Poor hydration has myriad adverse effects on the human body’s ability to resist infection and appears to be a common underlying factor in phenotypical states at high risk of COVID-19 (1, 4). Our findings across study sites in the US, Germany and India appear to be the first to specifically implicate laryngeal dehydration as a factor in the susceptibility to and worsening of symptoms in COVID-19.
Consistent with our previous work (43), we found that exhaled aerosol is highest in the elderly (Fig. 2B), the obese (Figs. 2C, 2D), and those infected by SARS-CoV-2 (Fig. 2E), relative to young and healthy (low BMI, non-infected) human subjects. We also find that exhaled aerosol is amplified in young and healthy human subjects following exercise-induced whole-body dehydration — reaching levels (Figs. 3A, 3B) otherwise observed in the non-infected elderly and obese human subjects (Figs. 2B, 2C). Topical hydration of young and healthy (exercise-induced dehydrated) subjects with salt droplets sized (over 80% of the droplets larger than 7 µm) to mostly deposit above the carina diminishes exhaled aerosol to normal low levels even while the subjects remain in a whole-body dehydrated state (Fig. 3C). These findings are consistent with recent findings (32) that healthy human subjects on moving from a dry air environment to a humid environment exhaled significantly fewer respiratory droplets. The central and lower airways being insensitive to the humidity of the environment, our results suggest that the breathing of humid air reduces dehydration of the upper airways, providing upper airway hydration similar to what we observe following hypertonic salt delivery in our exercise-induced dehydration study (Fig. 3C). Indeed in the previous work (32) we find that delivery of isotonic saline or hypertonic saline with 8-12 µm mean-diameter salt droplets diminishes exhaled aerosol to levels equivalent to the breathing of humid air — and consistent with what we observe in the present study.
Laryngeal respiratory droplet generation appears to be driven by the fact that on normal inhalation air passing through the larynx accelerates in the vicinity of the glottis (Fig. 1B) to attain turbulent flow conditions (Re up to 8000) at the center of a laryngeal jet of air that exits the glottis and enters the trachea (25, 26). This jet of air shears airway surface water lining the glottis, trachea and main bronchi closest to the carina threatening surface instability when Reynolds numbers exceed around 5000 (27). When the airway lining fluid is depleted either by the breathing of dry air or by systemic dehydration, its volume diminishes, salt concentrations increase, and surfactant concentration on mucus airway lining fluid increases. Enhanced surfactant concentration on airway lining fluid destabilizes surfaces and promotes droplet breakup (34, 35). On the other hand, delivery of hypertonic salt droplets to the trachea and main bronchi increases water content both by the delivery of water mass and the hypertonicity of the water delivered. Our recent work (32, 37-39) indicates that when the salts are divalent they prolong the stabilization effect of the mucus surface for 4 - 6 hours relative to 1-2 hours on the breathing of humid air or the delivery of sodium chloride salt to the same anatomical regions.
Analysis of respiratory droplet composition has revealed the presence of lung surfactant and the absence of mucin, implicating the smaller airways as a more probable site of generation than the upper airways (44). However, mucus exists in the upper airways as a hydrogel (45). Evaporating hydrogels naturally develop a thin film of water over free air surfaces in the process of seeking equilibrium with moisture in the air (46). In the first few minutes of evaporation, hydrogels therefore tend to evaporate at a rate that is indistinguishable from water itself — while with the thin water film disappearing, evaporation rate precipitously declines (47). In the upper airways, evaporation occurs from mucus surfaces on inhalation, hydrating inhaled air, while on exhalation highly humid air from the central and lower airways passes over upper airway mucus surfaces. The ability of the lungs to properly hydrate inhaled air prior to the penetration of air into the central and lower airways is contingent on the lungs releasing to the external environment approximately 1/2 liter of water per day (5). With the preponderance of airway lining fluid volume existing in the small airways and alveolar region of the lungs (5), movement of airway lining fluid from the lower to the upper airways is essential in the form of condensate from the fully saturated air exhaled out of the lungs, as well as deposition of respiratory droplets generated in the small airways (Fig. 1C). While further research is needed to clarify the nature of the phenomenon, conceivably the combination of water film over the surface of upper airway mucus, transfer of airway lining fluid from the lower airways to the upper airways on exhalation, and unlikelihood of hydrogel (mucin) molecules aerosolizing under the shear forces that otherwise easily breakup water surfaces, contribute to respiratory droplets, wherever they form in the airways, tending to be of similar composition.
Small airway respiratory droplet generation also occurs (31, 44). In our study we find particularly that when subjects deeply exhale and inhale, often referred to as residual volume breathing, exhaled aerosol numbers are much higher than on normal tidal breathing (see Supplemental Material), and as these numbers are not appreciably diminished by upper airway delivery of hypertonic salts (Supplemental Material), these residual-volume-breathing respiratory droplets appear to originate largely in the smaller airways. Within the airways there then appears to be a traffic of respiratory droplets from the upper airways to the lower airways, and from the lower airways to the upper airways, this traffic predominating in one direction or the other depending on many factors that should be further explored, including the degree of air volume expired, the rate of air flow, and the health and hydration state of the individual. We particularly find in the present study that the respiratory droplet generation in and movement from upper to lower airways is detrimental to lower respiratory tract disease, and notably to COVID-19
We observed a growth in exhaled respiratory droplets of 323% among the 87 patient volunteers (Figure 1B) from December 2020 to June 2021 as the delta variant grew from a small minority of cases to greater than 60% of sequenced infections (41). Differences in average age and disease severity of patients in our study (Table 1) were insignificant between December 2020 and June 2021, as were environmental factors including relative humidity and air quality, suggesting that increase in proportion of delta variant infections may be a cause of the amplification of respiratory droplet generation over the duration of our study and a factor in the increased contagiousness of the delta variant of SARS-CoV-2. In general we find that respiratory droplet numbers are higher in those infected by SARS-CoV-2 (Figs. 2E) than non-infected individuals (Fig. 2A), consistent with our previous report (43). Causes of this growth in exhaled aerosol with the advance of the delta variant infections may be multiple (48), including variances in upper airway dehydration and variances in surface activity — respiratory droplet generation having been shown to increase with the addition of surfactant to airway lining mucus (35), which increases propensity of airway lining mucus to break up (29).
Our observations of reduced symptoms and need for intravenous antibiotic and steroid intervention with laryngeal and tracheal hydration suggest that upper-airway respiratory droplet generation may contribute to the worsening of symptoms of COVID-19 owing to progression of the virus deeper into the lungs by the breakup of airway lining fluid in the upper airways where SARS-CoV-2 infection generally begins. This hypothesis is at least consistent with the preponderance of data pointing to the role of respiratory droplets in the airborne transmission of COVID-19 (49, 50). Diminution of respiratory droplet generation, and hydration of the glottis, by the delivery of upper airway hypertonic salts therefore reduces symptoms and improves oxygenation, both of which may contribute to a diminished need for intravenous drug intervention in those patients with high inflammation.
The impact of upper airway hydration on oxygenation needs further study. We find that oxygen saturation increases in moderately symptomatic COVID-19 patients over three days of daily administration of calcium-rich hypertonic salts (Fig. 4C), as well as during exercise-induced dehydration, although the latter conclusion is based on a small data set and must be validated by further study. Possibly glottal aperture, known to increase with increasing glottal hydration (24, 25), diminishes in early stage COVID-19 exercise-induced dehydration. In processes of physical exercise and progression of COVID-19, upper airway dehydration may add to the effects of elevated release of oxygen by hemoglobin (in states of exercise) and impairment of oxygen absorption in the gas exchange regions of the lungs (in symptomatic disease), thus contributing to low oxygen saturation before and after the latter play a predominant role. Further research is obviously needed.
Mechanistically, future research should explore the impact of laryngeal hydration by (monovalent, divalent, isotonic and hypertonic) salts in laryngeal models as well as on oxygenation and respiratory droplet generation in the elderly, the obese, diabetics, athletes and those with an airborne infection. Clinically, confirmation of the principal findings of our study in asymptomatic and symptomatic COVID-19 patients should be pursued in larger patient cohorts with particular attention to accurately quantifying viral RNA in exhaled breath.
We believe the growing global respiratory health crisis makes it especially important to explore prophylactic, therapeutic and anti-contagion benefits of regular daily upper airway hygiene or hydration or hygiene in populations at high risk of respiratory disease and in low-income environments where they are otherwise without access to proper hygiene (wearing of clean face masks, social distancing, and other modes of hygienic living including hand-washing), drugs and vaccination.
As a simple, safe, non-drug, daily hygiene strategy for improving upper airway hydration and natural clearance of inhaled contaminants, including SARS-CoV-2, divalent cation hypertonic salt delivery to the larynx and trachea appears a promising approach. A non-pathogen specific hygienic approach for respiratory disease could be an inexpensive and easily-adopted approach to maintaining global respiratory health in the light of the ongoing pandemic and the worsening of air quality associated with climate change.