High-speed video imaging reveals the two main components of a sneeze in colorful detail: a shower of larger droplets (shown in green) and a cloud made of a mixture of smaller droplets suspended in moist, warm gas (shown in red). Credit: Lydia Bourouiba/MIT
As a society, humans spend more than 90 percent of their time indoors. That number certainly decreased during the summer months of the COVID-19 pandemic, but once fall and winter hit again, people were forced back inside. We knew masks and proper indoor ventilation were key to reducing the spread of the virus, but at the time, little research was done on the influence of indoor conditions on the transmission of SARS-CoV-2.
Now, a new study from MIT finds that indoor relative humidity plays a significant role in the transmission of COVID-19. According to the study, maintaining an indoor relative humidity between 40 and 60 percent—or the “sweet spot”—is associated with lower rates of COVID-19 infections and deaths. Meanwhile, indoor conditions outside this range, whether lower or higher, are associated with worse outcomes. For perspective, most people are comfortable between 30 and 50 percent relative humidity.
For the study, published in Journal of the Royal Society Interface, Connor Verheyen, a PhD student in medical engineering and medical physics at MIT, and Lydia Bourouiba, director of the MIT Fluid Dynamics of Disease Transmission Laboratory, gathered global COVID-19 data from 121 countries, including case counts and reported deaths from January 2020 to August 2020.
The early period of the pandemic was chosen specifically to avoid vaccinated individuals as a variable. For each country, Verheyen and Bourouiba also tracked the local COVID-19 related policies, including isolation, quarantine and testing measures, and their statistical association with COVID-19 outcomes.
For each day that COVID-19 data was available, the researchers used meteorological data to calculate a country’s outdoor relative humidity. They then estimated the average indoor relative humidity, based on outdoor relative humidity and guidelines on temperature ranges for human comfort. For instance, humans are typically comfortable between 66 to 77˚ Fahrenheit indoors. They also collected experimental data, which they used to validate their estimation approach.
In warmer times, both outdoor and indoor relative humidity for each country was about the same, but they quickly diverged in colder times. The researchers found that while outdoor humidity remained around 50 percent throughout the year, indoor relative humidity for countries in the Northern and Southern Hemispheres dropped below 40 percent in their respective colder periods—when COVID-19 cases and deaths also spiked in these regions.
For countries in the tropics, relative humidity was about the same indoors and outdoors throughout the year, with a gradual rise indoors during the region’s summer season, when high outdoor humidity likely raised the indoor relative humidity over 60 percent. The researchers again found this rise mirrored the gradual increase in COVID-19 deaths in the tropics.
“We were very skeptical initially, especially as the COVID-19 data can be noisy and inconsistent,” said Bourouiba. “We thus were very thorough trying to poke holes in our own analysis, using a range of approaches to test the limits and robustness of 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.”
It’s still unclear exactly how strong, or the mechanisms by which, relative humidity affects COVID-19 outcomes; but the team’s follow-up studies suggest pathogens may survive longer in respiratory droplets in both very dry and very humid conditions.
“Our ongoing work shows that there are emerging hints of mechanistic links between these factors,” said Bourouiba. “For now however, we can say that indoor relative humidity emerges in a robust manner as another mitigation lever that organizations and individuals can monitor, adjust, and maintain in the optimal 40 to 60 percent range, in addition to proper ventilation.”