COVID-19 spread across the world with a speed and intensity that laid bare the limits of our understanding of the transmission pathways of such respiratory diseases. After much confusion and misinformation, there emerged a consensus that airborne transmission from very small respiratory droplets is the most important route for the spread of COVID-19. Each stage in this transmission pathway is mediated by complex flow phenomena, ranging from air-mucous interaction inside the respiratory tract, turbulence in the exhaled jet/ambient flow, to inhalation and deposition of these aerosols in the lungs. Given the emergence of the Delta-variant and the resurgence of infections in many communities, the importance of communicating infection risk across scientific disciplines, as well as to policy/decision makers, is more important than ever. Inspired by the Drake Equation that provides a framework to estimate the seemingly inestimable probability of advanced extraterrestrial life, I propose a relatively simple model for estimating the risk of airborne transmission of a respiratory infection such as COVID-19. The model couples ideas from fluid dynamics with factors involved in airborne transmission and predicts the effects of social distancing and facemask use on transmission risk. The model is designed to serve not only as a common basis for scientific inquiry across disciplinary boundaries, but to also be understandable by a broad audience outside science and academia.
