A new study found that very dry or very humid indoor air can worsen COVID-19 outcomes. Previous research has shown that proper ventilation can slow the spread of the virus, and now researchers at the Massachusetts Institute of Technology (MIT) have discovered that indoor relative humidity may also influence transmission of the virus.

According to MIT News, relative humidity is the amount of moisture in the air compared to the total moisture the air can hold at a given temperature before saturating and forming condensation. In a study published Wednesday in the Journal of the Royal Society Interface, the MIT team reported that maintaining an indoor relative humidity level between 40% and 60% is linked to lower rates of COVID-19 infection and deaths, while indoor conditions outside this range are associated with worse COVID-19 outcomes. Most people are comfortable with between 30% and 50% relative humidity, and airplane cabins operate with 20% relative humidity, to give the percentages perspective.

“There’s a protective effect of this intermediate indoor relative humidity,” says lead author Connor Verheyen, a Ph.D. student in medical engineering and medical physics at the Harvard-MIT Division of Health Sciences and Technology.

The researchers analyzed data of COVID-19 prevalence and meteorological measurements from 121 countries, from January 2020 through August 2020, and found a strong correlation between regional outbreaks and relative humidity. For each country the also tracked local COVID-19 measures such as isolation, quarantine, and testing measures along with statistical association with COVID-19 symptoms.

While much of the research on COVID-19 transmission has been focused on the virus’ virulent swings with the seasons, the MIT team noted that most societies spend 90% of their time indoors, where most transmissions occur. Indoor conditions can be very different from outdoor conditions because of climate control systems such as heaters that can significantly dry out indoor air.

The researchers measured both outdoor and indoor humidity in different hemispheres, noting that in the tropics, relative humidity was about the same indoors and outdoors throughout the year. However, during that region’s summer season, when high outdoor humidity raised the indoor humidity over 60%, that rise mirrored the gradual increase in COVID-19 deaths in the tropics.

“We saw more COVID-19 deaths on the low and high end of relative humidity, and less in the sweet spot of 40 to 60 percent,” said Verheyen. “This intermediate relative humidity window is associated with a better outcome, meaning fewer deaths and a deceleration of the pandemic.”

The team’s follow-up studies suggest that pathogens may survive longer in repository droplets in both very dry and very humid conditions. Monitoring indoor relative humidity may give us another valuable mitigation tool along with proper ventilation, the experts concluded