Indoor humidity level linked to transmission of COVID-19

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

According MIT News, relative humidity is the amount of moisture in the air relative to the total humidity the air can hold at a given temperature before it saturates and forms 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 infection and death -19, while indoor conditions outside this range are associated with worse COVID-19 outcomes. Most people are comfortable with a relative humidity between 30% and 50%, and airplane cabins operate with a relative humidity of 20%, to give the percentages some perspective.

“There is 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.

Researchers analyzed COVID-19 prevalence data and weather measurements from 121 countries, from January 2020 to August 2020, and found a strong correlation between regional outbreaks and relative humidity. For each country, they also tracked local COVID-19 measures such as isolation, quarantine, and testing measures, as well as statistical association with COVID-19 symptoms.

While much of the research on COVID-19 transmission has focused on the virulent fluctuations of the virus across seasons, the MIT team noted that most societies spend 90% of their time the interior, where most transmissions occur. Indoor conditions can be very different from outdoor conditions due to air conditioning systems such as radiators which can significantly dry out indoor air.

The researchers measured 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 this region’s summer season, when high outdoor humidity increased indoor humidity by more than 60%, this increase reflected the gradual increase in COVID-19 deaths in the tropics.

“We saw more COVID-19 deaths in the low and high relative humidity limits, and fewer in the 40-60% sweet spot,” Verheyen said. “This intermediate relative humidity window is associated with a better outcome, which means fewer deaths and a deceleration of the pandemic.”

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

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