands and stay socially distant and avoid crowded indoor spaces. “Any one of those alone is not enough,” he says. “But each one is like a card that you’re putting into a deck to stack the odds in your favor.”