A new study by the National Institutes of Health (NIH) which will appear in the Biophysical Journal finds the humidity built up when using masks triggers an immune response which could explain why cloth masks have been effective in limiting COVID-19 spread.
Researchers found normal breathing in a mask creates an absorption cycle where super-saturated air is absorbed by the mask fibers when you exhale, followed by evaporation when breathing in dry air from the environment. Double-layered cotton cloth masks, the temperature of air inhaled rises above room temperature which increases the relative humidity.
The humidity in the mask promotes respiratory epithelium, which studies have shown is beneficial to your immune system. As explained in a 2011 study published in the American Society for Microbiology, the "airway epithelium acts as a frontline defense against respiratory viruses, not only as a physical barrier and through the mucociliary apparatus but also through its immunological functions. It initiates multiple innate and adaptive immune mechanisms which are crucial for efficient antiviral responses."
Researchers this could be another explanation why masks lower disease severity. The researchers also note mask fit is important since the humidification efficiency could drop through gaps in the mask. The study examined tight fits for all the masks and researchers believe the N95 and KN95 mask is most effective due to the tight fit and recommended for day-to-day use.
Surgical masks, due to their leakage, are expected to perform a bit lower than the N-95 and cotton masks.