Mask Design for Covid-19

Simple Bernoulli and air permeability models are used to estimate the pressure developed inside a rigid covid-19 mask and the proportion of air flow passing through and leaking around the edges of the mask due to a sneeze or cough. Rather speculatively the results are extended to deal with the commonly used cloth masks with or without folds. This is done by introducing a constitutive law connecting the pressure developed within the space between the mask and the face and the volume of this space. The model identifies a dimensionless group and a design function that may be used to help evaluate the performance of masks in the covid-19 context.These measures may be simply measured in the laboratory. Simulations for rigid masks and cloth masks with or without folds are presented.