The Right Level of Humidity May Be Important Weapon in Fighting Coronavirus, New Studies Show

Artist's illustration of the coronavirus that causes COVID-19. iStock

A potentially important weapon against COVID-19 has emerged from two unrelated and seemingly unremarkable observations.

The first came several years ago when Walter Hugentobler, a Swiss physician who sometimes practices at a clinic in Zurich International Airport, noticed several years ago that pilots and flight attendants seemed unusually susceptible to the flu throughout the year, even though they were generally healthy.

More recently, Hazhir Rahmandad, an engineer at the Massachusetts Institute of Technology's Sloan School of Management, noticed that COVID-19 tended to spread in his native Iran at different rates from one region to the next, even when population densities were similar.

Both scientists followed up with studies that have converged on an important insight into COVID-19: the spread of the disease is likely to vary significantly with temperature and humidity.

This characteristic of the virus offers hope that we can mitigate the spread of the novel coronavirus with simple measures such as installing humidifiers in the home. In the dry winter months, cold air gets pulled into the home and heated, which lowers the relative humidity—in other words, the heated air is capable of holding more moisture than it actually contains. Such dry air impairs the lungs' ability to clear out invading viruses and the immune system's ability to keep the virus from replicating. "We spend 90 percent of our lives indoors, where the air is very dry in the winter," says Akiko Iwasaki, a Yale immunobiologist who led one of the studies, with Hugentobler as a co-author. "That's exactly when the virus best survives and transmits."

The research suggests that a relative humidity of 40 percent to 60 percent could help the body fight off the virus.

The finding has a downside, however. It supports the theory that the summer's heat and humidity will suppress SARS-CoV-2, leading to a drop in new cases and deaths. The respite would certainly come as a relief, but experts warn that it could lead to a dangerous complacency that sets the conditions for a destructive rebound of the pandemic, similar to what happened in the 1918 influenza outbreak.

The flu-prone plight of pilots and flight attendants casts new light on the long-known tendency of flus and some other respiratory infections to peak in the winter. The conventional wisdom held that colder temperatures support viruses and suppress our immune systems. But Hugentobler suspected that the ultra-low relative humidity in airplane cabins might be the bigger culprit. (Homes are similarly dry in the winter.) He found a series of studies from decades earlier that linked indoor humidifiers to reduced absenteeism in schools, workplaces and the military during the winter months. "But no one had paid attention to the studies," he says.

Late last year, a colleague pointed Hugentobler across the Atlantic to Yale's Iwasaki, who earlier in the year had published a groundbreaking study showing that the immune system of mice was compromised in lower humidities, leaving them less able to fight off respiratory illnesses. Hugentobler and a third researcher helped Iwasaki pull together an exhaustive review of all relevant research. They found strong and consistent evidence that the dry, warm air found indoors in winter in much of the world helps keep viruses intact, while at the same time stressing out the immune system and the protective hair-like "cilia" that line the lungs. "During the night, the dry air keeps the cilia from clearing out all the pollutants and viruses you've inhaled," says Iwasaki.

When the pandemic hit, the three researchers reworked their study to take into account the available data on the impact of low humidity on the novel coronavirus behind COVID-19, which supported their conclusions. They published a preliminary version of the paper online in late March, to mostly positive reviews from other scientists—though Iwasaki notes that social-media comments have included what she says is the standard dose of sexist trolling that accompanies most prominent work released by female scientists.

"Some people seem to feel threatened by it," she says. "You see criticisms that you don't see with similar work from male scientists." (For instance, critics sometimes say Iwasaki isn't qualified to comment on COVID-19 because she's not a physician, even though most medical advances emerge from Ph.D. labs. A tweet in which she complained about the abuse received over 11,000 likes, mostly from scientists and clinicians.)

Around the time Iwasaki's study was posted, MIT's Rahmandad was pondering the variation in COVID-19 infection rates throughout Iran's provinces. He noticed that the warmer, more humid parts of the country seemed less affected by the disease—as was much of warm-and-humid India and the rest of South Asia, even though the high population density and traffic with China should have made those areas ideal for fast spread of infections. "It all suggested that something related to the weather was important," says Rahmandad.

Having previously done research on modeling the spread of epidemics, Rahmandad in April joined up with scientists from Harvard, the University of Connecticut and Virginia Tech in order to figure out the impact of weather on COVID-19. Job one was to try to correct for the various significant errors and inconsistencies in infection-rate data that had been pouring in from different countries. "The official data from most countries grossly undercounted infections, and the numbers were anywhere from eight to fifteen days behind the disease," says Rahmandad. "So we had to do a lot of adjustment to get the true figures for each day so we could match them to the weather at the time."

Also on the team was Harvard's Mohammad Jalali, a data scientist who focuses on public-health questions. Part of his role in the project was finding ways to adjust the results to account for non-weather-related differences between different countries and regions. "We had to control for population density, social and cultural norms, and variations in government policy," explains Jalali.

After nearly two months of complex data organization, calculations and testing, the group released its preliminary findings in mid-May. Based on data from more than 3,700 locations around the world from December through April, the team found that infection rates start to fall off with temperatures over 77 degrees, with very high levels of bright sunlight and with rising humidity. "Combined, these factors mean that weather can affect transmission rates by between 15 to 40 percent, depending on the location," says Rahmandad.

The implications for summer in the U.S. are significant. Compared to the transmission rates of the disease from early in the pandemic during the winter, most of the U.S. will see a roughly 30-percent drop in transmission rates at the peak of summer heat, sunlight and humidity, predicts Rahmandad. (See the scientists' updated predictions for different locations.)

If you think that's good news, Rahmandad has a warning. Halting the pandemic entirely that would require a 70-percent drop in transmission rates. If people and governments get complacent as the warm, humid weather moves in, a deadly rebound could occur. "The summer weather by itself won't be nearly enough to quench the pandemic," he says. "Loosening our response strategies because we think the summer weather will change the picture would be a really terrible decision."

The research, though, should better equip scientists and public-health officials to advise the public to stay the course when it comes to social distancing and masks, even as the heat and humidity climb and new cases drop.

When it comes time to face a winter resurgence of the pandemic, the potential benefits of humidifiers, as shown in Iwasaki and Hugentobler's work, could prove important. In addition, says Iwasaki, longer-term protection against the winter spike in respiratory illnesses—including future pandemics—could come from changing building codes to force designs that better maintain humidity in the colder months. "That's especially important in hospitals and nursing homes," she says. She adds that she's been trying to push the World Health Organization to immediately back a high-priority drive for better indoor humidification, but hasn't had a response yet.

That's too bad. After a summer slowdown of the pandemic's brutal progress, we'll need every weapon we can muster heading into what threatens to be a disastrous fall and winter.