/Why might face masks reduce COVID-19 severity?

Why might face masks reduce COVID-19 severity?


  • Face masks reduce the spread of the new coronavirus and may also lessen the severity of COVID-19.
  • The present study finds that masks increase the humidity of the air that a person breathes.
  • The researchers propose reasons that explain why increased humidity may reduce the severity of COVID-19 cases.

Researchers have found that wearing a mask increases the humidity of the air a person breathes in.

They suggest this may explain why mask-wearing has links with reduced COVID-19 severity.

The research, which appears in the Biophysical Journal, adds further evidence for the value of wearing masks.

Face masks have been a crucial non-pharmaceutical intervention in the COVID-19 pandemic. Researchers believe they reduce the chances of a person passing the virus to another person, as well as acquiring the virus themselves.

Further, while healthcare professionals continue administering effective vaccines around the world, mask-wearing is still necessary.

The Centers for Disease Control and Prevention (CDC) point out there is not yet enough information on how these vaccines work in the real-world to recommend people stop wearing masks, even for those who have received two doses of the vaccines.

Recent research, which appears in the Journal of General Internal Medicine and the New England Journal of Medicine, found that face masks effectively reduce SARS-CoV-2 transmission and may also reduce its severity.

The researchers behind the articles suggest this is due to mask-wearers likely having exposure to reduced amounts of the virus. However, other scientists have contested this finding.

The researchers behind the present study are also skeptical that exposure to a small amount of the virus may account for a mask-wearer to have a less severe case of COVID-19.

Instead, they suggest the answer may lie in the humidity of the air that a person breathes while wearing a mask.

The researchers note that respiratory viruses, such as the common cold and influenza, often follow a seasonal pattern, with an increase in cases during the winter months. Some evidence also suggests this holds true with COVID-19.

Many factors may account for this, such as:

  • people spending more time indoors, where aerosols containing the virus can build up
  • people with a vitamin D deficiency
  • lower general levels of ultraviolet light, which may inactivate the virus
  • the virus being able to last longer in colder, less humid air

The researchers also point out that cold, less humid air can affect the physiology of the organism a virus might infect, inducing more severe infections. For example, researchers found that mice in low humidity developed more severe cases of influenza.

The researchers suggest that increased humidity may reduce the dehydration of a person’s lungs that can often occur during cold weather. They also believe increased humidity may improve a person’s mucociliary clearance — the way the lungs remove mucus and harmful particles.

Both of these could mean a reduction in the severity of a COVID-19 infection.

As a result, the researchers wanted to see whether the reduction in disease severity associated with mask-wearing could, in principle, be explained by masks increasing the humidity of the air a person breathes.

To find out if masks increase the humidity of the air a person breathes, the researchers asked participants to breathe into a sealed steel box wearing one of four common types of mask:

  • N-95 respirator
  • surgical mask
  • cotton-polyester mask
  • heavy cotton mask

The researchers then measured the box’s humidity, comparing this to the humidity measured when a person breathed into the box without wearing a mask.

This told the researchers how much moisture was being trapped by the mask, condensing on the inside of the mask and then evaporating and entering a person’s lungs as they inhaled.

The scientists carried out the test at three temperatures between 46–98oF (8–37°C).

The researchers found that all the masks increased the humidity of the air being inhaled by the participants, particularly at lower temperatures. The most effective was the heavy cotton mask.

The authors raise a note of caution that cotton masks might be the most effective. In their study, they ensured all face masks offered a sufficient seal against the face using a high density foam rubber rim.

However, in day-to-day use, N95 masks usually provide a much tighter fit and therefore are likely to perform better in increasing breathed air humidity.

According to the study’s co-lead author, Dr. Adriaan Bax, National Institutes of Health (NIH) Distinguished Investigator, “[w]e 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.”

“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,” Dr. Bax adds.

For National Institute of Diabetes and Digestive and Kidney Diseases Director Dr. Griffin P. Rodgers, the research adds further support to the need to wear masks:

“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.”

“This research supports the importance of mask-wearing as a simple yet effective way to protect the people around us and protect ourselves from respiratory infection, especially during these winter months when susceptibility to these viruses increases,” says Dr. Rodgers.

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