Environmental risks are inherent in the operation of any complex chemical process industry. The indoor release of hazardous chemicals that are denser than air is a topic of special concern, since dense clouds tend to persist at ground level or human breath level which leads to a magnification of their harmful potential. In the present work, we propose a computational fluid dynamics (CFD) based model for indoor risk assessment considering accidental release of a sustained, small, undetected leak of a dense toxic gas (chlorine) in an industrial indoor environment. Results from simulations show that the denser chlorine gas spreads like a liquid and flows all along the floor. At the same time, its concentration at a point away from the ground level increases slowly, thus showing that both stratification and dilution effects are present as the dense gas spreads. The implications of this spreading pattern from a risk assessment and risk mitigation point of view are discussed. (C) 2012 Elsevier BM. All rights reserved.