Masks help protect the people wearing them from getting or spreading SARS-CoV-2, the virus that causes COVID-19, but now researchers from the National Institutes of Health have added evidence for yet another potential benefit for wearers: The humidity created inside the mask may help combat respiratory diseases such as COVID-19.
The study, led by researchers in the NIH's National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), 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, the researchers suggest, 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.
"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 the study's lead author, Dr Adriaan Bax, NIH distinguished investigator. "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 can limit the spread of a virus to the lungs by promoting mucociliary clearance (MCC), a defence 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.
The study 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.
When the person wore a mask, the build-up of humidity inside the box greatly decreased, due to most of the water vapor remaining in the mask, becoming condensed, and being 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 three different air temperatures, ranging from about 46⁰F to 98⁰F.
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 increased level of humidity is something most mask-wearers probably felt without being able to recognise, and without realising that this humidity might actually be good for them," Bax said.
The researchers did not look at which masks are most effective against inhalation or transmission of the virus and defer to the CDC for guidance on choosing a mask. Earlier studies from Bax and his colleagues showed that any cloth mask can help block the thousands of saliva droplets that people release through simple speech – droplets that, if released, can remain in the air for many minutes.
While the current study did not examine respiratory droplets, it does offer more evidence as to why masks are essential to battling 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 NIDDK director Dr Griffin P Rodgers. "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."
The research was supported by the NIDDK Intramural Research Programme and the NIH Intramural Antiviral Target Programme.
Study details
Hydrating the Respiratory Tract: An Alternative Explanation Why Masks Lower Severity of COVID-19
Joseph M Courtney, Ad Bax
Published in Biophysical Journal on 2 February 2021
Abstract
Seasonality of respiratory diseases has been linked, among other factors, to low outdoor absolute humidity and low indoor relative humidity, which increase evaporation of water in the mucosal lining of the respiratory tract. We demonstrate that normal breathing results in an absorptiondesorption cycle inside facemasks, where super-saturated air is absorbed by the mask fibers during expiration, followed by evaporation during inspiration of dry environmental air. For double-layered cotton masks, which have considerable heat capacity, the temperature of inspired air rises above room temperature, and the effective increase in relative humidity can exceed 100%. We propose that the recently reported, disease-attenuating effect of generic facemasks is dominated by the strong humidity increase of inspired air. This elevated humidity promotes mucociliary clearance of pathogens from the lungs, both before and after an infection of the upper respiratory tract has occurred. Effective mucociliary clearance can delay and reduce infection of the lower respiratory tract, thus mitigating disease severity. This mode of action suggests that masks can benefit the wearer even after an infection in the upper respiratory tract has occurred, complementing the traditional function of masks to limit person-to-person disease transmission. This potential therapeutical use should be studied further.
[link url="https://www.nih.gov/news-events/news-releases/researchers-propose-humidity-masks-may-lessen-severity-covid-19"]NIH/National Institute of Diabetes and Digestive and Kidney Diseases material[/link]
[link url="https://www.cell.com/biophysj/pdf/S0006-3495(21)00116-8.pdf"]Biophysical Journal study (Open access)[/link]