Understanding the transport of hazardous airborne materials within buildings and other enclosed spaces is important for predicting and mitigating the impacts of deliberate terrorist releases of chemical and biological materials. Multizone models provide an approach to modelling the contamination levels in enclosed spaces but in certain cases they can be computationally expensive. Alternative methods are being explored at DSTL that involve the direct solution of the contaminant dynamics equation. The Study Group was asked to identify the limits to this alternative approach and to explore its extension.