key: cord-0958546-ry129kn8
authors: Kohanski, Michael A.; Palmer, James N.; Cohen, Noam A.
title: Aerosol or droplet: critical definitions in the COVID‐19 era
date: 2020-04-23
journal: Int Forum Allergy Rhinol
DOI: 10.1002/alr.22591
sha: 6c481e0125ced6c1ea2fe1ac9cc9f669ba35eefc
doc_id: 958546
cord_uid: ry129kn8

nan

To the Editor, In the recent publication by Workman et al., (1) entitled "Endonasal Instrumentation and Aerosolization Risk in the Era of COVID-19: Simulation, Literature Review, and Proposed Mitigation Strategies", the authors examine spread of fluorescent particles during a range of endonasal procedures using a cadaver model and propose guidance on risk of aerosolization with these procedures. This study is designed to measure droplet spread, not aerosol deposition or settling. Infectious aerosols are defined as particles under 100 microns in diameter that are suspended in a gas and can be respired (2, 3) . Aerosol particles between 10 and 100 microns tend to deposit in the upper airway whereas particles under 5-10 microns in size are the airborne particles that can bypass the upper airway and penetrate deep into the lungs (2, 4, 5) . Aerosol movement, deposition and surface settling times are generally influenced by air flow rates in the local environment (2, 4) . Simulation of aerosol movement in an exam room demonstrated that aerosol can spread throughout the room within 5 minutes and aerosol clearance is highly dependent on the number of air changes per hour (6) . These fundamental properties of aerosols together with characteristic spread patterns for droplets form the basis for guidelines for PPE use for airborne versus droplet precautions (3).

Workman et al., (1) utilize an atomizer to create a fluorescent layer of particles ranging in size from 30 to 100 microns in diameter. However, the particles produced during the simulation of aerosol generation by the atomizer alone and during the endoscopic procedures are not limited to this size. The fluorescent particles can attach to other larger particles through hydrostatic forces, including moisture inside the cadaver, or random sized particles generated by the described endoscopic procedures. These aggregate particles are then expelled from the nose as droplets based on the velocity of the initial spray or net velocity of air movement generated by the endoscopic procedure. This is supported by the results of Figure 3A in the manuscript. In Figure 3A , the size of particles detected outside the model system after expulsion of fluorescent particles generated by the atomizer are in the range of 100's of microns to about 1500 microns in size, well outside the defined range of infectious aerosols which are less than 100microns in diameter. This is also not a limitation of the reported detection limit, as the authors report an estimated detection limits down to 20 microns.

The above points are not merely a matter of semantics, rather, these definitions are critical to understanding the physical behavior of these micro and nanometer scale particles, and their propensity to linger in the air or spread across and contaminate the local environment. This is of particular importance for COVID-19 as the SARS-CoV2 spike protein receptor, ACE-2, is highly expressed on type II airway pneumocytes (7, 8) and

SARS-CoV2 can survive in a closed environment as an aerosol for at least 3-hours with an estimated half-life in aerosol of 1-hour (9) . When considering the guidance on risks associated with an aerosol generating procedure, we need to remain cognizant and account for the multiple components involved in the spread of infection with SARS-CoV2. Variables to consider include 1) the mechanism of spread (contact, droplet or aerosol), 2) the minimum viral titer and length of exposure required to cause an infection with these various modes of spread, 3) factors that would increase host susceptibility to infection by SARS-CoV2 and 4) host factors that would lead to a severe form of COVID-19. While we are learning more about COVID-19 daily, many of the answers to these questions are currently unknown. The adapted face covering intervention to limit droplet spread demonstrated by Workman et al.,

(1) is likely to be an effective, practical method for limiting droplet and contact spread of infectious particles in an analogous fashion to data supporting wearing masks in public reducing risk of disease transmission (10) . However, as Workman et al., note in their discussion, more rigorous studies are required before we can determine the relative safety of various aerosol-generating procedures.

Endonasal instrumentation and aerosolization risk in the era of COVID-19: simulation, literature review, and proposed mitigation strategies. Int Forum Allergy Rhinol

Preventing Transmission of Pandemic Influenza and Other Viral Respiratory Diseases: Personal Protective Equipment for Healthcare Personnel: Update

Aerosol transmission of infectious disease

Routes of influenza transmission. Influenza Other Respir Viruses

Dispersion and exposure to a cough-generated aerosol in a simulated medical examination room

Tissue distribution of ACE2 protein, the functional receptor for SARS coronavirus. A first step in understanding SARS pathogenesis

Cell cycle dependence of ACE-2 explains downregulation in idiopathic pulmonary fibrosis

Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1

Face masks to prevent transmission of influenza virus: a systematic review

This article is protected by copyright. All rights reserved.