  The utility of wearing simple cloth face masks is analyzed using
computational fluid dynamics simulations. We simulate the aerodynamic flow
through the mask and the spatial spread of droplet ejecta resulting from
respiratory events such as coughing or sneezing. Without a mask, a turbulent
jet forms, and droplets with a broad size distribution are ejected. Large
droplets (greater than about 125 {\mu}m in diameter) fall to the ground within
about 2 m, while turbulent clouds transport a mist of small aerosolized
droplets over significant distances (~ 5 m), consistent with reported
experimental findings. A loosely fitted simple cotton cloth mask (with a pore
size ~ 4 microns) qualitatively changes the propagation of the high velocity
jet, and largely eliminates the turbulent cloud downstream of the mask. About
12\% of the airflow leaks around the sides of a mask, considering a uniform gap
of only 1 mm all around, between the face and the mask. The spread of ejecta is
also changed, with most large droplets trapped at the mask surface. We present
the viral load in the air and deposited around the person, and show that
wearing even a simple cloth mask substantially decreases the extent of spatial
spread of virus particles when an infected person coughs or sneezes.
