Indoor humidity may slow coronavirus spread, Yale scientists say

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Researchers at Yale say that we may get some respite from the coronavirus pandemic as we move into spring, although this depends on how indoor environments adapt.

While the effectiveness of social distancing measures obviously plays a crucial role in battling the spread of COVID-19, the scientists are also eyeing changes in relative humidity indoors from winter to spring to summer.

Relative humidity measures water vapor relative to the temperature of the air.

“In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature,” explains the National Weather Service on its website. This differs from absolute humidity, which is a measure of the actual amount of water vapor in the air, regardless of its temperature.

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“The cold, dry air of winter clearly helps SARS-CoV2 -- the virus that causes COVID-19 -- spread among people,” explain the Yale researchers in a statement. “But as humidity increases during spring and summer, the risk of transmission of the virus through airborne particles decreases both outside and indoors in places such as offices.”

The research was published March 23 in the Annual Review of Virology.

“Ninety percent of our lives in the developed world are spent indoors in close proximity to each other,” said Yale immunobiologist and the research’s senior author Akiko Iwasaki, in the statement. “What has not been talked about is the relationship of temperature and humidity in the air indoors and outdoors and aerial transmission of the virus.”

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In an abstract of the research, scientists explain that the seasonal cycle of respiratory viral diseases has been widely recognized for thousands of years. “Annual epidemics of the common cold and influenza disease hit the human population like clockwork in the winter season in temperate regions,” they write. “Moreover, epidemics caused by viruses such as severe acute respiratory syndrome coronavirus (SARS-CoV) and the newly emerging SARS-CoV-2 occur during the winter months.”

The researchers note that, when cold outdoor air with little moisture is heated indoors, the air’s relative humidity drops to about 20 percent. “This comparatively moisture-free air provides a clear path for airborne viral particles of viruses such as COVID-19,” they explain in the statement. “Warm, dry air also dampens the ability of cilia, the hair-like projections on cells lining airways, to expel viral particles.” Additionally, the immune system’s ability to respond to pathogens is suppressed in drier environments, according to Iwasaki.

Scientists say there is a “sweet spot” in relative humidity for indoor environments, citing research involving rodents. “Mice in environments of between 40 percent and 60 percent relative humidity show substantially less ability to transmit viruses to non-infected mice than those in environments of low or high relative humidity,” the researchers state. Iwasaki notes that mice kept at 50 percent relative humidity were also able to clear an inhaled virus and mount robust immune responses.

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However, Iwasaki adds that the research only applies to aerosol transmission of the virus. “It doesn’t matter if you live in Singapore, India, or the Arctic, you still need to wash your hands and practice social distancing,” she said.

The University of Zurich also participated in the research.

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As of Monday afternoon, at least 1,292,564 coronavirus cases have been diagnosed worldwide, at least 338,995 of which are in the U.S. The disease has accounted for at least 70,798 deaths around the world, including more than 5,100 people in the U.S.

Follow James Rogers on Twitter @jamesjrogers