What aerosol physics tells us about airborne pathogen transmission

While a global pandemic by SARS-CoV in 2002–2003 was averted by fast identification, effective surveillance, and quarantine, such measures cannot be easily transferred to new situations, as the case of SARS-CoV-2 demonstrates. Knowledge of the fundamental biological and physical parameters affecting transmission pathways of respiratory pathogens is thus critical for the design of effective non-pharmaceutical intervention strategies. Respiratory pathogens are transmitted via respiratory droplets containing lung fluid laden with infectious pathogens. The generation, transport and eventual fate (deposition, inhalation) of respiratory droplets are key processes in the transmission pathways of respiratory pathogens. Respiratory droplets are generated within the human respiratory tract, thoracic or extrathoracic, with possibly different pathogen loads, or upon release from an infected person via lung-fluid fragmentation (Bourouiba, Dehandschoewercker, and Bush 2014). They are expelled by expiratory events that include violent events such as coughing or sneezing and quiescent ones such as talking, breathing, or laughing. Respiratory droplets have been associated with three modes of pathogen transmission: in the medical literature, these are referred to as “contact,” “droplet,” and “airborne” transmission modes (Weber and Stilianakis 2008a). Contact transmission, be it direct or indirect, occurs via contact with pathogen-laden droplets: transfer of pathogens via physical touch between a susceptible and an infected host (e.g., hand contact) is classified as direct contact transmission, whereas transfer mediated by fomites containing settled droplets is classified as indirect contact transmission. Droplet transmission refers to transmission by large droplets (diameter dp > 20 microns) that are transported by the turbulent air flow generated by a violent expiratory event. They are, subsequently, sprayed and directly deposited upon the conjunctiva or mucus membranes of a susceptible host. Since large droplets gravitationally settle rather quickly, droplet transmission is considered important at close range: in still air, a 50-micron droplet crosses a vertical 1.5m distance in 20 s (Drossinos and Housiadas 2006). Airborne transmission, also referred to as “aerosol transmission,” refers to pathogen transmission via inhalation of small respiratory droplets (typically smaller than 10 microns: a 10-micron droplet settles gravitationally in still air within approximately 9min). Being relatively small they may deposit deep into the respiratory tract, including the alveolar region. These droplets, often referred to by the confusing term “droplet nuclei,” are small enough to remain airborne for sufficient time to transmit the pathogen. Hence, airborne transmission does not require direct face-to-face contact. The demarcation between the three transmission modes is not clearly specified, as it is not based on welldefined droplet physical properties or their dynamics, often creating confusion. For example, droplets associated with droplet transmission may be transported by a turbulent jet and subsequently inhaled. The classification of transmission modes depends on the size of the droplets. The size distribution of expelled droplets has been a subject of considerable research and controversy (partly attributable to different instrumentation or collection methods). Nevertheless, it is reasonable to consider that respiratory-droplet diameters vary from 0.5 microns to 1000 microns (Duguid 1946; Loudon and Roberts 1967, Papineni and Rosenthal 1997; Chao et al. 2009; Morawska et al. 2009; Asadi et al. 2019). Care should be exercised in interpreting droplet sizes. Respiratory droplets are generated in a nearly 100% relative-humidity environment. Upon exhalation into the lower-humidity ambient environment they shrink by evaporation (a fast molecular process, of the order of seconds or less, depending on droplet size, composition, and relative humidity) to reach their equilibrium diameter. Some estimates suggest that droplets may shrink to about half their original size (Nicas, Nazaroff, and Hubbard 2005, Parienta et al. 2011). Not all pathogen transmission modes are relevant for all respiratory infections. The dominant transmission mode will depend on the interplay of a number of factors, including frequency of violent droplet-generating events (coughing, sneezing), droplet size distribution, ambient relative humidity, viral load, virus inactivation rates, deposition location of inhaled droplets in the airway, and infectious dose. The identification of the dominant transmission mode is essential for a proper and efficient strategy to control the spread of an epidemic, including the proper choice of personal protective equipment. The companion Aerosol Science and Technology Editorial (Asadi et al. 2020) argues cogently for the