Table 1.

Characteristic time scales and derived dimensionless parameters.

Time scale (s)		Dimensionless parameter
${\tau }_i=\frac{1}{{\mu }_i}$	Disease infectivity
${\tau }_t=\frac{{\alpha }_d}{{\beta }_d{\kappa }_d}$	Disease transmission	$R_0\equiv \frac{{\beta }_d{\kappa }_d}{{\alpha }_d{\mu }_i}=\frac{{\tau }_i}{{\tau }_t}$
${\tau }_r=\frac{1}{{\alpha }_d}$	Droplet removal (droplet infectivity)	$\lambda =\frac{{\mu }_i}{{\alpha }_d}=\frac{{\tau }_r}{{\tau }_i}$
${\tau }_c=\frac{l}{v}$	Convective	$\nu =\frac{v}{{\alpha }_{d}l}=\frac{{\tau }_r}{{\tau }_c}$
${\tau }_{d,p}=\frac{l^2}{D_{d,p}}$	Diffusion (droplet, person)	${\eta }_{d,p}=\frac{D_{d,p}}{{\alpha }_d{l}^2}=\frac{{\tau }_r}{{\tau }_{d,p}}$