Transient diffusion of indoor air and contaminant elements affects human comfort and health within short periods of time. This paper presents the method of exposure prediction under such conditions using the scales of accessibility of supplied air (ASA) and accessibility of contaminant source (ACS). These two scales describe the spreading performance of supplied air and indoor pollutants, respectively, and they can be figured out by computational fluid dynamics (CFD) simulation and then used to predict contaminant dispersion when flow field keeps unchanged. The new method in the form of an algebraic equation makes clear how the ventilating parameters affect the final indoor contaminant distribution. It can also be used to predict the possible health impact on occupants since this method covers both the concentration and the exposure duration of contaminants. A tracer gas measurement was conducted to validate the application model, and results show good agreement. Case study results show that transient diffusion differs very much from that under constant conditions, and this prediction method helps to flexibly control the ventilation system to meet specific requirements. (C) 2004 Elsevier B.V. All rights reserved.