One of the researchers who carried out the study breathes into a sealed box while wearing a mask. Courtesy of NIDDK.

What you need to know

Masks help protect the people wearing them from getting or spreading SARS-CoV-2, the virus that causes COVID-19. Now researchers have found evidence for yet another potential benefit for wearers: The humidity created inside the mask may help reduce the severity of respiratory diseases such as COVID-19.

A study led by researchers in 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.


What did the researchers do?

The study tested four common types of masks: an N95 respirator 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 buildup of humidity inside the box greatly decreased due to most of the water vapor remaining in the mask, becoming condensed, and being re-inhaled.

Why does humidity matter?

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, in 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 people are likelier to get respiratory infections in cold, dry weather.

Why is this research important?

The study offers more evidence for why masks are essential to battling COVID-19. It could also help explain why wearing masks has been linked to lower disease severity in people infected with SARS-CoV-2.

Where can I go to learn more?

Use Masks to Slow the Spread of COVID-19

The Centers for Disease Control and Prevention provides information about masks, including why and how to wear a mask, types of masks, and how to store and wash masks.


National Institutes of Health. (2021, February 12). Researchers propose that humidity from masks may lessen severity of COVID-19. Retrieved May 11, 2021, from

Courtney, J. M., & Bax, A. (2021). Hydrating the respiratory tract: An alternative explanation why masks lower severity of COVID-19. Biophysical Journal, 120(6), 994–1000.

Gandhi, M., & Rutherford, G. W. (2020). Facial masking for COVID-19 — Potential for “variolation” as we await a vaccine. New England Journal of Medicine, 383(18), e101.

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