Heterogeneity of Airborne Virus Transmission in the Built Environment: A Narrative Review

The COVID-19 pandemic accelerated recognition of airborne transmission of respiratory infections. Despite improved mechanistic understanding, our ability to predict the risk of infection in a given scenario remains limited, owing, in part, to high heterogeneity in factors involved in transmission. The goal of this work is to quantify variability in factors that control risk of airborne virus transmission in the built environment. Using SARS-CoV-2 as a model, we conducted a narrative review on transmission via inhalable respiratory particles (≤100 μm). We divided transmission into 3 key processes: emissions of virus in respiratory particles from an infected individual (the source) into the air; transport and decay of virus in the environment; and deposition and infection in a new host (the receiver). Among the source-related factors, we found large variabilities spanning 7 or more orders of magnitude. For example, the rate of respiratory emissions from an infected individual ranges from ~1 to > 10⁷ particles per second, depending in part on the type of respiratory activity and individual physiological factors. Unexplained inter-individual variations, such as those defining "superemitters," introduce additional uncertainties. We also found considerable variability in the physical and biological decay of virus-laden particles as they move from the source to the receiver. The air exchange rate, which controls physical loss of particles by ventilation, ranges from ~0.01 to > 20 air changes per hour in typical buildings. Lastly, upon inhalation, the efficiency and site of virus deposition vary with the physiological state of the receiver, whose immunological status further influences whether infection is established. Overall, we found the largest variability in the rate of respiratory particle production and viral load of respiratory fluid. These 2 factors determine the amount of virus released into the air by an infected person, which is a critical indicator of the potential for onward transmission. Predicting the risk of infection in a specific scenario remains challenging due to the combined effects of variability in key determinants.