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.
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.