Heterogeneity of Airborne Virus Transmission in the Built Environment

Since the COVID-19 pandemic, airborne transmission via respiratory particles has gained widespread recognition as a critical pathway for infectious disease spread. Airborne transmission can be divided into three key processes: generation of virus-laden respiratory particles from an infected person; transport of the particles through the air; and finally, deposition and infection in a susceptible new host. Airborne virus transmission can occur at both short- and long-range distances, and it involves particles with a diameter as large as 100 µm. Despite improved mechanistic understanding of transmission, our ability to predict the risk of infection in a given scenario remains limited owing to high heterogeneity across different transmission scenarios. Here, we identify factors that control the risk of airborne transmission and attempt to quantify the variability in each factor. Using SARS-CoV-2 as a model, we conducted a narrative review of selected literature on transmission via inhalable respiratory particles. We found that variability exceeding one order of magnitude was common among quantifiable factors in all three processes of transmission. Notably, substantial variability was found in the emission rate of respiratory particles (~1 to > 107 particles/s), depending in part on individual physiological factors and type of respiratory activity. We anticipate that this review will help improve our understanding of disease transmission. Models, including personal exposure risk assessment and population-level prediction of disease spread, should carefully consider the variability in key factors governing the risk of airborne transmission.