  A mathematical model for estimating the risk of airborne transmission of a
respiratory infection such as COVID-19, is presented. The model employs basic
concepts from fluid dynamics and incorporates the known scope of factors
involved in the airborne transmission of such diseases. Simplicity in the
mathematical form of the model is by design, so that it can serve not only as a
common basis for scientific inquiry across disciplinary boundaries, but also be
understandable by a broad audience outside science and academia. The caveats
and limitations of the model are discussed in detail. The model is used to
assess the protection from transmission afforded by face coverings made from a
variety of fabrics. The reduction in transmission risk associated with
increased physical distance between the host and susceptible is also quantified
by coupling the model with available data on scalar dispersion in canonical
flows. Finally, the effect of the level of physical activity (or exercise
intensity) of the host and the susceptible in enhancing transmission risk, is
also assessed.
