This paper describes the development of a computer simulation tool, NEOSim, capable of modelling small NEO impacts and their effect on the global population. The development of the tool draws upon existing models for the atmospheric passage and impact processes. Simulation of the land and ocean impact effects, combined with a population density model, leads to casualty estimation at both a regional and global level. Casualty predictions are based upon the intensity of each impact effect on the local population density, with consideration given to the population inside or outside local infrastructure. Two case studies are presented. The first evaluates the potential threat to the UK, and highlights coastal locations as being at greatest risk. Locations around Cornwall demonstrate an increase in casualties above the local average. The second case study concerns the potential impact of asteroid (99942) Apophis in 2036. Propagation of the possible orbits along the line of variance leads to an extensive path of risk on the Earth. Deflection of the asteroid, by a variety of means, will move the projected impact site along this path. Results generated by NEOSim for the path indicate that South American countries such as Colombia and Venezuela are at a greatest risk with estimated casualty figures in excess of 10 million. Applications of this software to the NEO threat are discussed, along with the next stage of NEO impact simulation development.