Accroding to the following paper: https://math.mit.edu/~bush/wordpress/wp-content/uploads/2014/04/Sneezing-JFM.pdf
The following figure demonstrates the spreading and settling of the particles from a cough cloud emitted horizontally. This cloud is of radius r(t) at position s(t) from the source, where r = alpha * S, while alpha is a constant known as the entrainment coefficient. and S = t^ 1/4
alpha was taken too be 0.03