The latest results of the study published in the biophysical Journal’, led by researchers in the NIH’s National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), showed that face masks significantly increased the humidity in the air that the mask-wearer breathes in. This higher level of humidity in inhaled air, the researchers indicated, could help explain why wearing masks has been linked to lessen the severity of COVID-19 in people infected with SARS-CoV-2, since hydration of the respiratory tract is known to support the immune system. 

High levels of humidity can restrict the spread of a virus to the lungs by encouraging mucociliary clearance (MCC), a protective mechanism that eliminates mucus and potentially dangerous particles inside the mucus from the lungs. High levels of humidity can also bolster the immune system by producing special proteins, called interferons, that battle against viruses, a process known as the interferon response. Low levels of humidity have been shown to affect both MCC and the interferon response, which may be one reason why people are more likely to get respiratory infections in cold weather.

The process of testing humidity 

The study evaluated four common styles of masks: an N95 mask, a three-ply disposable surgical mask, a two-ply cotton-polyester mask, and a heavy cotton mask. The researchers assessed the amount of humidity by making a volunteer breathe into a sealed steel box. To ensure no leakage, the masks were securely fitted against the volunteer’s face using high-density foam rubber. Measurements were taken at three different air temperatures, varying from around 46 to 98 degrees Fahrenheit.

The findings showed that all four masks increased the amount of humidity of inhaled air, albeit to different degrees. At lower temperatures, the humidifying effects of both masks significantly improved. At all temperatures, the dense cotton mask contributed to the most elevated level of humidity. The increased level of humidity is something most mask-wearers probably felt without being able to identify, and without recognizing that this humidity could actually be beneficial for them.

“We show that face masks strongly increase the effective humidity of inhaled air, thereby promoting hydration of the respiratory epithelium which is known to be beneficial to the immune system,” NIH investigators Joseph M. Courtney, Ph.D., and Adriaan Bax, Ph.D., wrote in Biophysical Journal. “Increased humidity of inspired air could be an alternate explanation for the now well-established link between mask-wearing and lower disease severity.”