Aerosol generation from the respiratory tract with various modes of oxygen delivery

Original: Am J Respir Crit Care Med 2020 Oct 15; 202: 1115-1124

Author(s): Nathaniel T. Gaeckle, Jihyeon Lee, Yensil Park et al

Respiratory digests
Respiratory infections
Aerosol generation from the respiratory tract with various modes of oxygen delivery - article image

Reviewer: Emer Kelly, Incoming e-Learning Director

Published: 22 Nov, 2020

There has been increased use of high-flow nasal cannula (HFNC) and non invasive ventilation (NIPPV) in the era of COVID-19 and in particular the severe acute respitatory syndrome coronavirus 2 (SARS-CoV-2). The implications of using these oxygen delivery modalities in a disease that is highly contagious with airborne and droplet transmission is concerning.

This small study sought to measure the size and concentration of particles and droplets generated from the respiratory tract of 10 healthy human volunteers on the following oxygen modalities: room air, 4 L/min nonhumidified nasal cannula (NC), 15 L/min nonhumidified face mask (FM), HFNC at 10 L/min, HFNC at 30 L/min, HFNC at 50 L/min, NIPPV with an inspiratory positive airway pressure / expiratory positive airway pressure of 12/5 and 20/10 cm H2O. The study was conducted in a negative pressure room. An aerodynamic particle spectrometer (APS) was used to take 5 separate measurements:

  1. background particle measurement of the room, with the funnel sampler directed away from the participant
  2. normal breathing
  3. talking which was reading at a normal volume
  4. deep breathing
  5. five deep coughs

The funnel of the APS was directed towards the participant’s mouth for measurements 2-5. Using the APS data, particle concentration and geometric mean diameter were calculated.

The average background particle concentration in the room was 0.060 particles / cm3. The median particle concentration exhaled from the respiratory tract ranged from 0.041 to 0.168 particles/cm3. With the exceptions of room air, participants inhaled particle-free oxygen gas, so the measured particles presumably originated from participants. After adjustments for the multiple comparisons, the number and size of particles measured from the respiratory tract did not significantly change with the oxygen modalities tested. Coughing produced a higher number of particles than the other manoeuvers tested and was the only manoeuver to significantly increase measured particle number above the background room concentration.


  • In this study of healthy individuals, NIPPV or HFNC did not produce higher- concentration aerosol from the respiratory tract than room air or nonhumidified oxygen conditions. In fact in some instances HFNC and NIPPV decreased aerosol. Different respiratory patterns and characteristics had more impact on aerosol generation than the mode of oxygen delivery.
  • This was a small study and describes findings in a healthy population and does not take into account the effect of relative humidity and local ventilation on particle distance travelled. It gives interesting information but the cannot be readily applied to the treatment of patients with SARS-CoV-2.