It is shown how the uniformity of a polydisperse aerosol can be assessed by means of a segregation index, defined in terms of the mean and the mean square of n(D-p;(r) over right arrow )dD(p), the number concentration of particles with diameter in the range (D-p, D-p + dD(p)) in a volume element centered at (r) over right arrow. The segregation index is zero for a uniform aerosol and attains positive values for any other situation, in such a manner that the larger the value of the index, the worse the aerosol uniformity. As an illustrative example, numerical calculations have been performed for a polydisperse aerosol undergoing simultaneous diffusion and electrostatic dispersion in a laminar flow tube. In this case, the segregation index, zero at the tube entrance (uniform aerosol), increases steadily as the aerosol approaches the tube outlet. Some practical implications for aerosol sampling from a duct are discussed. (C) 2014 Elsevier Ltd. All rights reserved.