Airborne transmission of respiratory viruses

Abstract

The COVID-19 pandemic has revealed critical knowledge gaps in our understanding of and a need to update the traditional view of transmission pathways for respiratory viruses. The long-standing definitions of droplet and airborne transmission do not account for the mechanisms by which virus-laden respiratory droplets and aerosols travel through the air and lead to infection. In this Review, we discuss current evidence regarding the transmission of respiratory viruses by aerosols-how they are generated, transported, and deposited, as well as the factors affecting the relative contributions of droplet-spray deposition versus aerosol inhalation as modes of transmission. Improved understanding of aerosol transmission brought about by studies of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection requires a reevaluation of the major transmission pathways for other respiratory viruses, which will allow better-informed controls to reduce airborne transmission.

Phases involved in airborne transmission of respiratory viruses.

Virus-laden aerosols (<100 I1/4m) are first generated by an infected individual through expiratory activities, through which they are exhaled and transported in the environment. They may be inhaled by a potential host to initiate a new infection, provided that they remain infectious. In contrast to droplets (>100 I1/4m), aerosols can linger in air for hours and travel beyond 1 to 2 m from the infected individual who exhales them, causing new infections at both short and long ranges.

Fig. 1.. Airborne transmission of respiratory viruses.

Phases involved in the airborne transmission of virus-laden aerosols include (i) generation and exhalation; (ii) transport; and (iii) inhalation, deposition, and infection. Each phase is influenced by a combination of aerodynamic, anatomical, and environmental factors. (The sizes of virus-containing aerosols are not to scale.)

Fig. 2.. Physicochemical properties of virus-laden aerosols.

The behavior and fate of virus-laden aerosols are inherently governed by their characteristic properties, including physical size, viral load, infectivity, other chemical components in the aerosol, electrostatic charge, pH, and the air-liquid interfacial properties.

Fig. 3.. How long can aerosols linger in air?

Residence time of aerosols of varying size in still air can be estimated from Stokes’ law for spherical particles (116). For example, the time required for an aerosol of 100, 5, or 1 μm to fall to the ground (or surfaces) from a height of 1.5 m is 5 s, 33 min, or 12.2 hours, respectively.

Fig. 4.. Factors affecting indoor airborne transmission.

Whereas the motion of large droplets is predominantly governed by gravity, the movement of aerosols is more strongly influenced by airflow direction and pattern, type of ventilation, and air filtration and disinfection.

Fig. 5.. Size-dependent aerosol deposition mechanisms to sites in the respiratory tract.

(A) Main deposition mechanisms and corresponding airflow regimes in different regions of the human respiratory tract. Large aerosols tend to deposit in the nasopharyngeal region as a result of inertial impaction, whereas small aerosols tend to deposit in the tracheobronchial and alveolar regions on the basis of gravitational sedimentation and Brownian diffusion. An enlarged view of tracheobronchial and alveolar regions illustrates the deposition mechanism. (B) The deposition efficiency of aerosols at different regions of the respiratory tract as a function of aerosol diameter based on the ICRP lung deposition model is shown (116). The majority of large aerosols deposit in the nasopharyngeal region; only aerosols that are sufficiently small can reach and deposit in the alveolar region.