Research Suggests Humidity Inside Face Masks May Lessen Severity of COVID-19


High levels of humidity promote mucocilliary clearance, which is a defense mechanism that removes mucus and potentially harmful particles from the lungs.

Although it is proven that wearing masks protects individuals and those around them from coronavirus disease 2019 (COVID-19), new research suggests that the humidity created inside the masks may help combat respiratory diseases in general.

The study, led by researchers in the National Institutes of Health’s National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), found that face masks significantly increase the humidity that the mask-wearer inhales. This humidity could help explain why wearing masks has been linked to lower disease severity in people infected with severe acute respiratory coronavirus 2 (SARS-CoV-2), because hydration of the respiratory tract is known to benefit the immune system.

“We found that face masks strongly increase the humidity in inhaled air and propose that the resulting hydration of the respiratory tract could be responsible for the documented finding that links lower COVID-19 disease severity to wearing a mask,” said lead author Adriaan Bax, PhD, in a press release. “High levels of humidity have been shown to mitigate severity of the flu, and it may be applicable to severity of COVID-19 through a similar mechanism.”

High levels of humidity promote mucocilliary clearance (MCC), according to the study authors, which is a defense mechanism that removes mucus and potentially harmful particles from the lungs. Humidity can also bolster the immune system by producing 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 more likely to get respiratory infections in cold weather.

The investigators tested for 4 common types of masks: an N95 mask, a 3-ply disposable surgical mask, a 2-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 they wore no mask, the water vapor of the exhaled breath filled the box, leading to a rapid increase in humidity inside the box.

When the person wore a mask, however, the buildup of humidity greatly decreased because most of the water vapor stayed inside the mask, became condensed, and was re-inhaled. To ensure no leakage, the masks were tightly fitted against the volunteer’s face using high-density foam rubber. Measurements were taken at 3 different temperatures ranging from approximately 46 to 98 degrees Fahrenheit.

The investigators found that all 4 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 greatest increase in level of humidity.

“The increased level of humidity is something most mask-wearers probably felt without being able to recognize, and without realizing that this humidity might actually be good for them,” Bax explained in the statement.

The investigators did not evaluate which masks are most effective against inhalation or transmission of the virus, although earlier studies from Bax and his colleagues found that any cloth mask can help block the saliva droplets released through simple speech. This study does offer more evidence as to why masks are essential in combatting COVID-19.

“Even as more people nationwide begin to get vaccinated, we must remain vigilant about doing our part to prevent the spread of the coronavirus that causes COVID-19,” said Griffin P. Rodgers, director of NIDDK, in the press release. “This research supports the importance of mask-wearing as a simple, yet effective, way to protect the people around us and to protect ourselves from respiratory infection, especially during these winter months when susceptibility to these viruses increases.”


Researchers propose that humidity from masks may lessen severity of COVID-19 [news release]. EurekAlert; February 12, 2021. Accessed February 17, 2021.

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