The problem of aerosol sampling under calm air conditions has been studied by solving the Navier-Stokes equations and integrating the equations of particle motion. Both particle inertia and settling are considered. The sampling efficiency of an inlet has been found to depend on two dimensionless parameters, the Stokes number and the relative settling velocity. A new criterion for accurate aerosol sampling in calm air is then derived. Experimental data of several researchers are discussed in the light of this new sampling criterion, and the data are found to be in accord with the theoretical predictions.