||Hazard assessment following challenge by ambient aerosols requires accurate estimation of exposure, that is, determination of the total mass of particulate matter deposited within the respiratory tract, and its intercompartmental distribution. Aerosol deposition studies utilizing human test subjects yield valuable information on the deposition of inhaled particles in man, but experimental regimens lack the resolution to quantitate 'hot spots' within the lung. Aerosol deposition experiments with respiratory tract surrogates, or models, can be designed to be complimentary to human studies. The authors have put experimental modelling efforts into perspective per their applicability for the risk assessment of inhaled ambient aerosols. Heterogeneity of deposition has been detected in in vitro experiments over a wide range of particulate sizes and flow rate magnitudes--in a variety of surrogate larynx-TB models, for both constant and time-dependent flow patterns, and for both phases (inspiration and expiration) of a breathing cycle.