Although snoring has long been linked to both chronic bronchitis in adults [1] and community acquired pneumonia in children [2], no mechanism for a causal relationship has yet emerged. Here, we present laboratory data supporting a mechanism whereby snoring generates pharyngeal fluid droplets that are carried, with their pathogens, deep into the lower respiratory tract (LRT) by the inspired airstream.

Radiotracer studies have shown that transfer of small amounts (≤ 100 µL) of oropharyngeal secretions into the lower respiratory tract (LRT) occurs at night during sleep in roughly half of healthy adults [3], a process generally referred to as microaspiration. Analogous import of oral microbiota into the lungs of healthy individuals has also been well documented [4]. Microaspiration is generally considered as the primary pathway for progression of upper airway infections to pneumonia, as documented for COVID-19 [5], but equally applicable to other bacterial and viral respiratory diseases [6]. Macroaspiration is commonly seen in obstructive sleep apnea (OSA) patients but involves the drawing of much larger quantities (> 0.5 mL) of fluid into the LRT [7], approximately doubling COVID-19 pneumonia risk [8].

While the word ‘microaspiration’ suggests a process analogous to macroaspiration, the sliding of tiny fluid droplets deep into the LRT, against the mucociliary clearance flow, defies the physics of fluids: Such micron-sized droplets would tightly adhere to the mucosal surface layer that covers the airways and not easily be moved by passing air. Moreover, deeper in the bronchial tree where the respiratory airflow invariably is laminar [9], surface-adhered particles cannot undergo net movements with the in- and outflow of passing air. By contrast, air can carry tiny fog-like droplets deep into the LRT. Only droplets smaller than about five micron will reach the lung parenchyma where their pathogen deposition can cause pneumonia [10]. Hence, we propose that both macro- and microaspiration include a pivotally important mode where droplets, generated by snoring, are carried by inspired air.

Snoring sounds result from soft tissue vibrations and the associated modulation of the airstream during inspiration, with anatomic details revealed by fiberoptic endoscopy [11]. Such video analysis showed periodic transient contacts between the back of the tongue and the soft palate, which flaps up and down driven by the inspired airstream. Fluid filaments are known to form between such transiently touching, wetted surfaces when they start to separate but are blown apart by the airstream, resulting in numerous tiny droplets. This droplet-generating mechanism was demonstrated to be highly productive for speech [12] but also is active during snoring. Because the inspired snoring airstream carries the particles first into the lung where they are inaccessible to traditional aerosol detectors, they could only be observed by generating an unnatural expiratory snoring sound [13]. Yet, following a regular inspiratory snoring sound, not all snoring-generated droplets “rain out” in the lung, and the fraction that remains airborne can be observed in subsequently exhaled breath. This exhaled fraction is analogous to cigarette smoke (particle size 0.1-1 micron) that is exhaled after first being drawn into the lung. Using chemical tracer technology, we here demonstrate that the exhaled snoring droplets are exclusively generated during inspiration when producing a snoring sound.