The dispersion of an aerosol bolus in models of lung airway with one, two or three bifurcations was analyzed numerically for steady expiratory flow. The broadening of the concentration peak was converted into an effective diffusivity D-eff. It was possible to express local D-eff for each generation as an approximately linear function of local mean flow velocity tl and diameter d. During exhalation, unlike in inhalation, the average proportional constant D-eff/u(0)d(0) does not change over several generations. In exhalation D-eff for particles is 30% lower than for gases just as in inhalation. Results of the present study on total dispersion were in excellent agreement with experiments when the difference in diffusivity between gases and particles were corrected for.