2. Methods SIENA’s purpose as a simulation environment to explore urban processes from an environmental and spatial health view is reflected in the data and methods used to build it. To allow maximum flexibility for the user, SIENA is made up of a two-tier data structure consisting of a core structure and contextual information (see Fig. 1). The SIENA core structure typify a medium-sized city in Great Britain. It is based on terrain, land cover, transport network and population structures of real-world cities. These four key urban components provide important information for environmental and health analysis because they interact, both with each other and with other, external influences. For example, there is a twofold impact of the terrain on traffic-related air pollution: firstly as a determinant of meteorological conditions (e.g. wind direction) and secondly as an important influence on the spatial distribution of the road network, available building land and consequently population density. Population density is essential to predict population exposures but is also related to urban expansion and determines land cover change mainly from agricultural to urban land use and consequently has indirect influence on the transport system. Particular focus is therefore given to the interactions between these four urban components to preserve in SIENA the complex interactions that can be observed in the real world. Contextual data is the second component of the SIENA structure. This is scenario-specific data that can be added by the user as the need arises for specific