Cryoelectron microscopy (cryo-EM) studies show that SARS-CoV-2 virions are particles near the larger end of the NP size range (70–90 nm).21,201 However, when dispersed into the air, the infectious particles exist as functionally larger particles. Initially, liquid droplets containing coronavirus virions originating from the respiratory tract of infected patients are emitted during normal breathing, forced expiration (e.g., coughing and sneezing), or aerosol-generating medical procedures (e.g., intubation and suctioning). Liquid droplets emitted into the air through these mechanisms originate from points throughout the respiratory tract and carry within them virions as well as other materials associated with the airways, including bacterial cells and epithelial cells (Figure 5). Droplets are emitted over a wide size range, and their potential viral burden is a cubic function of particle diameter (Figure 6). Thus, larger droplets have the potential to carry a significantly larger burden of virions according to their size and are substantially more hazardous than small droplets. Once emitted into the air, water from droplets immediately begins to evaporate. Water loss is associated with a rapid decrease in both particle diameter and terminal settling velocity (the equilibrium rate of fall of a particle in still air). The rate of evaporation of the largest droplets (>150 μm) is often insufficient to slow their swift descent in air, and they impact on nearby surfaces. However, the rapid water loss and sharp slowing of settling velocity of smaller droplets enables them to avoid a similar fate. Their constituent solid residues are instead drawn together during evaporation and cemented with dried respiratory secretions, and they remain aloft as droplet nuclei. Several epidemiological studies have supported the potential for droplet nuclei to be an important means of transmission for SARS-CoV-2.202,203