Research from the National Institute of Diabetes & Digestive & Kidney Diseases (NIDDK) and partners confirms the benefits of wearing masks to protect against COVID-19, but also suggests that other methods of hydrating the upper airway could also reduce the risk of COVID-19 incidence and symptoms.
In a randomized four-arm study of 21 healthy human volunteers, the researchers found that breathing humid air, wearing cotton masks, and depositing salt droplets in the nose, trachea, and main passages to the lungs similarly reduce the exhalation of respiratory droplets - by which the virus that causes COVID-19 is spread - by about half within 10 minutes. Respiratory droplet generation returned to relatively high baseline levels within 60–90 minutes of returning to dry air following the first two interventions, but after calcium and magnesium salt exposure, suppression continued for four to five hours.
The findings are consistent with earlier studies suggesting that cotton masks humidify inhaled air, and suggest that this humidification may be one factor underlying the success of mask wearing for reducing COVID-19 transmission.
"The breathing of moist, salty air may lower the transmission potential of SARS-CoV-2 by lowering respiratory droplet exhalation, acting in this sense as a mask," the authors wrote, while cautioning that the mechanics of this process need further study.
The authors, who included researchers from the Harvard School of Engineering and Applied Sciences and Massachusetts General Hospital as well as NIDDK, published their findings on August 31 in Molecular Frontiers Journal. The study was funded by olfaction technology start-up Sensory Cloud Inc. and in part by the National Institutes of Health.
To further explore the possible connection between humidity and COVID-19 risk, the researchers also used an ecological regression model to analyze county-level COVID-19 trends by climate zone between January 2020 and March 2021. The analysis suggested that exposure to elevated airborne salt on (Gulf and Pacifc) US coastlines appears to suppress COVID-19 incidence and deaths per capita by approximately 25%–30% relative to inland counties — after controlling for 10 potential confounding environmental, physiological, and behavioral variables including humidity.
"Practically, the safety, naturalness, and ease of access of exposure to moist, salty air, suggests that a range of approaches to upper airway hydration might be encouraged to improve respiratory health and well-being," the authors wrote. "Airway hygiene seems a special opportunity today given the realities of the COVID-19 pandemic and the longer-term global crisis of dirty air."