Aerosol transport of viruses during breathing and speaking, as is common during social interactions of people, is largely unstudied. There is significant recent evidence that the fluid dynamics behind these flows may play a significant role in the spread of Covid-19. The rapid spread of Covid-19 highlights the lack of effective guidelines and mitigation strategies for reducing the impact of a contagious virus in the absence of a vaccine. Recent evidence from hospitals, and even the press, suggests that the fluid dynamics of this transport pathway should be re-investigated. The focus of this research will be to quantify the complex flows associated with speech and breathing during a conversation or nearby encounter and show how transmission of particles between individuals occurs. The understanding gained from this work will provide actionable mitigation strategies to reduce transmission from asymptomatic people. These outcomes can be useful in the short term by offering routes to reducing potential viral transport and future infections.This research will show how the inherent structural flow difference between exhalation and inhalation during speech or simple breathing could be a potent yet unsuspected transport mechanism for pathogen transmission. The results will quantify an important aspect of this transmission, which in the case of Covid-19 is associated with asymptomatic carriers during relatively close social interactions. The quantitative characterization of this important topic surrounding viral transmission has largely been absent from the fluid mechanics and transport phenomena literatures, and even absent more generally from quantitative studies of public health. Using flow visualization and numerical simulations, the investigation will decipher the fluid dynamics in the neighborhood of the head of an individual during breathing and speaking, and the corresponding environmental mixing important to potential transmission (inhalation of particles by a nearby person). This approach will allow quantifying the fraction of particles exhaled by one person that are inhaled by a nearby person as a function of their separation distance. In these ways, the research will shed new light on a simple yet fundamental mechanism of transport associated with simple human activities, i.e., breathing and speaking, which can be generalized to many pathogens and aid in the identification of potential mitigation strategies for disease transmission by aerosols.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.