Bulk materials handling operations involving falling streams of the bulk material are common throughout industry. Proper design of any fugitive dust control system servicing such operations requires knowledge of the behaviour of the free-falling stream in terms of the air entrained by the falling material and the concentration of dust liberated. In this paper results of experiments on alumina powder free-falling from a hopper are presented. It was found that the material stream initially contracts in cross-section as it accelerates and a boundary layer of dusty air develops around the core stream of bulk material. Results of air entrainment per unit mass of parent material are also given. The authors describe a simple analytical model of the complex air entrainment process by treating the falling stream of material as a negatively buoyant plume of dusty air fully miscible with the ambient air. The experimental data are successfully correlated using this approach. The plume model provides a more realistic description for the situation considered here than previous theoretical treatments based on air movement induced by isolated particles independently falling through quiescent air. © 1995, Hosokawa Powder Technology Foundation. All rights reserved.