Filtering factor by an MMSN disk of planetesimals of size R_p,mean = 1 km and density in a MMSN gas disk at 1 AU, as a function of the dust stopping time τ_s for two values of the turbulence parameter α = 10^-4 (blue curves) and α = 10^-2 (red curves), respectively. The corresponding physical sizes of dust particles (top axis) assume a density of 1 g cm^-3 and the MMSN conditions at 1 AU. The solid lines are obtained from Eq. (60). The dashed lines indicate the results obtained when neglecting gas advection (τ_s,ν = 0). Small values of τ_s<α correspond to a situation in which the dust particles are mixed with the gas vertically and have a radial migration due to gas drag that is slower than advection by the gas. We note that the filtering factor is inversely proportional to R_p,mean, implying that for size distributions equivalent to that of the present-day asteroid belt (R_p,mean ~ 70 km), filtering would be very inefficient.