It's sort of like the Goldilocks principle — a room that's either too dry or too humid can influence transmission of COVID-19 and cause more illness or death, Massachusetts Institute of Technology researchers say.
Maintaining an indoor relative humidity between 40% and 60% is associated with lower rates of COVID-19 infections and deaths, they reported Nov. 16 in the Journal of the Royal Society Interface.
Indoor conditions outside that range are associated with worse COVID outcomes, according to the report.
“There's potentially a protective effect of this intermediate indoor relative humidity,” said lead author Connor Verheyen, a doctoral student in the Harvard-MIT Program in Health Sciences and Technology, in Cambridge, Mass.
The research team noted that most people are comfortable between 30% and 50% relative humidity. An airplane cabin is kept around 20%.
Until now, researchers have considered that COVID-19 could be influenced by the seasons, but they tended to examine the virus' patterns in the context of outdoor weather conditions.
The MIT team decided that other researchers might be looking in the wrong direction, given that people in most places spend more than 90% of their time indoors. Indoor conditions also are where most viral transmission occurs.
For the study, the investigators combined COVID data with meteorological measurements taken from 121 countries.
They gathered COVID case counts and deaths from between January and August 2020, before vaccines were available, and then compared each day of data with an average estimated indoor humidity on that day.
For example, they reasoned that if outdoor temperatures fell below the typical human comfort range of 66 to 77 degrees Fahrenheit, folks would crank on the heat — and thus cause indoor humidity to fall.
As a result, they found that indoor relative humidity tended to drop below 40% during colder periods, and that COVID cases and deaths also spiked at those times.
The team also found a gradual rise of indoor humidity during tropical countries' summer season reflected in a gradual increase in COVID deaths as humidity went past 60%.
COVID-19 cases and deaths tended to increase when a region's average estimated indoor humidity was lower than 40% or higher than 60%, regardless of the time of year.
Nearly all regions had fewer COVID infections and deaths when average indoor humidity hovered in the “sweet spot” between 40% and 60%, the study authors said.
“We were very skeptical initially, especially as the COVID-19 data can be noisy and inconsistent,” said co-researcher Lydia Bourouiba, director of the MIT Fluid Dynamics of Disease Transmission Laboratory. “We thus were very thorough trying to poke holes in our own analysis," she noted in an MIT news release.
Bourouiba said the team used a range of approaches to test the findings, including taking into account factors such as government intervention.
“Despite all our best efforts, we found that even when considering countries with very strong versus very weak COVID-19 mitigation policies, or wildly different outdoor conditions, indoor — rather than outdoor — relative humidity maintains an underlying strong and robust link with COVID-19 outcomes,” Bourouiba said.
The researchers aren't sure why indoor humidity might have such an influence over COVID's virulence, but follow-up studies have suggested that germs might survive longer in respiratory droplets in either very dry or very humid conditions.
The U.S. Centers for Disease Control and Prevention has more about COVID risk in specific settings.
SOURCE: Massachusetts Institute of Technology, news release, Nov. 15, 2022