Masks help protect the people wearing them from getting or spreading SARS-CoV-2, the virus that causes COVID-19.
In a new study, researchers found evidence for another potential benefit for wearers: The humidity created inside the mask may help combat respiratory diseases such as COVID-19.
They found that face masks substantially increase the humidity in the air that the mask-wearer breathes in.
This higher level of humidity in inhaled air could help explain why wearing masks has been linked to lower disease severity in people infected with SARS-CoV-2, because hydration of the respiratory tract is known to benefit the immune system.
The research was conducted by a team from the National Institutes of Health.
In the study, the team tested four common types of masks: an N95 mask, a three-ply disposable surgical mask, a two-ply cotton-polyester mask, and a heavy cotton mask.
The researchers measured the level of humidity by having a volunteer breathe into a sealed steel box.
When the person wore no mask, the water vapor of the exhaled breath filled the box, leading to a rapid increase in humidity inside the box.
The results showed that all four masks increased the level of humidity of inhaled air, but to varying degrees.
At lower temperatures, the humidifying effects of all masks greatly increased. At all temperatures, the thick cotton mask led to the most increased level of humidity.
The researchers proposed that the resulting hydration of the respiratory tract could be responsible for lower COVID-19 disease severity to wearing a mask.
High levels of humidity have been shown to mitigate the severity of the flu, and it may be applicable to the severity of COVID-19 through a similar mechanism.
According to the team, high levels of humidity can limit the spread of a virus to the lungs by promoting mucociliary clearance (MCC), a defense mechanism that removes mucus and potentially harmful particles within the mucus from the lungs.
High levels of humidity can also bolster the immune system by producing special proteins, called interferons, that fight against viruses a process known as the interferon response.
Low levels of humidity have been shown to impair both MCC and the interferon response, which may be one reason why people are likelier to get respiratory infections in cold weather.
One author of the study is Adriaan Bax, Ph.D., NIH Distinguished Investigator.