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Public health experts recommend wearing masks to reduce the risk of developing COVID-19. Evidence indicates that the masks serve as simple barriers to prevent viral transmission during high-risk scenarios. Findings from a new study suggest that the humidity inside masks contributes to reduced severity of COVID-19 outcomes.

Viral respiratory infections such as colds and influenza exhibit seasonal characteristics and commonly occur during colder weather, when indoor air humidity is often low. Studies in mice have shown that susceptibility to influenza increased in low humidity.

The current study measured the humidity of the respired air inside four types of commonly worn masks – an N95 mask, a disposable surgical mask, a two-layer cotton-polyester mask, and a heavy cotton mask. Volunteers breathed into a sealed steel box in which the humidity was measured. Air temperatures during the experiment ranged from 46° to 98° F.

The authors of the study found that all four mask types increased the humidity of the volunteers' respired air, especially at lower temperatures. Humidity inside the thick cotton mask was highest compared to the others, regardless of temperature.

These findings demonstrate that the protective effects of wearing a mask may be due the increase in humidity in the inspired air of the user. They also underscore the importance of wearing masks indoors where relative humidity may be low.

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