Experimental deposition patterns of cigarette smoke in surrogate human airway systems are very heterogeneous. Particle deposits are enhanced at predictable, well-defined morphological regions; most specifically, carinal ridges within bifurcation zones and along posterior sections of tubular airways. The efficiency of the mucociliary transport mechanism in vivo is also reduced at airway branchings. The geometrical sites of preferential particle deposition and impaired clearance can be correlated with clinically observed anatomical sites exhibiting increased incidences of bronchogenic carcinomas. These locations are not compatible with current theoretical models simulating only the usual particle deposition processes of inertial impaction, sedimentation, and diffusion, while intending to account for particle hygroscopicity. Moreover, data from human subject exposures indicate that heretofore unknown factors affect the distribution of inhaled cigarette smoke. Herein, a new mathematical model is presented that explains cigarette smoke deposition patterns, including bifurcation "hot spots," in terms of composition and cumulative density. The behavior of mainstream cigarette smoke can be related to physicochemical parameters of its particulate and vapor-gas phases and is a result of two distinct effects: (1) particle cloud motion and (2) vapor-gas behavior. In lung airways, Effect 1 is the most prominent. The high particle number,ns approximately equal to 3 x 10(9) cm-3, and mass,rho s approximately equal to 10(-4) g cm-3, concentrations of smoke dictate that a bolus of it has kinetic properties of an entity (Effect 1 above), independent of the aerodynamic size characteristics of individual constituent particles. This motion may be exacerbated by the bulk movement (Effect 2 above) of the vapor-gas phase density of smoke.