Yale Study: Humidity Inside House Critical To Curbing Airborne Transmission Of COVID-19


While summer heat will make it harder for the coronavirus to spread on an airborne level, infectious droplets are also more likely to fall onto indoor surfaces and remain contagious for long periods of time.


NEW HAVEN, Conn — Dating back to Ancient Greece, respiratory illnesses have always been more prevalent during the winter months before largely fading away as temperatures increase. Many are hopeful that COVID-19 will follow a similar pattern. Now, a new Yale study finds that the environment indoors may end being just as important as temperatures outside to slowing down the coronavirus pandemic.

SARS-CoV2, or the virus that causes COVID-19, is definitely more infectious during the winter, according to the study’s authors. As temperatures and humidity rise in the spring it should be harder for the coronavirus to transmit itself through airborne particles in both outdoor and indoor locations. That being said, changes in relative humidity indoors during warmer months should also cut back the risk of transmission. “Relative humidity” refers to the difference between outdoor humidity and temperatures and indoor conditions.

“Ninety percent of our lives in the developed world are spent indoors in close proximity to each other,” says Yale immunobiologist and senior author Akiko Iwasaki in a release. “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.”

When cold, dry winter air is heated indoors, that air’s relative humidity drops by roughly 20%. That even drier than before air is perfect path for airborne viral particles. Dry air is also known to diminish the ability of our cells to expel viral particles. Not to mention the fact that our immune systems aren’t as effective in drier environments.

The research team were interested in the effects of relative humidity in the winter on viral transmission. They noted that during colder months the level of relative humidity inside is very low; chilly, dry air is just reheated and cycled throughout homes and offices. Prior research has also shown that rodents infected with respiratory illnesses more easily transmitted viruses between one another within low-humidity settings.

“That’s why I recommend humidifiers during the winter in buildings,” Iwasaki adds.

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Now, that doesn’t mean we’ll all have no worries once things heat up. While it will be harder for viruses to spread on an airborne level, infectious droplets are also more likely to fall onto indoor surfaces and remain contagious for long periods of time.

“Many homes and buildings are poorly ventilated and people often live in close proximity, and in these cases, the benefits of higher humidity are mitigated,” Iwasaki explains.

So, in the Spring and Summer the coronavirus will have a much harder time spreading through the air, but everyone should be extra vigilant in cleaning surface areas in their home.

Researchers found that there is a sweet spot in terms of indoor relative humidity. Mice living in an environment of between 40%-60% relative humidity were much less capable of transmitting viruses to healthy rodents than those living in habitats featuring lower or higher levels of relative humidity.

To be clear, these viral fluctuations depending on humidity levels only apply to airborne transmission. The coronavirus is still going to be quite contagious all year long through people standing close to each other or touching infected surfaces.

“It doesn’t matter if you live in Singapore, India, or the Arctic, you still need to wash your hands and practice social distancing,” Iwasaki concludes

The study is published in Annual Review of Virology.

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